Introduction: Ecology, Evolution, and the Scientific Method

Searching for Life at the Bottom of the Ocean

1Introduction: Ecology, Evolution, and the Scientific Method A deep-sea vent. In some regions of the ocean floor, hot water containing sulfur compounds is released from the ground. The sulfur compounds provide energy for chemosynthetic bacteria, which then serve as food for many other species that live near the vents, including these rust-colored tube worms (Tevnia jerichonana) that have been stained orange by iron compounds emitted from the vents. Searching for Life at the Bottom of the Ocean In the early 1800s, scientists hypothesized that deep ocean waters— depths greater than 275 m where sunlight cannot penetrate—were devoid of life. Without sunlight there can be no photosynthesis, and without photosynthesis there can be no plants or algae to serve as food for other organisms. The cold temperatures and extreme

pressures of deep ocean waters were also thought to contribute to the absence of deep-sea life. Given that ocean depths can exceed 10,000 m, it was reasonable to hypothesize that the deepest areas of the ocean could not support life. At the time, however, scientists were not able to explore the deepest regions of the ocean. As exploration continued throughout the nineteenth century, scientists’ ideas about the limits of life began to change. In an 1873 expedition, scientists aboard the British research ship HMS Challenger dragged a large, open-sided heavy box suspended from long ropes behind the ship across the floor of the Atlantic Ocean. This box—known as a dredge—sampled the sea floor in different parts of the ocean at depths of up to 4,572 m. The scientists were astonished to discover nearly 5,000 new species. When it became clear that life flourished at depths beyond the penetration of light, scientists were forced to reject their earlier hypothesis that no life existed in the deep ocean waters. After discovering this rich abundance of deep-sea life, scientists needed to understand how it could exist. The lack of light suggested that deep-sea organisms were somehow sustained by energy that did not come from photosynthesis on the ocean floor. Scientists had observed that the surface waters of the ocean produced a steady descent of tiny particles—known as “marine snow”—that were produced by the death and decomposition of organisms living near the surface. In addition to marine snow, when large organisms such as whales died, they fell to the ocean floor. Scientists hypothesized that marine snow and the decaying bodies of large organisms must be the energy source needed to sustain organisms in deep ocean waters. In the 1970s, scientists were finally able to send small submarines to take a firsthand look at the deepest ocean areas. Their discoveries were shocking. They confirmed that much of the ocean floor supported living organisms, and furthermore that areas near openings in the floor of the ocean, called hydrothermal vents, contained a great diversity of deep-sea species. Hydrothermal vents release plumes of hot water with high concentrations of sulfur compounds and other mineral nutrients. A tremendous number of species surrounded these hydrothermal vents, including clams, crabs, and fish. Indeed, the total

amount of life at these depths rivaled that seen in some of the most diverse places on Earth. It became clear that the amount of energy contained in the descending marine snow was not sufficient to support such a diverse and abundant set of life forms. That hypothesis now had to be rejected. “How could so much life exist at the bottom of the ocean?” The observation that life existed near the hydrothermal vents suggested that the vents were somehow supplying the energy for these species. Scientists had known for a long time that some species of bacteria could obtain their energy from chemicals rather than from the Sun. The bacteria use the energy in chemical bonds, combined with carbon dioxide (CO2), to produce organic compounds—a process known as chemosynthesis—similar to the way that plants and algae use the energy of the Sun and CO2 to produce organic compounds through photosynthesis. Based on this knowledge, scientists hypothesized that the hot vents, which release water with dissolved hydrogen sulfide gas and other chemicals, provided a source of energy for bacteria and that these bacteria could be consumed by the other organisms living around the vents. After several years of investigations, researchers found that the immediate area around the hot vents contained a group of organisms known as tubeworms, which can grow to more than 2 m long. These animals possess specialized organs that house vast numbers of chemosynthetic bacteria that live in a close relationship with the tubeworms. The tubeworms capture the sulfide gases and CO2 from the surrounding water and pass these compounds to the bacteria. The bacteria then use the sulfide gases and CO2 to produce organic compounds. Some of these organic compounds are passed to the tubeworms, which use the organic compounds as food. These bacteria also represent a food source for many of the other animals

that live near the vents. In turn, these bacteria-consuming animals can be consumed by larger animals, such as fish. Research on deep-sea vents continues today, and our hypotheses continue to be revised. Searching for deep-sea vents has historically been difficult and researchers estimated that the vents were relatively rare and widely spaced at 12 to 200 km apart. In 2016, researchers using advanced sensors that could detect the chemicals produced by the vents discovered that the deep-sea vents are actually three to six times more abundant than previously thought and are spaced only 3 to 20 km apart. The story of the deep-sea vents demonstrates how scientists work: They make observations, devise hypotheses, test the hypotheses to confirm or reject them, and, if a hypothesis is rejected, devise a new hypothesis. As you will see throughout this chapter and subsequent chapters, science is an ongoing process that often leads to fascinating discoveries about how nature works. SOURCES: Baker, E. T., et al. 2016. How many vent fields? New estimates of vent field populations on ocean ridges from precise mapping of hydrothermal discharge locations, Earth and Planetary Science Letters 449:186–96. Dubilier, N., et al. 2008. Symbiotic diversity in marine animals: The art of harnessing chemosynthesis, Nature Reviews Microbiology 6:725–740. Dunn, R. R. 2002. Every Living Thing (HarperCollins).

Learning Objectives

After reading this chapter, you should be able to:

1.1 Illustrate how ecological systems exist in a hierarchical organization.

1.2 Understand how ecological systems are governed by physical and biological principles.

1.3 Describe how different organisms play diverse roles in ecological systems.

1.4 Distinguish the many approaches scientists use to studying ecology.

1.5 Explain how humans influence ecological systems. The story of deep-sea vents offers an excellent introduction to the science of ecology. Ecology is the scientific study of the interactions among organisms and the environment. Ecology The scientific study of the abundance and distribution of organisms in relation to other organisms and environmental conditions. Although Charles Darwin never used the word ecology in his writings, he appreciated the importance of beneficial and harmful interactions among species. In his 1859 book On the Origin of Species, Darwin compared the large number of interactions among species in nature to the large number of interactions among consumers and businesses in human economic systems. With this metaphor in mind, he described species interactions as “the economy of nature,” which a century later inspired the title of this ecology textbook. The word ecology came into general use in the late 1800s. Since that time, the science of ecology has grown and diversified. Professional ecologists now number in the tens of thousands and have produced an immense body of knowledge about the world around us. Ecology is an active, modern science that continues to yield fascinating new insights about the environment and our impact on it. As we saw in the chapter opening story about life in the ocean depths, science is an ongoing process through which our understanding of nature constantly changes. Scientific investigation uses a variety of tools to understand how nature works. This understanding is never complete or absolute. It changes constantly as scientists make new discoveries and as growing human populations and technological advances cause major environmental changes, often with dramatic consequences. With the knowledge that ecologists provide through their study of the natural world, we are in a better position to develop effective policies to manage environmental concerns related to land use, water, natural catastrophes, and

public health. This chapter will start you on the road to thinking like an ecologist. Throughout this book, we will consider the full range of ecological systems —biological entities that have their own internal processes and interact with their external surroundings. Ecological systems exist at many different levels, ranging from an individual organism to the entire globe. Despite tremendous variations in size, all ecological systems obey the same principles with regard to their physical and chemical attributes and the regulation of their structure and function. Ecological systems Biological entities that have their own internal processes and interact with their external surroundings. We begin this journey by examining the many different levels of organization for ecological systems, the physical and biological principles that govern ecological systems, and the different roles species play. Once we understand these basics, we will consider the many approaches to studying ecology and then consider the importance of understanding ecology when faced with the wide variety of ways that humans affect ecological systems.

1.1 Ecological Systems Exist in a Hierarchy of Organization

1.1 Ecological systems exist in a hierarchy of organization An ecological system may be an individual, a population, a community, an ecosystem, or the entire biosphere. As you can see in Figure 1.1, each ecological system is a subset of the next larger one, forming a hierarchy. In this section, we will examine the individual components of ecological systems and how we study ecology at different levels in the ecological hierarchy.

Figure 1.1 The hierarchy of organization in ecological systems. At each level of complexity, ecologists study different processes.

Individuals

An individual is a living being—the most fundamental unit of ecology. Although smaller units in biology exist—for example, an organ, a cell, or a macromolecule—none of them has a separate life in the environment. Every individual has a membrane, or other covering, across which it exchanges energy and materials with its environment. This boundary separates the internal processes and structures of the ecological system from the external

resources and conditions of the environment. In the course of its life, an individual transforms energy and processes materials. To accomplish this, it must acquire energy and nutrients from its surroundings and rid itself of unwanted waste products. This process alters the conditions of the environment and affects the resources available for other organisms. It also contributes to the movement of energy and chemical elements. Individual A living being; the most fundamental unit of ecology.

Populations and Species

Scientists assign organisms to particular species. Historically, the term species was defined as a group of organisms that naturally interbreed with each other and produce fertile offspring. Over time, scientists have realized that this definition does not fit all species. In fact, no single definition can apply to all organisms. For example, some species of salamanders are all female and only produce daughters, which are clones of their mothers. In this case, individuals do not interbreed, but we consider these individuals to be of the same species because they are genetically very similar to each other. In addition, some organisms that we consider distinct species can interbreed. In such cases, we cannot use a lack of interbreeding to draw the line between species. Species Historically defined as a group of organisms that naturally interbreed with each other and produce fertile offspring. Current research demonstrates that no single definition can be applied to all organisms. Defining a species becomes even more complicated when we consider organisms such as bacteria. Scientists increasingly appreciate that bacteria can transfer bits of their DNA to other bacteria that are not closely related, a process known as horizontal gene transfer. This can happen in a number of ways: when a bacterium engulfs genetic material from the environment, when two bacteria come into contact and exchange genetic material, or when a virus transfers genetic material between two bacteria. Such cases make it difficult to group bacteria into distinct species. Despite these difficulties, the term species has still proven useful to ecologists.

A population consists of individuals of the same species living in a particular area. For example, we might talk about a population of catfish living in a pond, a population of wolves living in Canada, or a population of tubeworms living near a hydrothermal vent on the ocean floor. The boundaries that determine a population can be natural, for example, where a continent meets the ocean. Alternatively, a population might be defined by other criteria such as a political boundary. For example, a scientist might want to study the population of bald eagles (Haliaeetus leucocephalus) that resides in Pennsylvania, whereas biologists of the U.S. Fish and Wildlife Service might wish to study the bald eagle population of the entire United States. Population The individuals of the same species living in a particular area. Populations have five distinct properties that are not exhibited by individuals: geographic range, abundance, density, change in size, and composition. The geographic range of a population—also known as its distribution—is the extent of land or water within which a population lives. For example, the geographic range of the North American grizzly bear (Ursus arctos) includes western Canada, Alaska, Montana, and Wyoming. The abundance of a population refers to the total number of individuals. The density of a population refers to the number of individuals per unit of area. For instance, we might count the grizzly bears in an area and determine that there is 1 bear/100 km2. The change in size of a population refers to increases and decreases in the number of individuals in an area over time. Finally, composition of a population describes the makeup of the population in terms of gender, age, or genetics. For example, we can ask what proportion of the grizzly population is male versus female, or juvenile versus adult.

Communities

At the next level of the ecological hierarchy, we identify an ecological community, which is composed of all populations of species living together in a particular area. The populations in a community interact with each other in various ways. Some species eat other species, while others, for example, bees and the plants they pollinate, have cooperative relationships that benefit

both parties. These types of interactions influence the number of individuals in each population. A community may cover large areas, such as a forest, or may be enclosed within a very small area, such as the community of tiny organisms that live in the digestive systems of animals, or in the tiny amount of water found in tree holes. In practical terms, ecologists who study communities do not study every organism in the community. Instead, they generally study a subset of the species in the community, such as the trees, the insects, or the birds, as well as the interactions among those species. Community All populations of species living together in a particular area. The boundaries that define a community are not always rigid. For example, if you were to examine the community of plants and animals that live at the base of a mountain versus the top of a mountain in Colorado, you would find that the two locations contain many different species. However, if you were to walk up the mountain, you would notice that some species of trees, such as Douglas firs (Pseudotsuga menziesii), are abundant at the beginning of your hike and then gradually dwindle as you move higher. Other species, such as subalpine firs (Abies lasiocarpa), begin to take their place as the number of Douglas firs declines. In other words, the boundaries of the upper and lower forest communities are not distinct. Because of this, scientists must often decide on the boundaries of a community they want to study. For instance, an ecologist might decide to study the community of plants and animals on a large desert ranch in New Mexico, or the community of aquatic organisms that lives along a designated stretch of coastline in California. In these cases, no distinct boundary separates the studied community from the area that surrounds it.

Ecosystems

From communities we move on to ecosystems. An ecosystem is composed of one or more communities of living organisms interacting with their nonliving physical and chemical environments, which include water, air, temperature, sunlight, and nutrients. Ecosystems are complex ecological systems that can include thousands of different species living under a great variety of conditions. For example, we may speak of the Great Lakes ecosystem or the

Great Plains ecosystem. Ecosystem One or more communities of living organisms interacting with their nonliving physical and chemical environments. At the ecosystem level, we typically focus on the movement of energy and matter between physical and biological components of the ecosystem. Most energy that flows through ecosystems originates with sunlight and eventually escapes Earth as radiated heat. In contrast, matter cycles within and between ecosystems. With the exception of places such as deep-sea vents where energy is acquired through chemosynthesis, the energy for most ecosystems comes from the Sun and is converted to organic compounds by photosynthetic plants and algae. These organisms can then be eaten by herbivores—animals that eat plants—which, in turn, are eaten by carnivores —animals that eat other animals. In addition, dead organisms and their waste products can be consumed by detritivores, which themselves can be consumed by other animals. In all these cases, each step results in some of the energy originally assimilated from sunlight being converted into growth or reproduction of consumers; the remainder of the energy is lost to the surroundings as heat and is eventually radiated back into space. When considering matter in an ecosystem, we often look at the most common elements that organisms use, such as carbon, oxygen, hydrogen, nitrogen, and phosphorus. These elements comprise a major portion of the most important compounds for living organisms, including water, carbohydrates, proteins, and DNA. These elements can be held in many different places, or pools, on Earth, including in living organisms, and in the atmosphere, water, and rocks. The movement of these elements among these pools is known as the flow of matter. For instance, many nutrients that are in the soil are taken up by plants, and these plants are consumed by animals. The nutrients exist in an animal’s tissues, and many leave as excreted waste. When the animal dies, the nutrients in its tissues are returned to the soil, thereby completing the cycling of nutrients. The boundaries of ecosystems, like those of populations and communities, are often not distinct. Scientists generally distinguish ecosystems by their relative isolation with respect to flows of energy and materials, but, in reality,

few ecosystems are completely isolated. Even aquatic and terrestrial ecosystems exchange materials and energy by runoff from the land and the harvesting of aquatic organisms by terrestrial consumers, as when bears capture salmon on their upstream spawning runs.

The Biosphere

At the highest level of the ecological hierarchy is the biosphere, which includes all the ecosystems on Earth. As Figure 1.2 illustrates, distant ecosystems are linked together by exchanges of energy and nutrients carried by currents of wind and water and by the movements of organisms, such as migrating whales, birds, and fish. Such movement connects terrestrial, freshwater, and marine ecosystems by carrying soil, nutrients, and organisms.

Figure 1.2 The biosphere. The biosphere consists of all ecosystems on Earth, which are linked together by movements of air, water, and organisms. Biosphere All the ecosystems on Earth. The biosphere is the ultimate ecological system. All transformations of the biosphere are internal, with two exceptions: the energy that enters from the Sun, and the energy that is lost to space. The biosphere holds practically all materials that it has ever had, and retains whatever waste materials we generate.

#### Studying Ecology at Different Levels of Organization

Organization

Each level in the hierarchy of ecological systems is distinguished by unique structures and processes. As a result, ecologists have developed different approaches for exploring these levels and for answering the questions that arise. The five approaches to studying ecology match the different levels of hierarchy: the individual approach, the population approach, the community approach, the ecosystem approach, and the biosphere approach. The individual approach to ecology emphasizes the way in which an individual’s morphology (the size and shape of its body), physiology, and behavior enable it to survive in its environment. This approach also seeks to understand why an organism lives in some environments but not in others. For example, an ecologist studying plants at the organism level might ask why trees are dominant in warm, moist environments, whereas shrubs with small, tough leaves are dominant in environments with cool, wet winters and hot, dry summers. Individual approach An approach to ecology that emphasizes the way in which an individual’s morphology, physiology, and behavior enable it to survive in its environment. Ecologists who use the individual approach are often interested in adaptations—the characteristics of an organism that make it well-suited to its environment. For example, desert animals have enhanced kidney function to help them conserve water. The cryptic coloration of many animals helps them avoid detection by predators. Flowers are shaped and scented to attract certain kinds of pollinators. Adaptations are the result of evolutionary change through the process of natural selection, which we will consider later in this chapter. Adaptation A characteristic of an organism that makes it well suited to its environment. The population approach to ecology examines variation over time and space in the number of individuals, the density of individuals, and the composition of individuals, which includes the sex ratio, the distribution of individuals among different age classes, and the genetic makeup of a

population. Changes in the number or density of individuals can reflect the balance of births and deaths within a population, as well as immigration and emigration of individuals from a local population. This can be influenced by a number of factors, including interactions with other species and the physical conditions of the environment, such as temperature or the availability of water. In the process of evolution, genetic mutations may alter birth and death rates, genetically distinct types of individuals may become common within a population, and the overall genetic makeup of the population may change. Because other species might serve as food, pathogens, or predators, interactions among species can also influence the births and deaths of individuals within a population. Population approach An approach to ecology that emphasizes variation over time and space in the number of individuals, the density of individuals, and the composition of individuals. The community approach to ecology is concerned with understanding the diversity and relative abundances of different kinds of organisms living together in the same place. The community approach focuses on interactions between populations, which can either promote or limit the coexistence of species (Figure 1.3). For example, in studying the Serengeti Plains of Africa, an ecologist taking the community approach might ask how the presence of zebras, which consume grasses, might affect the abundance of other species, such as gazelles, which also consume the grasses.

Figure 1.3 The community approach to ecology. Ecologists using the community approach study interactions among plants and animals that live together. For example, on an African plain, ecologists might ask how cheetahs affect the abundance of gazelles and how the gazelles affect the abundance of the plants that they consume. Community approach An approach to ecology that emphasizes the diversity and relative abundances of different kinds of organisms living together in the same place. The ecosystem approach to ecology describes the storage and transfer of energy and matter, including the various chemical elements essential to life, such as oxygen, carbon, nitrogen, phosphorus, and sulfur. These movements of energy and matter occur through the activities of organisms and through the physical and chemical transformations that occur in the soil, atmosphere, and water. Ecosystem approach An approach to ecology that emphasizes the storage and transfer of energy and matter, including the various chemical elements essential to life. The biosphere approach to ecology is concerned with the largest scale in the hierarchy of ecological systems. This approach tackles the movements of

air and water—and the energy and chemical elements they contain—over Earth’s surface. Ocean currents and winds carry the heat and moisture that define the climates at each location on Earth, which, in turn, govern the distributions of organisms, the dynamics of populations, the composition of communities, and the productivity of ecosystems. Biosphere approach An approach to ecology concerned with the largest scale in the hierarchy of ecological systems, including movements of air and water—and the energy and chemical elements they contain—over Earth’s surface. We have described these five approaches as distinct. However, most ecologists use multiple approaches to study the natural world. A scientist who wants to understand how an ecosystem will respond to a drought, for example, will likely want to know how individual plants and animals respond to a lack of water, how these individual responses affect the populations of plants and animals, how a change in the populations might affect interactions among species, and how a change in species interactions might affect the flow of energy and matter.

Concept Check

1. What is ecology? 2. Why do ecologists consider both individuals and ecosystems to be ecological systems? 3. What are the unique processes that are examined when taking the individual, population, community, and ecosystem approaches to studying ecology?

1.2 Ecological Systems Are Governed by Physical and Biological Principles

1.2 Ecological systems are governed by physical and biological principles Although ecological systems are complex, they are governed by a few basic principles. Life builds on the physical properties and chemical reactions of matter. The diffusion of oxygen across body surfaces, the rates of chemical reactions, the resistance of vessels to the flow of fluids, and the transmission of nerve impulses all obey the laws of thermodynamics. Within these constraints, life can pursue many options and has done so with astounding innovation. In this section, we briefly review the three major biological principles that you may recall from your introductory biology course: conservation of matter and energy, dynamic steady states, and evolution.

Conservation of Matter and Energy

The law of conservation of matter states that matter cannot be created or destroyed, but can only change form. For example, as you drive a car, gasoline is burned in the engine; the amount of fuel in the tank declines, but you have not destroyed matter. The molecules that comprise gasoline are converted into new forms, including carbon monoxide (CO), carbon dioxide (CO2), and water (H2O). Law of conservation of matter Matter cannot be created or destroyed; it can only change form. Another important biological principle, the law of conservation of energy —also known as the first law of thermodynamics—states that energy cannot be created or destroyed. Like matter, energy can only be converted into different forms. Living organisms must constantly obtain energy to grow, maintain their bodies, and replace energy lost as heat. Law of conservation of energy Energy cannot be created or destroyed; it can only change form. Also known as The first law of thermodynamics. These two laws imply that ecologists can track the movement of matter and energy as it is converted into new forms through organisms, populations, communities, ecosystems, and the biosphere. At every level of organization,

we should be able to determine how much energy and matter enter the system and account for its movement. For example, consider a field full of cattle (Bos taurus) eating grass. At the organismal level, we can determine how much energy an individual animal consumes and then calculate the proportion of this energy that is converted into growth of its body, the maintenance of its physiology, and its waste. At the population level, we can calculate how much energy the entire herd of cattle consumes by eating grass. At the community level, we can evaluate how much energy each species of grass creates via photosynthesis and how much of this energy is passed on to cattle and other plant-eating species, such as rabbits, that might coexist with the cattle. At the ecosystem level, we can estimate how elements such as carbon flow from the grasses to herbivores (cattle and rabbits) and then on to predators. We can then track how dead grass, the waste products of herbivores and predators, and the dead bodies of herbivores and predators all decompose and return to the soil. At the biosphere level, we can examine how the energy flows among the many ecosystems and moves around the world.

Dynamic Steady States

Although matter and energy cannot be created or destroyed, ecological systems continuously exchange matter and energy with their surroundings. When gains and losses are in balance, ecological systems are unchanged and the system is said to be in a dynamic steady state. For example, birds and mammals continuously lose heat in a cold environment. However, this loss is balanced by heat gained from the metabolism of foods, so the animal’s body temperature remains constant. Similarly, the proteins of our bodies are continuously broken down and replaced by newly synthesized proteins, so our appearance remains relatively unchanged. Dynamic steady state When the gains and losses of ecological systems are in balance. The principle of the dynamic steady state applies to all levels of ecological organization, as illustrated in Figure 1.4. For individual organisms, assimilated food and energy must balance energy expenditure and metabolic breakdown of tissues. A population increases with births and immigration, and it

decreases with death and emigration. At the community level, the number of species living in a community decreases when a species becomes extinct, and increases when a new species colonizes the area. Ecosystems and the biosphere receive energy from the Sun, and this gain of energy is balanced by heat energy radiated by Earth back out into space. One of the most important questions ecologists ask is how the steady states of ecological systems are maintained and regulated. We will return to this question frequently throughout this book.

Figure 1.4 Dynamic steady states. At all levels of organization, the inputs to the systems must equal the outputs.

An understanding of dynamic steady states helps provide insights regarding the inputs and outputs of ecological systems. Of course, ecological systems also change. Organisms grow, populations vary in abundance, and abandoned fields revert to forest. Yet all ecological systems have mechanisms that tend to maintain a dynamic steady state.

Evolution

Although matter and energy cannot be created or destroyed, what living systems do with matter and energy is as variable as all the forms of organisms that have ever existed on Earth. To understand the variation among organisms—the diversity of life—we turn to the concept of evolution. An attribute of an organism, such as its behavior, morphology, or physiology, is the organism’s phenotype. A phenotype is determined by the interaction of the organism’s genotype, or the set of genes it carries, with the environment in which it lives. For example, your height is a phenotype that is determined by your genes and the nutrition you received in the environment where you were raised. Phenotype An attribute of an organism, such as its behavior, morphology, or physiology. Genotype The set of genes an organism carries. Over the history of life on Earth, the phenotypes of organisms have changed and diversified dramatically. This is the process of evolution, which is a change in the genetic composition of a population over time. Evolution can happen through a number of different processes that we will discuss in detail in later chapters. Perhaps the best known process is evolution by natural selection, which is a change in the frequency of genes in a population through differential survival and reproduction of individuals that possess certain phenotypes. As outlined by Charles Darwin in his book On the Origin of Species, evolution by natural selection depends on three conditions: 1. Individual organisms vary in their traits. 2. Parental traits are inherited by their offspring.

3. The variation in traits causes some individuals to experience higher fitness, which we define as the survival and reproduction of an individual. Evolution Change in the genetic composition of a population over time. Natural selection Change in the frequency of genes in a population through differential survival and reproduction of individuals that possess certain phenotypes. Fitness The survival and reproduction of an individual. When these three conditions exist, an individual with higher survival and reproductive success will pass more copies of his or her genes to the next generation. Over time, the genetic composition of a population changes as the most successful phenotypes come to predominate. As a result, the population becomes better suited to the surrounding environmental conditions. Phenotypes that are well suited to their environment and, in turn, confer higher fitness are known as adaptations. Consider the example in Figure 1.5 in which some individuals in a population of caterpillars are colored in such a way that they blend in with their surroundings and escape the notice of predators while other individuals are not. If color is inherited, over time the population will consist of a progressively larger proportion of caterpillars that blend in with their environment.

Figure 1.5 Evolution by natural selection. In this example, the caterpillar population is initially quite variable in color (a). Individuals that better match the twig are less obvious to the bird hunting for food and therefore more likely to survive. If color is genetically inherited, the next generation (b) of the caterpillar population will be better matched to resemble twigs. As this natural selection continues over many generations, the color of the caterpillar population will closely match the twigs (c). At this stage, the color of the caterpillar represents an adaptation against predation. Species do not evolve in isolation. Rather, evolution in one species opens up new possibilities for other species with which the evolving species interacts. For example, milkweed plants have evolved the ability to produce a sap that is toxic to most herbivores. However, caterpillars of the monarch butterfly (Danaus plexippus) have evolved the ability to eat the leaves of milkweed plants and tolerate the toxic chemicals. Monarch caterpillars not only tolerate these toxic compounds, but also sequester these compounds in their bodies and use the toxins to defend themselves, as larvae as well as

adults, against bird predators. In addition, both larvae and adults have evolved conspicuous “warning” coloration to advertise their toxicity. After the caterpillar species evolved these defensive abilities, predatory birds evolved a new ability to discriminate between caterpillars and butterflies that were toxic and those that were edible. In this case, the evolution of toxic chemicals in milkweed plants led to the evolution of chemical tolerance by the monarch butterflies, which further led to birds evolving the ability to discriminate between the unpalatable monarch butterflies and other species of palatable butterflies. As you can see from these examples, the complexity of ecological communities and ecosystems builds on, and is fostered by, existing complexity. Ecologists seek to understand how these complex ecological systems came into being and how they function in their environmental settings.

1. Describe how ecological systems are governed by physical and biological principles. 2. What does it mean when we say that ecological systems are in a dynamic steady state? 3. What are the three conditions required for evolution by natural selection to occur?

1.3 Different Organisms Play Diverse Roles in Ecological Systems

1.3 Different organisms play diverse roles in ecological systems Transformations of matter and energy in ecological systems are performed by both small and large forms of life, and these different life forms can play unique roles in ecological systems. In this section, we will examine how organisms interact with each other and the environment. We will see how ecologists categorize species based on how they obtain their energy and how they interact with other species. We will also describe the major groups of organisms that have evolved, discuss the diverse ecological roles found within each group, and examine the concepts of habitat and niche.

Broad Evolutionary Patterns

Early in the history of Earth, ecosystems were dominated by bacteria. The ancient evolution of these groups is still debated by scientists, but a leading hypothesis, illustrated in Figure 1.6, is that bacteria are the oldest group of organisms. Over time, bacteria gave rise to the archaea. Bacteria and archaea are both prokaryotic organisms, which are single-cell organisms that do not possess distinct cellular organelles, such as a nucleus. These evolutionary events likely happened in the ocean and may have occurred near the deep-sea vents that we discussed in the chapter opener. If so, it may be that the first bacteria used chemosynthesis and this later gave rise to the evolution of photosynthesis.

Figure 1.6 The evolution of life on Earth. Bacteria are the oldest forms of life on Earth. The engulfing of one bacterial species by another led to the rise of eukaryotes containing cell organelles such as mitochondria and chloroplasts. Over time, eukaryotic organisms evolved, which possess distinct cellular organelles. The key event in the evolution of eukaryotes occurred when one bacterium engulfed another. The engulfed bacterium became what we now know as the mitochondrion, an important organelle for cellular respiration in eukaryotic organisms. This ancestor subsequently gave rise to all modern organisms that contain mitochondria, including red algae, green algae, plants, fungi, and animals. As the evolution of the eukaryotes progressed, there was a second key event. A eukaryotic cell engulfed another bacterium that was capable of photosynthesis, which evolved into what we now know as the chloroplast. The group of eukaryotic organisms that contained chloroplasts subsequently gave rise to modern-day red algae, green algae, and plants. Those species that didn’t contain chloroplasts gave rise to modern-day fungi and animals. Bacteria also modified the biosphere, making it possible for other forms of life to exist. For example, photosynthetic bacteria that were present more than 3 billion years ago produced oxygen as a by-product of photosynthesis. The higher concentrations of oxygen in the atmosphere and oceans favored the

evolution of additional life forms that required oxygen, such as plants and animals. Despite all these changes, bacteria have persisted to the present day. As we shall see, their unique biochemical capabilities allow them to consume resources that their more complex descendants cannot use, and to tolerate ecological conditions that are beyond the capacities of other organisms. Ecosystems depend on the activities of many forms of life. Each major group fills a unique and necessary role in the biosphere. We will now briefly review the major groups of organisms, including bacteria, protists, plants, fungi, and animals. Bacteria Although bacteria are microscopic, their enormous range of metabolic capabilities enables them to accomplish many unique biochemical transformations and to occupy parts of the biosphere where plants, animals, fungi, and most protists cannot survive. Some bacteria can assimilate molecular nitrogen (N2) from the atmosphere, which they use to synthesize proteins and nucleic acids. Other species of bacteria, such as those living in deep-sea hydrothermal vents, can use inorganic compounds such as hydrogen sulfide (H2S) as sources of energy in chemosynthesis. Furthermore, many bacteria can live under anaerobic conditions, such as in mucky soils and sediments, in which free oxygen is lacking, where their metabolic activities release nutrients that can be taken up by plants. Finally, some bacteria, including cyanobacteria (colloquially known as blue-green algae), can conduct photosynthesis. Cyanobacteria account for a large fraction of the photosynthesis that occurs in aquatic ecosystems. When bodies of water contain high amounts of nutrients, cyanobacteria can form dense populations that turn the water green, an event known as an algal bloom (Figure 1.7). Later in the book, we will have much more to say about the special place of bacteria in ecosystems.

Figure 1.7 Cyanobacteria. Also known as blue-green algae, cyanobacteria are bacteria capable of conducting photosynthesis. These organisms can grow rapidly under highly fertile conditions, producing large floating mats in the water that can be toxic to animals, as illustrated in this photo of Lake Mendota, Wisconsin. Algal bloom A rapid increase in the growth of algae in aquatic habitats, typically due to an influx of nutrients. Protists Protists are a highly diverse group of mostly single-celled eukaryotic organisms that includes the algae, slime molds, and protozoans. This bewildering variety of protists fills almost every ecological role. For instance, algae are the primary photosynthetic organisms in most aquatic ecosystems. Some algae can form large plantlike structures, such as the seaweed known as kelps, which can grow up to 100 m in length. Because of this large size, regions of the ocean containing large amounts of kelps, such as that shown in

Figure 1.8, are called kelp forests. Although kelps may resemble large plants, the actual organization of kelp tissues is less structurally complex than that of trees and other plants.

Figure 1.8 Kelp forests. Although most protists are very small organisms, some protists such as seaweeds can grow very large and look like large plants. This kelp forest is off the coast of Southern California. Other protists are not photosynthetic. Foraminifera and radiolarians are protists that feed on tiny particles of organic matter or absorb small dissolved organic molecules. Some of the ciliate protozoans are effective predators of other microorganisms. Many protists live in the guts or other tissues of a host organism where they might be helpful or cause harm. For example, termites are a type of insect that consume large amounts of cellulose. Cellulose is a very difficult substance for animals to digest, but the termite has a community of protists—as well as bacteria—in its gut that are very effective at breaking down the cellulose. Some of the best known harmful protists include Plasmodium, which causes malaria in humans, and Trypanosoma brucei, which causes sleeping sickness. Plants Plants are well known for their role in using the energy of sunlight to synthesize organic molecules from CO2 and H2O. On land, most plants have structures with large exposed surfaces—their leaves—to capture the energy from sunlight (Figure 1.9a). Leaves are thin because surface area is more important than leaf thickness for capturing light energy. To obtain carbon, terrestrial plants take up gaseous CO2 from the atmosphere. At the same time, they lose large amounts of water by evaporation from their leaf tissues to the

atmosphere. Thus, plants need a steady supply of water to replace the water lost during photosynthesis. Most plants are firmly rooted in the ground and in constant contact with water in the soil. Others, including orchids and a variety of tropical “air plants” (epiphytes), typically grow by attaching themselves to other plants—often the trunks of trees—and can photosynthesize only in humid environments that are bathed in clouds (Figure 1.9b). Although we typically envision plants as organisms that obtain their energy from sunlight, plants can also obtain energy in other ways. For example, several groups of plants have evolved to be simultaneously photosynthetic and carnivorous. These plants include the Venus fly trap (Dionaea muscipula), several species of sundews, and several species of pitcher plants (Figure 1.9c). They often live in locations that are low in nutrients, so the invertebrates they trap and consume provide an additional source of nutrients and energy. In addition, more than 400 species of plants— including more than 200 species of orchids—lack chlorophyll and therefore cannot photosynthesize to obtain energy. Scientists once thought that these plants were acting as decomposers and obtained their organic carbon from dead organic matter, but we now know that many of these plants are, in fact, acting as parasites on fungi, which are the real decomposers in the ecosystem. These parasitic plants obtain the vast majority of their organic carbon from fungi. Other plants, such as dodder, have little chlorophyll and no roots (Figure 1.9d). Instead, the stringy plant—also known as strangle weed or devil’s guts—winds around other plants, penetrates their tissues, and sucks up water, nutrients, and products of photosynthesis. The dodder is a serious pest for many farm crops.

Figure 1.9 Plants. Plants can play many roles in an ecosystem. (a) Most plants, such as this garlic mustard (Alliaria petiolata), are rooted in the soil and make organic compounds by photosynthesis. (b) Epiphytes, such as this Haraella odorata, also conduct photosynthesis but grow above the ground and are attached to other plants. (c) Carnivorous plants, such as this Venus flytrap (Dionaea muscipula), can both photosynthesize and obtain nutrients by trapping and digesting invertebrates. (d) Some plants, such as dodder, act as parasites by taking nutrients from other plants. Fungi Fungi assume unique roles in the biosphere because of their distinctive growth form. Whereas some fungi such as yeasts are unicellular, most other fungi are multicellular. Most fungal organisms consist of threadlike structures called hyphae that are only a single cell in diameter. These hyphae may either form a loose network, which can invade plant or animal tissues or dead leaves and wood on the soil surface, or grow together into reproductive structures such as mushrooms (Figure 1.10). Because fungal hyphae are able to penetrate deeply into tissue, they readily decompose dead plant material, eventually making nutrients available to other organisms. Fungi digest their foods externally and secrete acids and enzymes into their immediate surroundings. Such digestion allows fungi to decompose dead organisms and dissolve nutrients from soil minerals.

Figure 1.10 Fungi. The mushrooms produced by this sulphur tuft fungus (Hypholoma fasciculare) in Belgium are fruiting bodies of much larger, unseen masses of threadlike hyphae that penetrate decaying wood and leaf litter. Fungi are effective decomposers. Although most fungi function as decomposers, they can interact with other species in both positive and negative roles. Many fungi live in mutualistic relationships with plants, living either around or within the roots of plants. Using their extensive network of hyphae, they obtain scarce nutrients from the surrounding soil and provide them to the plant. In exchange, the plant provides the products of photosynthesis. Other fungi, though, can act as pathogens. Several closely related species of fungus cause Dutch elm disease, which has caused widespread death in several species of elm trees throughout North America and Europe during the past 100 years. Fungal pathogens are also a major problem for many crops, including potatoes, wheat, and rice. Animals Animals play a wide range of roles as consumers in ecological systems. Some animals, for example, elephants, gazelles, and voles, eat plants. Other animals, such as mountain lions, rattlesnakes, and frogs, eat other animals. Ticks, lice, and tapeworms are animals that live on or in other organisms. Finally, animals such as flies, bees, butterflies, moths, and bats can serve as

Categorizing Species Based on Sources of Energy

Ecologists often categorize organisms according to how they obtain energy, as illustrated in Figure 1.11. Organisms that use photosynthesis to convert solar energy into organic compounds or use chemosynthesis to convert chemical energy into organic compounds are known as producers or autotrophs. Organisms that obtain their energy from other organisms are known as consumers or heterotrophs. There are many different kinds of heterotrophs. Some consume plants, some consume animals, and some consume dead organic matter. In the next section, we will discuss these various interactions in more detail.

Figure 1.11 Categories of species based on their energy sources. Species that obtain their energy from photo synthesis or chemosynthesis are known as producers or autotrophs. Species that obtain their energy from consuming other species are heterotrophs. Species that can take a mixed strategy of being producers and heterotrophs are mixotrophs. Producer An organism that uses photosynthesis to convert solar energy into organic compounds or uses chemosynthesis to convert chemical energy into organic compounds. Also known as Autotroph. Consumer An organism that obtains its energy from other organisms. Also known as Heterotroph. Not all species fit neatly into categories of autotrophs or heterotrophs. Some species can obtain their sources of carbon through a variety of ways. Because these species take a mixed approach to obtaining their energy, they are called mixotrophs. Mixotrophs are quite common in nature. For example,

some bacteria can switch back and forth between photosynthesis and chemosynthesis. In addition, many species of algae can photosynthesize and obtain organic carbon by engulfing bacteria, protists, and bits of organic carbon that exist in the water. Other mixotrophs include the carnivorous plants that we discussed earlier in this chapter. These plants obtain their energy both from photosynthesis and from consuming invertebrates. Mixotroph An organism that obtains its energy from more than one source.

Types of Species Interactions

In considering the diversity of species that exist on Earth, we are often interested in the roles that they play. Ecologists categorize species by the types of interactions they have with other species, as you can see from the examples in Figure 1.12. Below is a brief introduction to these interactions, beginning with the various types of consumers, each of which will be covered in much greater detail in later chapters.

Figure 1.12 The four types of consumers. Predators can be broken down into predators such as mountain lions, parasitoids such as braconid wasps shown here on a tomato hornworm, parasites such as winter ticks, and herbivores such as bison. Predation Predators are organisms that kill and partially or entirely consume another individual. The mountain lion, for example, is a predator that kills whitetailed deer (Odocoileus virginianus) and many other species of prey.

Predator An organism that kills and partially or entirely consumes another individual. Parasitoids represent a special kind of predator. Parasitoids lay their eggs on or inside other animals, particularly insects, and the eggs hatch into larvae that consume the host individual from the inside, eventually killing it. Most parasitoid species are wasps and flies. Parasitoid An organism that lives within and consumes the tissues of a living host, eventually killing it. Parasitism Parasites are organisms that live in or on another organism, called the host. An individual parasite rarely kills its host, although some hosts die when they are infected by a large number of parasites. Common parasites include tapeworms and ticks. When a parasite causes a disease, it is called a pathogen. Pathogens include several species of bacteria, viruses, protists, fungi, and a group of worms called helminths. Parasite An organism that lives in or on another organism, but rarely kills it. Pathogen A parasite that causes disease in its host. Herbivory Herbivores are organisms that consume producers, such as plants and algae. When an herbivore consumes a plant, it typically consumes only a small portion of the plant and does not kill the plant. For example, caterpillars consume a few leaves or parts of leaves, which the plant can regenerate. Cattle consume the tops of grass leaves but do not destroy the growing region, located at the base of the plant. Herbivore An organism that consumes producers such as plants and algae. Competition

Competition can be defined as an interaction with negative effects between two species that depend on the same limiting resource to survive, grow, and reproduce. For example, two species of grass might compete for nitrogen in the soil. As a result, the survival, growth, and reproduction of each are reduced when living with the other species of grass in the same area compared to when they are living in the area alone. Similarly, coyotes and wolves might compete for the same prey animals in the forest, such that they survive, grow, and reproduce better when they are living alone than when the other species is present. Competition for limited resources is a very common interaction in nature. Competition An interaction resulting in negative effects between two species that depend on the same limiting resource to survive, grow, and reproduce. Mutualism When two species interact in a way such that each species receives benefits from the other, their interaction is a mutualism. The lichens in Figure 1.13, for example, are composed of a fungus living together with green algal cells or cyanobacteria as a single organism. The fungus provides nutrients to the algae and the algae provide carbohydrates from photosynthesis to the fungus. Other examples of mutualisms include the bacteria that help digest plant material in the guts of cattle, fungi that help plants extract mineral nutrients from the soil in return for carbohydrate energy from the plant, and honeybees that pollinate flowers as they obtain nectar.

Figure 1.13 Mutualism. A lichen is a symbiotic association of a fungus and algal cells. Mutualism An interaction between two species in which each species receives benefits from the other. Commensalism Commensalisms are interactions in which two species live in close association and one species receives a benefit, while the other experiences neither a benefit nor a cost. For example, plants such as burdock (Arctium lappa) produce fruits containing tiny barbs that stick to the hair of mammals that brush up against it. The burdock receives a benefit of having its seeds dispersed, while the mammal is neither helped nor harmed by carrying these fruits on its body. Commensalism An interaction in which two species live in close association and one species receives a benefit, while the other experiences neither a benefit nor a cost. Because organisms are specialized for particular ways of life, many different types of organisms are able to live together in close association. A

close physical relationship between two different types of organisms is referred to as a symbiotic relationship. Many parasites, parasitoids, mutualists, and commensal organisms live in symbiotic relationships. Symbiotic relationship When two different types of organisms live in a close physical relationship. When considering the different types of interactions among species, it can be helpful to categorize interactions between the two participants as positive (+), negative (−), or neutral (0). Table 1.1 provides a summary of species interactions using this approach.

TABLE 1.1 The Outcome of Interactions Between Two Species Type of interaction Species 1 Species 2 Predation/parasitoidism + − Parasitism + − Herbivory + − Competition − − Mutualism + + Commensalism + Interactions that provide a benefit to a species are indicated by a “+” symbol, interactions that cause harm to a species are indicated by a “−” symbol, and interactions that have no effect on a species are indicated by a “0” symbol. Consumers of Dead Organic Matter Consumers of dead organic matter—including scavengers, detritivores, and decomposers—also play important roles in nature. Scavengers, such as vultures, consume dead animals. Detritivores, such as dung beetles and many species of millipedes, break down dead organic matter and waste products—known as detritus—into smaller particles. Decomposers, such as many species of mushrooms, break down dead organic material into simpler elements and compounds that can be recycled through the ecosystem. Scavenger An organism that consumes dead animals. Detritivore

An organism that feeds on dead organic matter and waste products that are collectively known as detritus. Decomposer Organisms that break down dead organic material into simpler elements and compounds that can be recycled through the ecosystem.

Habitat Versus Niche

In addition to knowing how a species makes its living by interacting with other species, we also need to consider the physical setting in which it lives. For example, if you were to walk through a meadow in the eastern or central United States, you would likely come across the eastern cottontail rabbit (Sylvilagus floridanus). This species thrives in abandoned farm fields that are full of grasses and other tall wildflowers interspersed with shrubs. These plants provide food for the rabbit and protection from its many predators, including coyotes (Canis latrans) and several species of hawk. In considering species in nature, ecologists find it useful to distinguish between where an organism lives and what it does. The habitat of an organism is the place, or physical setting, in which it lives. In the case of the rabbit, the habitat consists of old fields that contain grasses, wildflowers, and shrubs. Habitats are distinguished by physical features, often including the predominant form of plant or animal life. Thus, we speak of forest habitats, desert habitats, stream habitats, and lake habitats (Figure 1.14).

Figure 1.14 Habitats. Terrestrial habitats are distinguished by their dominant vegetation, whereas aquatic habitats are distinguished by their depth and the presence or absence of flowing water. (a) Freshwater streams contain flowing water. (b) Lakes are typically large bodies of water that have very little flow. (c) In the tropical rainforest, warm temperatures and abundant rainfall support the highest productivity and biodiversity on Earth. (d) Tropical grasslands, which develop where rainfall is sparse, support vast herds of grazing herbivores during the productive rainy season. Habitat The place, or physical setting, in which an organism lives.

During the early years of ecological research, scientists developed a complex system of classifying habitats. For example, they began by distinguishing between terrestrial and aquatic habitats. Among aquatic habitats, they identified freshwater and marine habitats. Among the marine habitats, they classified coastal habitats, the open ocean, and the ocean floor. As such classifications became more finely divided, the distinctions began to break down since scientists found that habitat types overlap and that absolute distinctions rarely exist. However, the idea of habitat is nonetheless useful because it emphasizes the variety of conditions to which organisms are exposed. For example, inhabitants of extreme ocean depths and tropical rainforest canopies experience altogether different conditions of light, pressure, temperature, oxygen concentration, moisture, and salt concentrations, as well as differences in food resources and predators. An organism’s niche includes the range of abiotic and biotic conditions it can tolerate. In the case of the cottontail rabbit, its niche includes the ranges of temperature and humidity that it can tolerate, the plants that it eats, and the predatory hawks and coyotes with which it coexists. An important ecological principle states that each species has a distinct niche. No two species have exactly the same niche because each has distinctive attributes of form and function that determine the conditions it can tolerate, how it feeds, and how it escapes enemies. Consider the hundreds of insect species that might live in a garden; each has a unique niche in terms of the food that it eats (Figure 1.15). For example, the caterpillar of the cabbage white butterfly (Pieris rapae) feeds on the group of plants that have been cultivated from the wild mustard plant (Brassica oleracea), including cabbage, broccoli, and cauliflower. However, the Colorado potato beetle (Leptinotarsa decemlineata) feeds almost exclusively on the leaves of potato plants (Solanum tuberosum). Similarly, the European corn borer (Ostrinia nubilalis) feeds primarily on corn plants (Zea mays). The variety of habitats and niches holds the key to much of the diversity of living organisms.

Figure 1.15 Niche. Even within a group of similar organisms, such as insects, each species has a distinct niche. In the case of insects, the food they consume is only one aspect of their niche. (a) The European corn borer is specialized to feed on corn plants. (b) The Colorado potato beetle is specialized to feed on the leaves of potato plants. (c) The caterpillar of the cabbage white butterfly is specialized to feed on the leaves of cabbage, broccoli, and cauliflower. Niche The range of abiotic and biotic conditions that an organism can tolerate.

Concept Check

1. How do the sources of energy acquired by plants, animals, and fungi differ? 2. What are the major types of species interactions? 3. Compare and contrast an organism’s habitat and an organism’s niche.

1.4 Scientists use several approaches to studying ecology

1.4 Scientists use several approaches to studying ecology Scientists have investigated the diverse roles that organisms play in the environment for more than a century. Ecologists investigate their subject matter using a systematic procedure, often referred to as the scientific method. The three steps of this process, shown in Figure 1.16, are (1) observations regarding a pattern in nature, (2) development of a hypothesis and its associated predictions, and (3) testing the hypothesis.

Figure 1.16 The scientific method. The scientific method begins with observing patterns in nature and developing a hypothesis that explains how or why the pattern exists. Predictions from a hypothesis are tested with manipulative experiments, natural experiments, mathematical models, or additional observations.

Observations, Hypotheses, and Predictions

Most research begins with a set of observations about nature that invite explanation. Usually, these observations identify and describe a consistent pattern. As we learned in the history of research on the deep-sea vents, once it was discovered that the diversity and abundance of species living around

the vents could not be sustained by the relatively small amount of organic matter drifting down from the sunlit surface, new hypotheses regarding chemosynthetic bacteria had to be developed and tested. In such cases, some hypotheses will be supported, while others will be rejected and require new hypotheses. This process is the scientific method. To help you better understand the scientific method, imagine that you are walking around a pond on a warm, spring night following a rainstorm. You would likely hear male frogs making frog calls. If you returned to the same pond on cooler nights following a dry period, you would be less likely to hear frogs calling. If you traveled to many different ponds, you would observe this pattern over and over again. That is, you would observe and describe a consistent pattern in nature. Repeated natural patterns lead scientists to hypothesize about the causes of these patterns. Hypotheses are ideas that potentially explain a repeated observation. In the case of the frogs, we consistently observed that they called only on warm nights after a rainstorm. Having established the existence of this pattern, we want to understand it better. We might want to explain how frogs sense changes in temperature and rainfall, and how sensing these environmental changes stimulates frogs to call. We also might want to explain why frogs call on warm nights after it rains. How do the frogs benefit from calling—perhaps by attracting mates—and what, if any, are the costs of calling? Hypothesis An idea that potentially explains a repeated observation. Hypotheses about how and why organisms respond to the environment represent different types of explanations. The “how” explanation addresses the details of the animal’s sensory perception and changes in its hormone concentrations, nervous system, and muscular system. In the case of the frogs, we might hypothesize that the frog’s nervous system detects warm temperatures and rain. This initiates changes in a male frog’s hormones and physiology that cause him to contract muscles that make him call. Hypotheses that address the immediate changes in an organism’s hormones, physiology, nervous system, or muscular system are known as proximate hypotheses. If these hypotheses are supported by our observations, then we can make predictions. Predictions are statements that

arise logically from hypotheses. For example, if our hypothesis about how a rainy night causes male frogs to call is correct, then we can predict that any frogs exposed to warm rain will respond by changing the concentration of specific hormones that stimulate the brain to send a signal to the muscles of the vocal apparatus to contract. Proximate hypothesis A hypothesis that addresses the immediate changes in an organism’s hormones, physiology, nervous system, or muscular system. Prediction A logical consequence of a hypothesis. Ultimate hypotheses address why an organism has evolved to respond in a certain way to its environment—that is, the fitness costs and benefits of a particular response. For example, we might hypothesize that male frogs call to attract females. Furthermore, if we suspect that male frogs call to attract females, then perhaps males sing on warm nights after a rainstorm because such nights produce the best conditions for laying eggs, which is when females are most interested in mating. The males benefit from calling on a warm, wet night because they will be more likely to attract females and therefore father more offspring. If the males call at other times, they will attract fewer female mates and receive a much smaller benefit. But what about costs? We might hypothesize that when male frogs call to attract females, they risk attracting the attention of predators. The increased risk of death represents a high fitness cost to male frogs. Ultimate hypothesis A hypothesis that addresses why an organism has evolved to respond in a certain way to its environment in terms of the fitness costs and benefits of the response. We have now generated a number of predictions that logically follow from our ultimate hypotheses about male frog calling: (1) Males that call will attract females; (2) females actively search for males only on warm, wet nights because that produces the best conditions for laying eggs; and (3) if singing imposes a cost, then males should save their singing for times when it will provide the greatest benefit.

Testing Hypotheses with Manipulative Experiments

A particular hypothesis can be supported by our observations, but it can rarely be confirmed beyond a doubt. However, our confidence increases as we continue testing a hypothesis and repeatedly find that our observations support it. Although the methods of acquiring scientific knowledge appear to be straightforward, many pitfalls exist. For example, an observed relationship between two factors does not necessarily mean that one factor causes the other to change. The cause must be determined independently. To accomplish this, we can design manipulative experiments in which a hypothesis is tested by altering the factor that is hypothesized to be an underlying cause of the phenomenon. Manipulative experiment A process by which a hypothesis is tested by altering a factor that is hypothesized to be an underlying cause of the phenomenon. To understand the process of a manipulative experiment, consider the observation that herbivorous insects often consume less than 10 percent of a plant’s tissues. Ecologists have proposed several hypotheses to explain this. One hypothesis is that predators consume insect herbivores at such a high rate that insect populations remain low. This low insect population cannot eat very much of the plant tissue. This seems like a reasonable hypothesis, but how do we test it with a manipulative experiment? Researchers working on this question decided to explore whether the predation hypothesis applies to insects feeding on oak trees in Missouri. They observed that birds consume many insects on oak leaves and then hypothesized that birds reduce the populations of insect herbivores. If this hypothesis is correct, when birds are absent, insect populations should increase and consume more leaf biomass. Confirmation of this prediction would support the hypothesis; a lack of confirmation would lead them to reject the hypothesis and propose a new one. To test the hypothesis that predation by birds lowers the abundance of insects on oak trees, the researchers decided to conduct a manipulative experiment in which they used cages that excluded birds from the trees (Figure 1.17a). The manipulation, also known as the treatment, is the factor that we want to vary in an experiment. Often one of the manipulations used is a control. A control is a manipulation that includes all aspects of an

experiment except the factor of interest. In the oak tree experiment, the caged trees served as the treatment, whereas the uncaged trees served as the control.

Figure 1.17 A manipulative experiment. Manipulative experiments provide the strongest tests of hypotheses. (a) In a study that tested if birds are an important factor in determining the number of insects on oak trees in Missouri, ecologists placed cages around some white oak saplings to exclude birds and left other oak saplings uncaged to serve as a control. (b) From this experiment, the researchers measured the number of insect herbivores per leaf and

the amount of leaf tissue that was consumed in each of the two treatments. After R. J. Marquis and C. J. Whelan, Insectivorous birds increase growth of white oak through consumption of leaf-chewing insects, Ecology 75 (1994): 2007–2014. Manipulation The factor that we want to vary in an experiment. Also known as Treatment. Control A manipulation that includes all aspects of an experiment except the factor of interest. Once we decide on which manipulations we wish to do, we have to assign each manipulation to a specific experimental unit. An experimental unit is the object to which we apply the manipulation. In the case of the oak tree experiment, the researchers decided that they would use groups of three white oak tree saplings as their experimental units. After making this decision, each experimental unit was either caged—by surrounding the group of three saplings with bird-proof netting—or left uncaged to allow bird access. Experimental unit The object to which we apply an experimental manipulation. A manipulation to a single experimental unit might provide exciting results, but the results may not be reliable unless the experiment is repeated and demonstrates a similar outcome. Being able to produce a similar outcome multiple times is known as replication, which is an integral feature of most experimental studies. In the oak tree study, the investigators decided to add cages to 10 groups of trees and leave 10 groups of trees uncaged. In doing so, they replicated their experiment 10 times. Replication Being able to produce a similar outcome multiple times. When we assign different manipulations to our experimental units, the assignments must be made using randomization, which means that every experimental unit has an equal chance of being assigned a particular manipulation. In the oak tree experiment, the researchers randomly assigned groups of trees to be caged or left uncaged as controls. In this way, they could be assured that the caged trees were initially no different from the controls.

Randomization An aspect of experiment design in which every experimental unit has an equal chance of being assigned to a particular manipulation. Once the researchers set up the experiment, they collected data on the number of herbivorous insects and the percentage of leaf tissue that had been consumed. They found that caged trees had about twice as many insect herbivores as the control trees. Moreover, the percentage of leaf tissue consumed at the end of the growing season was nearly twice as high on caged trees as on the control trees (Figure 1.17b). These findings led the researchers to conclude that the experiment supported their hypothesis. Although many experiments are conducted in natural settings such as oak forests or lakes (Figure 1.18a), other experiments use smaller experimental venues. Many experiments make use of microcosms, which are simplified ecological systems that attempt to replicate the essential features of an ecological system in a laboratory or field setting. In the case of experiments that study aquatic systems, for example, microcosms might consist of large outdoor tanks of water. These tanks would include many of the features of natural water bodies, including soil, vegetation, and a diversity of organisms (Figure 1.18b). The use of microcosms assumes that a response to manipulations in a microcosm is representative of responses that would occur in a natural habitat. For example, we might wish to understand how species of fish compete for food. Observing competition among the fish species in a murky lake might not be feasible, but a large tank of water that included many features of the lake might work well, providing that the fish behave similarly under both conditions. If so, the results of the microcosm experiment may yield results that can be generalized to the larger, natural system. Experiments can also be conducted at very small scales, such as Petri dishes in the laboratory (Figure 1.18c). Choosing the proper venue for a given experiment represents a trade-off to researchers from the very natural outdoor experiments in which many factors are difficult to manipulate, to the very artificial but highly controlled lab experiments where a wide variety of factors can be manipulated.

#### Alternative Approaches to Manipulative Experiments

Figure 1.18 Experimental venues. The choice of experimental venue often represents a trade-off between the complexity of natural conditions and the more highly controlled conditions of a laboratory experiment. (a) Manipulative experiments of entire lakes, such as this lake in Wisconsin, include natural conditions, but such experiments are difficult to replicate. (b) Microcosm experiments, such as this experiment at the University of Pittsburgh’s Pymatuning Lab of Ecology, include many features of a lake by containing communities of aquatic organisms. Using microcosms allows the manipulations to be replicated many times. (c) Laboratory experiments, such as this pesticide experiment conducted in Petri dishes, allow researchers to conduct highly controlled experiments, but they are conducted under very unnatural conditions. Microcosm A simplified ecological system that attempts to replicate the essential features of an ecological system in a laboratory or field setting.

Experiments

Many hypotheses cannot be tested by experiments, either because the amount of area or length of time needed to test the hypotheses is simply too large, or because it is not possible to isolate particular variables and devise a suitable

control. These limitations are common when we are trying to understand patterns that have occurred over long periods, or in systems such as entire populations or ecosystems that are too large to be manipulated. Several different hypotheses might explain a particular observation equally well, so investigators must make predictions that distinguish among the alternatives. For example, many ecologists have observed a decrease in the number of species as one moves north or south, away from the equator. This repeated pattern has many potential explanations. As one travels north from the equator, average temperature and precipitation decrease, sunlight decreases, and seasonality increases. Each of these factors alone or together could affect the number of species that can coexist in a specific locality. Indeed, dozens of hypotheses have been proposed to explain the observed decrease in the number of species as one moves away from the equator. Isolating the effect of each factor has proved difficult because all the other factors change together. Ecologists have several alternative approaches that address these difficulties. One option, the natural experiment, relies on natural variation in the environment to test a hypothesis. For example, consider the hypothesis that the number of species on an island is influenced by the size of the island because larger islands have more available niches, support larger populations that resist extinction, and are easier for organisms to find and colonize. A manipulative experiment to test this hypothesis would be impossible since it would require both a massive manipulation of many islands as well as the ability to observe a difference in the number of colonizing species over hundreds or even thousands of years. Instead, we can test the hypothesis by comparing the number of species living on islands of different sizes that have been formed over shorter periods by changes in sea or lake levels. Although a manipulative experiment is not possible in such cases, a natural experiment like this still allows researchers to determine if patterns in nature are consistent or inconsistent with hypotheses about the underlying causes. Natural experiment An approach to hypothesis testing that relies on natural variation in the environment. Ecologists also use mathematical models to explore the behavior of ecological systems. In a mathematical model, an investigator designs a

#### Analyzing Ecology: Why Do We Calculate Means and Variances?

representation of a system with a set of equations that corresponds to the hypothesized relationships among each of the system’s components. For example, we might use a mathematical model to represent how births and immigration add to the growth rate of a population, and how deaths and emigration subtract from that growth rate. In this sense, a mathematical model is a hypothesis; it provides an explanation of the observed structure and functioning of the system. Mathematical model A representation of a system with a set of equations that correspond to hypothesized relationships among the system’s components. We can test the accuracy of a mathematical model by comparing the predictions it yields with observations in nature. For example, epidemiologists have developed models to describe the spread of communicable diseases. These models include such factors as the proportions of a population that are susceptible, exposed, infected, and recovered from infections. The models also include the rates of transmission and the probability that the organism will cause a disease in an infected organism. By including all these factors, such models can make predictions about the frequency and severity of disease outbreaks. These predictions can then be tested by comparing them with real-world observations of disease outbreaks. This approach is being used for a number of important wildlife diseases, including the transmission of rabies in such animals as bats, raccoons, skunks, and foxes, and the transmission of Lyme disease in wildlife and human populations.

Analyzing Ecology

Why Do We Calculate Means and Variances? As we saw in the oak tree experiment, when testing hypotheses, ecologists make observations, including measurements that are collected from organisms

or the environment. These observations, also known as data, are then used to test hypotheses. In the case of the oak trees, the researchers collected data on the density of herbivorous insects and the amount of leaf tissue they consumed. In asking questions in ecology, we often want to know the average value, or mean, of the data collected from different treatments or obtained under different conditions. In the case of the oak tree experiment, the researchers wanted to compare the mean density of insects on caged trees versus uncaged trees to determine whether birds depressed the numbers of insects consuming the tree leaves. Observations Information, including measurements, that is collected from organisms or the environment. Also known as Data. While a comparison of the different means tells us about the central tendencies of the data, ecologists often want to know if the data used to generate the mean have high or low variability. For example, if the mean density of insects on leaves was 10 insects per square meter of leaf surface in both of the following sets of data, which group is more variable? Group A: 10, 9, 11, 10, 8, 12, 9, 11, 8, 12Group B:  10, 5, 15, 10, 6, 14, 5, 15, 7, 13 Although both groups of data have the same mean, observations in Group A range from 8 to 12, whereas in Group B they range from 5 to 15. Therefore, the data in Group B are more variable. Why do we care about the variability of the data that we collect? Given that each mean is calculated from a set of data that has either a narrow or wide range, the variability gives us an idea of how much the distributions of data overlap with each other. If two groups of data have different means but the distributions of data overlap a great deal, then we cannot be confident that the two groups are actually different from each other. In contrast, if two groups of data have different means but the distributions of data do not overlap, then we might be very confident that the two groups are different. One way to measure how widely the data points are spread around the

mean is to calculate the variance of the mean. The variance of the mean is a measurement that indicates the spread of data around the mean of a population when every member of the population has been measured. Data points that are more widely spread around the mean will have a higher variance. The easiest way to calculate the variance in a set of data (denoted as σ2) is to do it in two steps: Variance of the mean A measurement that indicates the spread of data around the mean of a population when every member of the population has been measured. 1. Square each observed value (denoted as χ ) and calculate the mean of these squared values (where E indicates that we are taking the mean of several values): E[ χ2 ] 2. From this mean, subtract the square of the mean observed value: σ2=E[ χ2 ]-[ E(χ) ]2 In words, E[χ2] is the mean of the squared observed values and [E(χ)]2 is the square of the mean observed value. When we calculate the variance of the mean, the calculation is based on the assumption that we have measured every member of a population. In reality, we often cannot measure every member, but instead measure a sample of the population. In the oak tree study, for example, the researchers did not measure the insects on all the oak trees, but instead used a sample of 10 groups of oak trees. When we measure a sample of the population, the variation in the data is called the sample variance. The sample variance is very similar to the variance of the mean, except that we now account for how many samples of the population we measured (denoted as n). The sample variance (denoted as s2) is calculated as s2=nn-1×σ2 or

s2=nn-1×(E[χ2)-[E(χ)2) Sample variance A measurement that indicates the spread of data around the mean of a population when only a sample of the population has been measured. As you might notice from this equation, as the number of samples becomes very large, the value of the sample variance approaches the value of the variance of the mean for the entire population. To help you understand how to calculate the sample variance, consider the following set of observations on the abundance of insects per leaf on caged and uncaged trees: Caged trees Uncaged trees For the caged trees, we can calculate the mean of the values as (8+6+7+9+5)÷5=7 and the mean of the squared values as (82+62+72+92+52)÷5=51 We can then calculate the sample variance for the caged-tree data as s2=nn-1×(E [ χ2)- [ E(χ)2 ] )s2=55-1×(51-(7)2)s2=(1.25)×(51-49)=2.5 YOUR TURN Using the data from the five replicates of uncaged trees, calculate the mean and sample variance of insect abundance. Mathematical models can be used on any scale. For example, at a larger scale, ecologists have created mathematical models to investigate how

burning fossil fuels affects the CO2 content of the atmosphere. To manage human impacts on our environment, it is critically important to understand this relationship. Models of global carbon content include, among other factors, equations that describe the uptake of CO2 by plants and the dissolution of CO2 in the oceans. The earlier versions of these models failed to match observations and overestimated the annual increase in atmospheric CO2 concentrations. The Earth evidently contains CO2 “sinks” such as regenerating forests that remove CO2 from the atmosphere. By including the effects of these CO2 sinks, the refined carbon models more accurately describe observed atmospheric data and are more likely to predict future changes accurately. For any model, we can support or reject the hypothesis by comparing the model’s predictions against our observations. Rejected models can be further refined to incorporate additional complexities and better fit our observations.

Concept Check

1. Explain the scientific method. 2. Compare and contrast proximate hypotheses versus ultimate hypotheses. 3. In what ways do manipulative experiments differ from natural experiments?

1.5 Humans Influence Ecological Systems

1.5 Humans influence ecological systems For more than a century, ecologists have labored passionately to understand how nature works, from the level of the organism to the level of the biosphere. The wonders of the natural world summon our curiosity about life and our environment. For many ecologists, a curiosity about how nature works is reason enough to study ecology. Increasingly, however, ecologists find themselves struggling to understand how the rapidly growing human population, now more than 7 billion people, is affecting the planet. Our need to understand nature is becoming more and more urgent as the growing human population stresses the functioning of ecological systems. Environments dominated by humans or created by them—including urban and suburban living places, agricultural fields, tree farms, and recreational areas—are also ecological systems. The welfare of humanity depends on maintaining the proper functioning of these systems. The human population currently consumes massive amounts of energy and resources and produces large amounts of waste. As a result, virtually the entire planet is strongly influenced by human activities (Figure 1.19). These influences include the degradation of the natural environment and the disruption of many important functions that natural environments provide to humans. Growing human consumption of natural resources has caused a number of ecological problems. For example, removing plants from their natural environment to use as house plants and exploiting animals for human consumption and the pet trade have caused the decline of many species in their native habitats. The species affected are diverse, including the cacti of the American Southwest that are collected for sale as house plants, several species of reptiles and amphibians that are sold in the pet trade, and many species of fish and whales that are overexploited by commercial fishing.

Figure 1.19 Human impacts on ecological systems. The growth of human population, particularly over the past two centuries, has altered much of the planet. Humans have destroyed habitats, converted land to agriculture, created air and water pollution, burned large amounts of fossil fuel, and overharvested plants and animals. As commerce has become more global, species have been introduced unintentionally to new locations at an increasing rate. Some of these species, such as rats, snakes, and pathogens, can have devastating effects on local species. To feed 7 billion people, we have converted a large amount of land for agricultural use. This conversion has brought with it a number of challenges, including loss of natural habitats, pollution from fertilizers and pesticides, and questions about growing genetically modified crops. Some crops, such as corn, are now increasingly being used as sources of fuel, also known as biofuels, causing even more land to be converted to agricultural use. Humans also need land for housing, business, and industry. This has further reduced the amount of natural habitat available for other species and has been a major contributor to the decline and extinction of many species. We will deal with these issues in greater detail throughout the book. Another suite of ecological challenges is caused by wastes produced by human activity. For example, untreated sewage and industrial processes can damage the air, water, and soil. In addition, the use of nuclear power plants to generate electricity produces substantial amounts of nuclear waste. Of all human wastes, perhaps none has a higher public awareness than the greenhouse gases responsible for global

warming. Greenhouse gases are compounds in the atmosphere that absorb the infrared heat energy emitted by Earth and then emit some of the energy back toward Earth. In doing so, the gases prevent much of this energy radiated from the surface of Earth from escaping into space. Greenhouse gases include many different compounds, but an important player is CO2, which is produced by burning fossil fuels in the cars we drive and in the fossil fuel–powered, electricity-generating plants that provide electricity to so many of our homes and businesses. As the human population continues to grow and demands for energy increase, we burn more fossil fuels and produce more greenhouse gases. The more greenhouse gases we put into the atmosphere, the warmer our Earth becomes. Greenhouse gases Compounds in the atmosphere that absorb the infrared heat energy emitted by Earth and then emit some of the energy back toward Earth. Because ecological systems are inherently complex, it is difficult to predict and manage the effects of a growing human population on ecological systems at every level. At the level of the organism, for example, we might want to know how a pesticide sprayed in the environment could affect each of the many tissues and organ systems of an animal’s body, leading to changes in behavior, growth, and reproduction. At the community level, we might ask how a decrease in abundance of one species caused by commercial harvesting could affect the populations of many other species in that community. At the biosphere level, we would like to quantify the large number of sources that emit CO2 into the atmosphere and understand the processes that take CO2 out of the atmosphere. Each of these cases presents a set of complex questions that are not easy to answer. Yet we need a solid understanding of how the ecological system operates before we can predict the outcome of human impacts on the system and recommend ways to minimize damages. In the chapters that follow, you will develop that understanding.

The Role of Ecologists

The plight of individual species headed toward extinction arouses us emotionally. However, ecologists increasingly realize that the only effective means of preserving the species of the world is through the conservation of

ecosystems and the management of large-scale ecological processes. Individual species, including those that humans rely on for food and other products, are themselves dependent on the maintenance of environmental support systems. Local effects of human activities on ecological systems can often be managed once we understand the underlying mechanisms responsible for change. Increasingly, however, our activities have led to multiple, widespread effects that are more difficult for scientists to characterize and for legislative and regulatory bodies to control. For this reason, a sound scientific comprehension of environmental problems is a necessary prerequisite to action. The media is filled with reports of environmental problems: disappearing tropical forests, depleted fish stocks, emerging diseases, global warming, and wars that cause environmental tragedies and human suffering. But it is important to know that there are success stories as well. Many countries have made great strides in cleaning up their rivers, lakes, and air. Fish are once again migrating up major rivers in North America and Europe to spawn. Acid rain has decreased, thanks to changes in the combustion of fossil fuels. The release of chlorofluorocarbons, which damage the ozone layer that shields the surface of Earth from ultraviolet radiation, has decreased dramatically. The inevitability of global warming caused by increasing atmospheric CO2 concentrations has provoked global concern and set off an international research effort. Conservation programs, including breeding endangered species in captivity, have saved some animals and plants from certain extinction. They have also heightened public awareness of environmental issues, and sometimes sparked public controversy. These successes would not have been possible without a general consensus founded on evidence produced by scientific study of the natural world. Understanding ecology will not by itself solve our environmental problems, because these problems also have political, economic, and social dimensions. However, as we contemplate the need for global management of natural systems, our effectiveness in this enterprise critically depends on our understanding of their structure and functioning—an understanding that depends on knowing the principles of ecology. This book introduces you to the study of ecology by building an

understanding of all aspects of the discipline. We begin by looking at the individual level, including how species have adapted to the challenges of the aquatic and terrestrial environments. We will then explore the topic of evolution, including how species have evolved various strategies for mating, reproducing, and living in social groups. Next, we move to the population level with a discussion of population distributions, population growth, and population dynamics over space and time. With a firm understanding of populations, we move on to examine species interactions, communities, and ecosystems. Finally, we consider ecology at the global level and investigate global patterns of biodiversity and global conservation.

1. In what ways have humans altered ecological systems? 2. How can our knowledge of ecological systems help humans manage these systems?

Concepts

The California Sea Otter The California sea otter. This once abundant marine mammal has

experienced large fluctuations in numbers as a result of human activities during the past three centuries. In this first chapter, we have examined a wide range of topics, including the hierarchy of perspectives in ecology, the biological and physical principles that govern natural systems, the variety of roles that different species play, the multiple approaches to studying ecology, and the influence of humans on ecological systems. To help you see how these topics interconnect, let’s examine a case study of the sea otter (Enhydra lutris) off the Pacific coast. Humans have affected sea otter populations for hundreds of years. Several scientific approaches have been taken to understand these impacts and to help reverse them. The sea otter was once abundant, with a geographic range that extended around the northern Pacific Rim, from Japan up to Alaska and down to Baja California. However, in the 1700s and 1800s, intense hunting for otter pelts reduced the population to near extinction and the fur industry subsequently collapsed. When a small population was discovered off the coast of central California in the 1930s, the otters were placed under protection. As a result, the population increased to several thousand individuals by the 1990s, though in more recent years, the otter has again experienced population declines. These changes in the size of otter populations presented an opportunity for scientists to examine a natural experiment in action. Ecologists quickly realized that to understand the causes and consequences of the sea otter’s fluctuations in abundance, they needed to use a range of ecological approaches, from the individual to the ecosystem. Taking an individual approach, ecologists established that the sea

otter was a predator on a wide range of prey species, including abalone, spiny lobsters, small fish, crabs, sea urchins, and small snails. Among these prey items, observations of otter feeding behavior revealed that otters prefer certain prey such as abalone, a large species of sea snail. They will only eat other small species of snails when their preferred prey becomes rare. Once scientists understood the sea otter’s niche, they were better able to protect it. However, not everyone was happy about the resurgence of sea otters through the 1990s. California anglers became upset; they argued that the growing otter population would cause a dramatic change in the marine community, including a drastic reduction in the populations of commercially valuable fish, abalone, and spiny lobsters—all harvested for human consumption. However, scientists who took a community approach to ecology found that an increasing otter population was also having other dramatic effects on the marine community. For example, the otter’s consumption of sea urchins—marine invertebrates that eat kelps—was causing an increase in kelps (Figure 1.8). Kelps can be harvested for making fertilizer, food, and pharmaceuticals. Thus, the growing otter population caused sea urchins to decrease, kelps to increase, and the commercial harvesting of kelps to increase. It turns out that the increase in kelps also provided young fish with a refuge from predators and a place to feed. Thus, the sea otter plays a key role in determining the community composition of coastal marine ecosystems.

Sea otters and the species with which they interact. Once scientists determined the major species in the ocean that affected the abundance of otter populations, they could better protect the otter from extinction. Solid arrows indicate consumption of one species by another. In the 1990s, the sea otter population mysteriously began to decline. To understand these declines, scientists used individual, community, and ecosystem approaches. In 1998, researchers showed that populations of otters in the vicinity of the Aleutian Islands, Alaska, had declined precipitously during the 1990s. The reason was that killer whales, or orcas (Orcinus orca), which previously had not preyed on otters, had begun to come close to shore where

they consumed large numbers of otters. Why did killer whales adopt this new behavior? The researchers pointed out that populations of the principal prey of killer whales— seals and sea lions—collapsed during the same period, perhaps causing the whales to hunt the otters as an alternative food source. Why did the seals and sea lion populations decline? One can only speculate at this point, but intense human fisheries have reduced fish stocks exploited by the seals to levels low enough to seriously threaten seal populations. There also were declines in otter populations along the California coast. Initially, declines in sea otters were attributed to the use of gill nets along the coast to exploit a new fishery that inadvertently killed otters in substantial numbers. Subsequent legislation moved the fishery farther offshore to help protect the otters. In this same region, the otters were also dying from infections by two protist parasites, Toxoplasma gondii and Sarcocystis neurona. These parasites cause a lethal inflammation of the brain. In 2010, for example, 40 dead and dying sea otters were found near Morro Bay, California, and 94 percent were infected with S. neurona. This was a surprising observation because the only known hosts of these parasites are opossums (Didelphis virginiana) and several species of cats. Given that these mammals live on land, how did sea otters become infected? Scientists hypothesized links between the terrestrial and marine ecosystems that allowed the parasites to infect sea otters. To date, two potential links have been suggested. First, house cats that spend time outside defecate on land and their feces contain the parasites; when it rains, the parasites get washed into local streams and rivers, and

eventually make their way to the ocean. Second, when humans flush cat feces and kitty litter down the toilet and into the sewer system, the waste water eventually enters the ocean. Although manipulative experiments found that the protists do not infect marine invertebrates and cause illness, the invertebrates can take the parasites into their bodies inadvertently while feeding. When invertebrates infested with parasites are consumed by otters, the otters get infected. New research indicates that abalone do not carry the parasites, whereas small marine snails do. Thus, when otters have an abundance of their preferred food, such as abalone, they have a low risk of being infected by the deadly parasite. When abalone is scarce, however, the otters are forced to feed on small snails that carry the parasite, which dramatically increases the risk of infection and death. The story of the sea otter highlights the importance of understanding ecology from multiple approaches using both manipulative and natural experiments. It also underscores the multiple roles that species can play in communities and ecosystems and how humans can dramatically influence the outcome. This understanding can then be used to take action to reverse harmful impacts on the environment. In the case of the sea otter, education campaigns now encourage the public to keep their cats inside more and to put used cat litter into the trash rather than flushing it down the toilet. SOURCES: Johnson, C. K., et al. 2009. Prey choice and habitat use drive sea otter pathogen exposure in a resource-limited coastal system, Proceedings of the National Academy of Sciences 106:2242–

2247. Miller, M. A. 2010. A protozoal-associated epizootic impacting marine wildlife: Mass mortality of southern sea otters (Enhydra lutris nereis) due to Sarcocystis neurona infection. Veterinary Parasitology 172:183–194. Estes, J. A. 2016. Serendipity: An Ecologist’s Quest to Understand Nature (University of California Press).

Summary of Learning Objectives

1.1 Ecological systems exist in a hierarchical organization. The hierarchy begins with individual organisms and moves up through higher levels of complexity, including populations, communities, ecosystems, and the biosphere. At each of these levels, ecologists study different types of processes. Key Terms: Ecology, Ecological systems, Individual, Species, Population, Community, Ecosystem, Biosphere, Individual approach, Adaptation, Population approach, Community approach, Ecosystem approach, Biosphere approach

1.2 Ecological systems are governed by physical and biological principles. These principles include the conservation of matter and energy, dynamic steady states, a requirement to expend energy, and the evolution of new phenotypes and new species. Key Terms: Law of conservation of matter, Law of conservation of energy (First law of thermodynamics), Dynamic steady state, Phenotype, Genotype, Evolution, Natural selection, Fitness

1.3 Different organisms play diverse roles in ecological systems. The major groups of organisms are plants, animals, fungi, protists, and bacteria. These organisms are involved in numerous species interactions, including competition, predation, mutualism, and commensalism. Each organism lives in specific habitats and has a particular niche. Key Terms: Algal bloom, Producer (Autotroph), Consumer (Heterotroph), Mixotroph, Predator, Parasitoid, Parasite, Pathogen, Herbivore, Competition, Mutualism, Commensalism, Symbiotic relationship, Scavenger, Detritivore, Decomposer, Habitat, Niche

1.4 Scientists use several approaches to studying ecology. Like all scientists, ecologists use the scientific method of developing and testing hypotheses. The testing of proximate and ultimate hypotheses can be accomplished using manipulative experiments, natural experiments, or mathematical models. Key Terms: Hypothesis, Proximate hypothesis, Prediction, Ultimate hypothesis, Manipulative experiment, Manipulation, Control, Experimental unit, Replication, Randomization, Microcosm, Observations (Data), Variance of the mean, Sample variance, Natural experiment, Mathematical model

1.5 Humans influence ecological systems. The rapid growth of the human population during the past two centuries has increased human influence on ecological systems, particularly as the result of resources they consume and waste products they release. Key Term: Greenhouse gases

Critical Thinking Questions

1. How might understanding one level of ecological organization help us understand processes occurring at a higher level of ecological organization? 2. At the population level, what would happen to a population of animals that was not in a dynamic steady state over long time periods? 3. Why are phenotypes the product of both their genes and their environments? 4. When we consider the major forms of life on Earth in Figure 1.6, what are the characteristics that connect the various types of organisms in a given group and suggest that they share a common ancestor? 5. If natural selection favors adaptive phenotypes, in what ways might prey populations evolve if they experience predators over many generations? 6. In the experiment on herbivore insects consuming the leaves of oak trees (p. 22), describe how the researchers could have also conducted a natural experiment in addition to their manipulative experiment. 7. In the Northern Hemisphere, many species of birds fly south during the autumn months. Propose a proximate and an ultimate cause for this behavior. 8. When experimental manipulations are conducted to test a hypothesis, what is the purpose of including a control? 9. Given the difficulty in conducting a manipulative experiment to identify the effects of elevated CO2 across the globe, how might we be able to validate the mathematical models that have been created? 10. Using the data for the caged trees from the “Analyzing Ecology” exercise (p. 24), calculate the sample variance if the variance of the

mean stayed the same but the sample size (n) were to increase from 10 to 100 to 1,000. How does sample size affects the estimate of the sample variance relative to the variance of the mean?