23 Conservation of Global Biodiversity
23Conservation of Global Biodiversity Conserving biodiversity. Efforts are being made to protect the world’s biodiversity, including this tiger longwing butterfly (Heliconius ismenius) from Costa Rica. Protecting Hotspots of Biodiversity The biodiversity of the world faces a wide range of threats from a growing human population that has caused species to go extinct at a rapid rate. To reverse this downward spiral, conservationists seek ways to protect aquatic and terrestrial ecosystems so that threats from human activities can be reduced or eliminated. A common approach to protecting species is to protect their habitats. But there are limits to how much habitat can be protected: habitat protection often means that the habitat must be purchased and not all habitats are available for purchase. Also, limited funds are available for purchasing habitats, and political and economic
factors often play a role in determining whether or not particular habitats can be set aside. With all these limitations, how should we prioritize the protection of the world’s biodiversity? In 1988, Norman Myers noted that small isolated areas such as tropical islands contain a large proportion of endemic species, which are species that have a relatively restricted distribution and are not found in other parts of the world. Therefore, a large proportion of the world’s terrestrial species is located in a relatively small proportion of the world’s land. Myers argued that we should concentrate our conservation efforts on these species-rich areas, which he defined as biodiversity hotspots, because saving these areas should save the most species. Myers identified 10 locations on Earth as biodiversity hotspots. Shortly afterward, the group Conservation International adopted Myers’s approach and decided that hotspots should be defined as areas containing at least 1,500 endemic plant species and experiencing at least 70 percent vegetation loss due to human activities. These criteria would identify areas of high species richness facing substantial threats. The group assumed that regions with high plant diversity also contained high animal diversity. Conservation International identified 34 biodiversity hotspots around the world, including the Caribbean Islands, Central America, the coast of California, the island of Madagascar, and several sites in the islands of Southeast Asia. Collectively, these locations represent 2.3 percent of the Earth’s land surface, but contain 50 percent of the world’s plants (more than 150,000 species) and 42 percent of the world’s vertebrate animals (nearly 12,000 species). A similar effort is under way for aquatic hotspots, with a particular focus on oceans. In the oceans, scientists have argued for hotspots in the open ocean, deep thermal vents (see Chapter 1), and coral reefs. However, many of the same debates occur in how to define marine hotspots. This can be particularly difficult in regions of the ocean where hot spots are seasonal, such as the seasonal upwelling of nutrients that causes periods of high productivity and greater biodiversity.
“A large proportion of the world’s terrestrial species is located in a relatively small proportion of the world’s land.” While identifying hotspots based on high numbers of endemic species is certainly reasonable, some scientists have suggested other approaches. One such approach is to focus on areas that have high species richness without a focus on endemic species. For example, the Amazon rainforest has a very high number of species, but most are not endemic to small geographic areas. Another approach is to prioritize species-rich locations facing the highest current or projected threats of species extinctions, such as locations that have, or are projected to have, a rapidly growing human population. Each approach supports a different list of protection priorities. Focusing conservation priorities on places with high diversity, however, automatically excludes low-diversity locations with species that people care about, such as the bison, wolves, and grizzly bears that live in western North America. It also places an emphasis on species richness, rather than on the important functions that many ecosystems provide. As an example, although wetlands typically have low plant diversity, they are incredibly important for flood control and water filtration.
Biodiversity hotspots. Thirty-four biodiversity hotspots have been identified for terrestrial sites around the world. These sites contain at least 1,500 endemic plant species and have experienced at least a 70 percent decline in their vegetation. While the criteria used are based on plants, these areas also contain a high diversity of animals. Outer limits on the map indicate hotspot regions that include oceanic islands. It is clear that we can take a variety of approaches in how we prioritize the conservation of biodiversity. All these approaches seek to help us invest our limited resources to save the most biodiversity that we possibly can. In this chapter, we will focus on how biodiversity serves us, the causes of the decline in biodiversity, and the efforts that are being made to save it. SOURCES: Bacchetta, G., et al. 2012. A new method to set conservation priorities in biodiversity hotspots. Plant Biosystems 146: 638–648. Marchese, C. 2015. Biodiversity hotspots: A shortcut for a more complicated concept. Global Ecology and Conservation 3: 297–309. Myers, N., et al. 2000. Biodiversity hotspots for conservation priorities. Nature 403: 853–858.
Learning Objectives
23.1 Identify the value of biodiversity based on social, economic, and ecological considerations.
23.2 Explain why the current rate of extinction is unprecedented.
23.3 Describe the ways in which human activities are causing the loss of biodiversity.
23.4 Identify conservation efforts that can slow or reverse declines in biodiversity. Throughout this book, we have examined factors that affect the distribution of species around the world. We have seen that these distributions are a result of the abiotic conditions a species can tolerate, the positive and negative interactions that occur among species, the ability to disperse to suitable habitats, and geologic processes that include the movement of the world’s continents. We have also examined how human activities affect particular species and how conservation efforts try to minimize these impacts. In this final chapter, we take a broad view of the decline in biodiversity around the globe. We begin by considering the many different ways in which people value biodiversity. We will then compare current rates of biodiversity decline with historic rates and examine the ways in which human activities contribute to these declines. Finally, we will discuss efforts under way to slow or even reverse the decline of biodiversity.
#### 23.1 the Value of Biodiversity Arises from Social, Economic, and Ecological Considerations
23.1 The value of biodiversity arises from social, economic, and ecological considerations A case for conserving the world’s biodiversity can reflect a range of different values. For example, the instrumental value of biodiversity focuses on economic values that species can provide, such as lumber for building or crops for eating. In contrast, the intrinsic value of biodiversity recognizes that species have inherent value that is not tied to any economic benefit. Of course, species and ecosystems can have both instrumental and intrinsic values. Instrumental value of biodiversity The economic value a species can provide. Intrinsic value of biodiversity A focus on the inherent value of a species, not tied to any economic benefit.
Instrumental Values
The total economic benefit of biodiversity is difficult to estimate because many of the world’s species remain undiscovered and the values of each species and ecosystem can be difficult to estimate. For example, the total economic benefit of biodiversity in the United States is estimated at $319 billion per year. For perspective, this is about 10 percent of U.S. annual gross domestic product. At the global level, estimates of the total benefit of biodiversity, including all of the ecosystem services provided, is $125 trillion. We can group the instrumental values of biodiversity into four categories of services: provisioning, regulating, cultural, and supporting. Provisioning Services Provisioning services are benefits of biodiversity that provide products humans use, including lumber, fur, meat, crops, water, and fiber. In many cases, plants and animals from the wild have been cultivated or domesticated and then selectively bred to enhance their valuable qualities. Provisions also include pharmaceutical chemicals that come from plants and animals; nearly 70 percent of the top 150 pharmaceutical drugs originate in chemicals that are produced in nature. A prominent example of the economic benefits of such a
drug is the chemical Taxol, which is used to fight cancer. Today, Taxol is synthesized in the laboratory, but it originally came from the Pacific yew tree (Taxus brevifolia) (Figure 23.1). This single chemical currently generates more than $1.6 billion in annual sales around the world. Over the past 25 years, more than 800 natural chemicals have been identified in the search to provide treatments for everything from cancer to contraception, and there is no indication that the pace of these discoveries is slowing down.
Figure 23.1 Provisioning services. The Pacific yew is one of many species that has served as a critical source of pharmaceutical chemicals that improve human health. Provisioning services
Benefits of biodiversity that humans use, including lumber, fur, meat, crops, water, and fiber. Regulating Services Regulating services are benefits of biodiversity that include climate regulation, flood control, and water purification. For example, wetlands absorb large amounts of water and so prevent flooding from water runoff during rainy periods. The plants living in the wetlands also remove contaminants from the water and make it more suitable for drinking. The CO2 that is taken out of the air by producers on land and in the ocean is another regulating service. Of the 8 gigatons of carbon that are put into the air each year by human activities, about 4 gigatons are taken out of the air by producers, which reduces the effect humans have on global temperatures due to global warming. Regulating services Benefits of biodiversity that include climate regulation, flood control, and water purification. Cultural Services Cultural services are benefits of biodiversity that provide aesthetic, spiritual, or recreational value. For example, cultural services include the benefits that people obtain when they go hiking, camping, boating, or birdwatching. People pay to visit beautiful natural areas, such as the Florida Everglades in the United States or Banff National Park in Canada. Sometimes areas are preserved because income from tourists can exceed what would be received from clearing a forest or from using the land for housing and industry. Many tropical countries have capitalized on this attraction by establishing parks and support services for tourists. In Palo Verde National Park in Costa Rica, for example, monkeys and beautiful tropical birds draw tourists to areas where the species are protected (Figure 23.2). Diversity itself is often the attraction in tropical rainforests and coral reefs because these ecosystems contain hundreds of different species of trees, birds, corals, or fish.
Figure 23.2 Attracting tourists to Palo Verde National Park. Striking examples of biodiversity, such as this resplendent quetzal (Pharomachrus mocinno), attract tourists from around the world. Cultural services Benefits of biodiversity that provide aesthetic, spiritual, or recreational value. Supporting Services Supporting services are benefits of biodiversity that allow ecosystems to exist, such as primary production, soil formation, and nutrient cycling. As we
have seen in Chapters 20 and 21, these processes are essential for the existence of species and ecosystems. There would be no ecosystems without producers that capture the energy of the Sun and then transfer it to all other trophic levels. Similarly, the formation of soil and the cycling of nutrients both play key roles in the persistence of existing ecosystems. Supporting services Benefits of biodiversity that allow ecosystems to exist, such as primary production, soil formation, and nutrient cycling.
Intrinsic Values
In contrast to instrumental values, the intrinsic values of biodiversity do not provide any economic benefits to humans. Instead, people who place intrinsic value in biodiversity feel religious, moral, or ethical obligations to preserve the world’s species. For example, a major motivation for efforts to bring back the bald eagle from the brink of extinction in the 1970s was that it was the national symbol of the United States and we had a moral obligation to prevent its extinction. However, it becomes very difficult to prioritize conservation efforts only by arguing that all species are intrinsically valuable. In reality, we can consider both instrumental and intrinsic values when deciding how to focus our conservation efforts.
Concept Check
1. What are three provisioning services of biodiversity? 2. Why are regulating services considered to be an instrumental value of biodiversity? 3. Why is it difficult to assign an economic value to intrinsic values of biodiversity?
#### 23.2 the Current Rate of Extinction Is Unprecedented
23.2 The current rate of extinction is unprecedented The current number of species on Earth is difficult to estimate. We do know that 1.3 million species have received Latin names and about 15,000 new species are described each year. While estimates for the total number of species range from 3 to 100 million, depending on the assumptions used, most scientists agree that there are about 10 million species. Some species are declining in abundance and facing extinction as humans continue to alter terrestrial and aquatic ecosystems. However, as we have seen throughout this book, some extinctions are natural. Therefore, we need to understand the historic versus modern rates of extinctions. In this section, we will explore the past and present rates of extinction and then examine how specific groups of organisms are faring. As part of this discussion, we will consider both declines in species diversity and declines in genetic diversity.
Background Extinction Rates
Over the past 500 million years, the world has experienced five mass extinction events, which are defined as events in which at least 75 percent of the existing species go extinct within a 2-million-year period. During these events, large numbers of species, genera, and families around the world went extinct. For simplicity, Figure 23.3a illustrates the number of families that have gone extinct.
Figure 23.3 Mass extinctions. (a) Over the past 500 million years, there have been five mass extinctions. During these periods, the world experienced a significant decline in the number of families, which also means there were declines in the number of genera and species. Continued speciation during subsequent years has helped to offset these extinctions. (b) The fifth extinction is hypothesized to have been caused by volcanic eruptions, cooler climates, and a massive asteroid that struck the Yucatan Peninsula and put massive amounts of dust into the air and blocked the Sun’s rays. Mass extinction events Events in which at least 75 percent of the existing species go extinct within a 2-millionyear period.
During the first mass extinction, about 443 Mya, most species lived in the oceans. An ice age caused sea levels to drop and the ocean chemistry to change, which resulted in 86 percent of species going extinct. The second mass extinction happened 359 Mya when much of the ocean lacked oxygen —for reasons that are unclear—and 75 percent of all species went extinct. During the third mass extinction—248 Mya—an astounding 96 percent of all species then present on Earth went extinct. Although researchers have constructed multiple hypotheses to explain this third mass extinction, we are still uncertain about the cause. The fourth mass extinction, which occurred 200 Mya, caused 80 percent of the world’s species to go extinct. Hypotheses for the causes of this fourth extinction include increased volcanic activity, asteroid collisions with Earth, and climate change. The fifth mass extinction happened 65 Mya and is best known as the one that led to the extinction of dinosaurs. This event is attributed to several factors. First, volcanic eruptions and changes in climate caused long periods of cold weather. This was followed by a massive asteroid that struck the Yucatan Peninsula in Mexico. The asteroid is estimated to have been 10 km wide and struck with a force more than 1 billion times that of the atom bomb dropped on Hiroshima during World War II (Figure 23.3b). The explosion created a massive, 180-km-wide crater. Scientists hypothesize that the explosion put so much dust into the atmosphere that it blocked the Sun’s rays, making Earth much less hospitable to dinosaurs along with many other groups, such as flowering plants. During this time, 76 percent of the species on Earth went extinct. As you can see from this history of natural mass extinction events, only a small percentage of all species that ever lived on Earth are present today. In fact, over the past 3.5 billion years, it is estimated that 4 billion species have existed on Earth and 99 percent of these species are now extinct. However, as you can see in Figure 23.3, after each mass extinction event, new species evolved and, overall, the number of species has increased with time.
A Possible Sixth Mass Extinction
We have seen that a mass extinction is defined as the extinction of 75 percent of species within a 2-million-year period. It is a widely held view in the scientific community that the increase in the human population during the past 10,000 years may have initiated a sixth mass extinction event. To
evaluate this hypothesis, we need to quantify the rates of extinction during the first five mass extinctions and then compare them with the current rate of extinction. Researchers have addressed this question by looking at groups of organisms, such as mammals, for which there are very good fossil data on extinctions. When the extinction rate for mammals during the most recent 500 years is compared to the rate of mammal extinctions over 500-year intervals in the past, we see that the current extinction rate exceeds the historic extinction rate. In fact, the United Nations Convention on Biological Diversity estimates that the extinction rate during the last 50 years has been as much as 1,000 times higher than the historic rate. In the next section, we will explore the reasons for this change in rate. Should this rate continue for hundreds or thousands of years, it could qualify as a mass extinction event.
Global Declines in Species Diversity
When we think about the decline in the world’s biodiversity, we often focus on the last few centuries, from the Industrial Revolution to today—a time in which we have drastically altered our world. However, human impacts on biodiversity can be seen much farther back in time. For example, based on a rich fossil history of mammals, researchers have defined different geographic regions of North America and determined the number of fossil species in each region. With these data they created species–area curves for different time periods in North America, which we discussed in Chapter 22. Compared to the period before the arrival of humans—150,000 to 11,500 years ago—the species–area curve for the period after human arrival—11,500 to 500 years ago—was significantly lower, as you can see in Figure 23.4. Simply put, the arrival of humans coincided with a 15 to 42 percent decline in mammal diversity, depending on the geographic region examined. The losses include 56 species and 27 genera of large mammals, including a giant ground sloth, the sabertoothed tiger, and several species of horses, camels, elephants, and lions.
Figure 23.4 Declining North American mammals. When researchers created species–area curves of North American mammals from different geographic regions before and after human arrival, they discovered that from 11,500 to 500 years ago (i.e., after humans arrived), the number of mammals declined anywhere from 15 to 42 percent, depending on the geographic region examined. Explanations for these extinctions include rapid climate change following the retreat of the glaciers, hunting pressure from the human population, and epidemic diseases carried from Asia by domesticated animals. Many scientists hypothesize that most of the large mammals were driven extinct by humans who hunted them. We know that the diversity of mammal species had been substantially reduced during the period of human occupation of North America prior to the Industrial Revolution, although we can’t be sure of the reason. Any current impacts are adding to previous extinctions. Whether the world’s current extinction rate of species will approach the magnitude of a mass extinction depends on how many of the species currently on Earth go extinct during the next few centuries. To assess how different groups of organisms are currently faring, the International Union for Conservation of Nature (IUCN) has defined categories that describe whether a species is abundant, threatened, or extinct. Extinct describes a situation in which a species was known to be in the wild in the year 1500 but no
individuals remain alive today. Extinct in the wild is a category used when the only individuals remaining are in captivity, such as animals living in zoos. Threatened species are those whose populations face a high risk of extinction in the future. This category includes species that are considered “endangered.” Near-threatened species are those that will likely become threatened in the future. In contrast, least-concern species are those that have abundant populations and are not likely to become threatened in the future. In some cases, the status of a species has not been determined or there are simply insufficient data to make a reliable determination. Assessing the status of species from a large taxonomic group is not easy. Many groups contain thousands of species, and often a large percentage of them have not been studied well enough to know if a species is abundant or declining. Making such a determination requires substantial time and money for each species. Currently, our best data to assess the decline in biodiversity are for conifers, birds, mammals, amphibians, fish, and reptiles. Assessments for these taxonomic groups were produced by the IUCN in 2017, and you can see a summary of these data in Figure 23.5.
Figure 23.5 The global status of conifers, birds, reptiles, mammals, amphibians, and fish. Species threatened by the risk of future extinction include 34 percent of conifers, 13 percent of birds, 25 percent of mammals, 42 percent of amphibians, 18 percent of fish, and 24 percent of reptiles. Conifers
Conifers include pines, spruces, firs, cedars, and redwoods, and the survival prospects of 95 percent of these species have been assessed. This high level of assessment is possible, in part, because the group has a relatively low number of species and large trees and shrubs are more easily assessed for population declines. Of the 606 species of conifers, none has gone extinct. Of those species that have sufficient data for assessment, 50 percent are categorized as of least concern, 16 percent are near-threatened, and 34 percent are threatened. Birds Some of the best data on species assessments come from birds because they are relatively easy to monitor and they have been studied for a long time. The 2017 assessment found sufficient data for assessment of 99 percent of the more than 11,000 species of birds on Earth. Since the year 1500, 156 of these (1.4 percent) have gone extinct. Of the remaining species for which there are reliable data, 77 percent have sufficiently abundant populations to be categorized as least concern, 9 percent are near-threatened, and 13 percent are threatened with extinction. Reptiles Reptiles include snakes, lizards, and turtles. Of the more than 5,000 species of reptiles, 86 percent have sufficient data for assessment. Twenty-eight species (0.4 percent) have gone extinct during the last 500 years. Of those remaining with sufficient data for assessment, 68 percent are of least concern, 8 percent are near-threatened, and 24 percent are threatened. Mammals Of the 5,560 species of mammals that have lived on Earth since the year 1500, 86 percent have sufficient data to assess their status. During the past 500 years, 83 mammal species (1.5 percent) have gone extinct. Of those remaining for which there are reliable data, researchers found that 67 percent of the species are categorized as of least concern, 8 percent are nearthreatened, and 25 percent are threatened. Amphibians Amphibians have been particularly hard hit in recent decades from several
causes that include habitat loss and the spread of the deadly chytrid fungal disease that we discussed in Chapter 15. Of the more than 6,500 species of amphibians assessed, only 76 percent contained sufficient data to assess their conservation status. During the past 500 years, 33 species (0.5 percent) have gone extinct. Of the remaining species with sufficient data, 50 percent are of least concern, 8 percent are near-threatened, and a staggering 42 percent of amphibian species are threatened. Because amphibians are not as visible as birds and mammals, scientists are still discovering new species at a rapid rate. For example, more than 3,000 new species of amphibians have been discovered in the past 25 years, which represents nearly half of the world’s described amphibian species. This translates to a discovery of a new amphibian species every 2.5 days. One challenge with so many new discoveries is that little is known about the population status of these species. Researchers expect hundreds of more species to be discovered in the future, and therefore our estimates of each category will continue to be updated. Fish Like amphibians, fish have also experienced a tremendous number of declines. The IUCN considered more than 16,000 species of fish in its 2017 assessment and found that more than 3,000 species were found to be data deficient. Since the year 1500, 64 fish species are now extinct. Of those remaining with reliable data, 77 percent of the species are categorized as of least concern, 4 percent are near-threatened, and 18 percent are threatened. The data from conifers, birds, reptiles, mammals, amphibians, and fish suggest that if human impacts on these species continue, we should expect a large number of species extinctions in future centuries. Although our best data come from these six groups, it is predicted that the patterns of decline observed in these groups are representative of many other groups for which data on the status of species are relatively poor. This prediction is supported by preliminary efforts to assess the status of other major groups. Although less than 10 percent of all species of flowering plants and insects have been assessed, about half of the currently assessed species are categorized as threatened. As we have discussed in previous chapters, these declines in species richness are a concern not only because of the risk of losing species, but also
because of the effect these declines have on communities and ecosystems. As we discussed in Chapter 17, decreases in the number of mycorhizal fungi cause decreases in the biomass of plants (see Figure 17.22). In Chapter 18, we also observed that a decline in species richness can cause communities to be less stable over time by affecting community resistance or resilience (see
Figure 18.22). Declines in species richness can also cause a decline in the functioning of ecosystems. For example, a review of all research examining patterns between experimental manipulations of plant species richness and the aboveground biomass of plants—just one measure of an ecosystem’s function—has found that commonly a positive relationship exists, although there are exceptions. You can see examples of these relationships in Figure 23.6.
Figure 23.6 Effects of species richness on ecosystem function. Researchers have observed positive relationships between species richness and ecosystem function in seven of eight locations in Europe, although the shape of the relationship differs among locations.
Global Declines in Genetic Diversity
In addition to the decline in species diversity around the world, we have also seen a decline in the genetic diversity of many species. As we discussed in Chapter 7, causes of decline in genetic diversity include declining population sizes, inbreeding depression, and the bottleneck effect. Smaller populations do not possess the same amount of
genetic diversity as large populations. These declines in genetic diversity reduce the probability that a population contains genotypes able to survive changing environmental conditions, including changes in climate and emerging infectious diseases. The decline in the genetic diversity of livestock and crops has a direct and immediate effect on humans. The primary livestock that we consume or use for labor and transportation include just seven species of mammals (cattle, pigs, sheep, goats, buffalo, horses, and donkeys) and four species of birds (chickens, turkeys, ducks, and geese). Humans have bred these species for a wide variety of different traits, including size, strength, quality of meat, and ability to persist under challenging environmental conditions such as drought and diseases (Figure 23.7).
Figure 23.7 Genetic diversity of chickens. Over the past few centuries, people have bred a wide variety of livestock breeds, such as chickens, to suit their particular needs or local environmental conditions. Today, most of these varieties are extinct because large livestock operations have focused on just a few breeds to maximize the production of livestock. During the past century, many livestock varieties have not been maintained because larger, modern livestock operations favor relatively few breeds that are the most productive in terms of meat or milk. In Europe, for instance, about half of the livestock breeds present in 1900 are now extinct. Of those
breeds remaining, 43 percent are at a serious risk of extinction. In North America, 80 percent of the livestock breeds that have been evaluated are either declining in abundance or facing extinction. Around the world, of the 7,000 breeds of 35 domesticated species of birds and mammals, more than 10 percent are already extinct and another 21 percent are at risk of extinction. Such a rapid decline in genetic diversity means there is considerably less diversity to draw from should we need to breed domesticated animals that can live in new locations or in changing environments, or that can withstand new diseases. Simply put, reduced genetic variation reduces our future options. Declines in genetic diversity are also occurring in plant species that are important to humans. Humans historically consumed more than 7,000 species of plants, but today consume only about 150 species. Moreover, just 12 species of plants make up the vast majority of people’s diets; among these are wheat, rice, and corn. In some cases—for example, corn—modern varieties look very different from their ancestors (Figure 23.8). In earlier times, humans bred varieties that grew well under particular local environmental conditions. However, as agricultural practices changed, irrigation and fertilizer made it possible to reduce the harshness of the growing environment, and small farms gave way to much larger operations that favored only the top-producing varieties. As a result, many of the older, local varieties of crops are no longer available. For example, U.S. farmers grew about 8,000 varieties of apples in 1900, but today, 95 percent of these varieties are extinct. Similarly, 80 percent of the corn varieties that existed in Mexico in 1930 and 90 percent of the wheat varieties that existed in China in 1949 are now gone.
Figure 23.8 Genetic diversity of crops. (a) Corn originated from a wild plant in Mexico, known as teosinte, shown on the left. Modern cultivated corn is shown on the right. An ear of the F1 hybrid is shown in the center. (b) From the teosinte ancestor, humans have bred a wide range of genetic varieties to perform well under different conditions, including these varieties grown in Oaxaca State, Mexico. Today, the tremendous variety of genetic crops is at risk of being lost. Losing the genetic diversity of these crops reduces our options when we need to respond to challenges such as new pathogens that attack a crop. For example, in the 1970s, a fungus attacked cornfields in the southern United States and killed half the corn crop because the plants all came from a single variety. Fortunately, another variety of corn possessed a fungus-resistant gene, and breeders were able to produce a new variety that is resistant to the fungus. To protect the genetic diversity of plant varieties, many countries have been archiving seed varieties of different crop species in thousands of storage facilities around the world. Concern that such facilities could be destroyed by natural disasters or war led to the construction of the Svalbard Global Seed Vault. Located on an island in the Arctic region north of the Norwegian mainland, the facility is a 125-m tunnel built into a mountain with rooms for seeds on each side of the tunnel, as illustrated in Figure 23.9. This facility protects seeds from virtually all catastrophes. The vault has a total capacity of 1.5 million samples; as of 2013, the facility contained more than 700,000 samples of crop seeds from nearly every country. The Svalbard Global Seed Vault and many other seed storage facilities around the world preserve the ability to call upon the genetic diversity of plant species far into the future.
Figure 23.9 The Svalbard Global Seed Vault. This facility was built on an island north of the Norwegian mainland to preserve the genetic diversity of crop plants so that humans will be able to use this genetic variation long into the future.
Concept Check
1. What data can we use to determine if we are in the midst of a sixth extinction? 2. Which groups of animals are the most threatened? 3. Why have we lost the genetic diversity of many crop plants?
#### 23.3 Human Activities Are Causing the Loss of Biodiversity
23.3 Human activities are causing the loss of biodiversity The current rapid decline in biodiversity is caused by the rapid increase in human populations and our many activities. Virtually all areas within temperate latitudes that are suitable for agriculture have been plowed or fenced, 35 percent of the land area is used for crops or permanent pastures, and countless additional hectares are grazed by livestock. Tropical forests are being logged at a rate of 10 million ha each year. Semiarid subtropical regions, particularly in sub-Saharan Africa, have been turned into deserts by overgrazing and harvesting firewood. Rivers and lakes are badly contaminated in many parts of the world. Gases from chemical industries and the burning of fossil fuels pollute our atmosphere. In this section, we take a global view of human impacts, including habitat loss, overharvesting, introduced species, pollution, and global climate change. While each of these factors is important, keep in mind that many of these factors occur simultaneously.
Habitat Loss
The destruction and degradation of habitat have been the largest cause of declining biodiversity. In the United States, for example, most old-growth forests were cut down in the eighteenth century and only a fraction of the original forest remains today. Of course, many of these forests have regrown. Logging of these new forests has typically continued using sustainable practices, although these younger forests do not provide habitat to all the same species as the original oldgrowth forests did. Today, many areas of the tropics are experiencing a similar pattern of deforestation. For instance, humans have cleared large forests on the island of Sumatra in Southeast Asia, such that only a small fraction of the original forest remains, as you can see in Figure 23.10. This deforestation has critically endangered many endemic birds and mammals, such as the Sumatran tiger (Panthera tigris sumatrae), the Sumatran ground cuckoo (Carpococcyx viridis), and the Sumatran orangutan (Pongo abelii;
Figure 23.11). Because such endemic species live nowhere else in the world, intense conservation efforts have focused on saving them from extinction by
Figure 23.10 Deforestation of a tropical forest. The island of Sumatra was once widely covered in forests. Even during the past few decades, much of the forest has been cleared. Today, only a small fraction of the island contains intact primary forest (i.e., forests that have not been logged) and degraded primary forest (i.e., forests that have experienced some amount of logging). Because the forest cover is based on images from satellites, some land cover cannot be determined due to the presence of clouds.
Figure 23.11 Sumatran orangutan. Habitat destruction of the island of Sumatra has caused the decline of many endemic species, including the Sumatran orangutan. To gain insight into the global scale of habitat change, researchers have assessed how forests have been changing in more modern times. From 1980 to 2000, continued loss of forests has occurred in many regions, including the Amazon, Russia, and Southeast Asia. However, there has been an increase in
forests in the United States, Europe, and Northeast Asia. You can view a map of these changes in Figure 23.12. The species composition that currently exists in regions that have experienced an increase in forest cover, however, is often quite different from what existed originally, particularly in cases where a single species of tree is planted due to its high commercial value.
Figure 23.12 Changes in forest cover. While some regions of the world experienced a decline in forest cover from 2001 to 2015, other regions experienced an increase. As we have discussed in previous chapters, habitat loss also leads to smaller habitat sizes and increased habitat fragmentation. In Chapter 22, we saw that a reduction in habitat size can lead to reduced population sizes and make it more likely a population will go extinct. This process is thought to be a significant reason that many national parks have lost species of mammals over the past 50 years, despite being protected from most harmful human activities. In addition, fragmented habitats have a high proportion of edge habitat that can alter the abiotic conditions of the interior habitat and favor edge species. We saw an example of this in Chapter 22, when we discussed the nest parasite known as the bronzed cowbird, an edge specialist that parasitizes the nests of other songbirds and leads to their decline. Forests are not the only habitats that have been changing as a result of human activities. For example, according to the National Park Service, the original tallgrass prairie once covered 69 million ha across the middle of North America. Less than 4 percent remains today. Because the remaining
areas are small fragments, many local populations of prairie plants and animals have gone extinct. A similar story exists for wetland habitats. Scientists estimate that in the 1600s, wetlands covered more than 89 million ha in the lower 48 states, but drainage for agriculture and other uses has reduced the area of wetlands by more than half. In some places such as California, 90 percent of the original wetlands have been lost. As we have seen, these habitat losses have a large negative effect on the world’s biodiversity.
Overharvesting
Human advances in techniques for logging trees, plowing grasslands, and capturing animals more efficiently have allowed us to harvest species at rapid rates and drive some species to extinction. For instance, during the past 3 centuries, commercial hunters in North America have hunted to extinction the Steller’s sea cow (Hydrodamalis gigas), great auk (Pinguinus impennis), passenger pigeon (Ectopistes migratorius), and Labrador duck (Camptorhynchus labradorius). Each of these once-abundant species was valued for food or feathers, and they were easily killed. Extinction caused by overhunting and overfishing is not a recent phenomenon. Wherever humans have colonized new regions, some elements of the fauna have suffered. For example, researchers examined the skeletal remains of animals in archaeological sites around the Mediterranean region to see how human diets changed over thousands of years. At a site in currentday Italy, they found that early human populations initially ate large quantities of tortoises and shellfish, which were easy to catch. As you can see in Figure 23.13, as supplies of those foods were depleted over time, people switched to hunting hares, partridges, and other small mammals and birds.
Figure 23.13 Historic overharvest. By examining the bones of consumed animals found in archaeological excavations, researchers have determined that early humans in Italy initially consumed animals that were easy to catch, such as tortoises and shellfish. Once these animals became rare due to overharvesting, people turned to eating hares, partridges, and other species of mammals and birds. Similar scenarios occurred when humans colonized other parts of the world. When Australia was colonized 50,000 years ago, several species of large mammals, flightless birds, and a species of tortoise soon disappeared. On Madagascar, a large island off the southeastern coast of Africa, the arrival of humans about 1,500 years ago caused the demise of 14 species of lemurs and between 6 to 12 species of elephant birds—giant flightless birds found only on that island. At about the same time, a small population of fewer than 1,000 Polynesian colonists in New Zealand hunted 11species of moas— another group of large flightless birds—so that these birds went extinct in less than a century. In each of these cases, humans encountered island species that were unaccustomed to humans and any other predation pressure. A
failure to recognize danger and the lack of defensive strategies made these species particularly vulnerable to human hunting. Overharvesting of species continues in modern times. In some cases, the harvesting is part of an illegal trade in plants and animals, an endeavor that is valued at $5 billion to $20 billion annually. For example, many animal skins are sold for furs and some cultures believe that certain animal body parts have medicinal value. Some species of rare trees, such as big-leaf mahogany (Swietenia macrophylla), are sold for their lumber, while species of rare flowers, such as endangered species of orchids, are sold for their beauty. Governments frequently regulate the harvest of plants and animals in the wild to ensure that the harvested species will persist for future generations to enjoy. However, harvesting regulations must balance not only the good of the harvested population but also the human employment that the harvest supports. As a result, some regulations set harvest levels that do not stop populations from declining. In marine environments, for example, modern fishing techniques have made it much easier to harvest enormous numbers of fish and shellfish. These techniques include fishing lines that are several kilometers long with thousands of baited hooks, nets that can surround schools of fish up to 2 km in diameter and 250 m deep, and huge trawlers that can scrape large areas of the ocean bottom. The ability to fish more efficiently and to cover larger areas has caused a decline of many fish populations around the world. When a commercially important fish species no longer has a population that can be fished, it is called a collapsed fishery. Collapsed fishery When a fishery no longer has a population that can be fished. For example, consider the case of the Atlantic cod, a species of fish caught by commercial trawlers. On the Grand Banks fishery off the coast of Newfoundland in Canada, the amount of cod caught from 1850 to 1960 slowly increased from 100,000 to 300,000 metric tons, as shown in Figure 23.14. During the 1970s and 1980s, new technologies—including advanced sonar, GPS, and larger trawlers—allowed a rapid increase in the number of cod caught, with a peak catch of 800,000 metric tons. However, the population crashed to very low levels in the early 1990s, leading the
Canadian government to close the fishery in 1992. Despite low cod numbers, the Canadian government was under pressure from cod fishermen to allow continued fishing. The government agreed, but soon the number of fish fell so much that cod fishing had to be completely stopped and 35,000 Canadian fishermen lost their livelihood. In 2012, 20 years after the ban was imposed, scientists reported that there were finally signs that the cod population was beginning to rebound.
Figure 23.14 A collapse of the Atlantic cod fishery. From 1850 to 1960, there was a slow increase in the catch of Atlantic cod by commercial fishermen off the coast of Newfoundland in eastern Canada. New technologies in the 1970s and 1980s allowed much larger catches of cod, but this led to an overharvest of the fish and a collapse of the population in 1992 that persists today. After Millennium Ecosystem Assessment, Ecosystems and human well-being: Synthesis (Island Press, 2005). The decline in cod also occurred in New England. Because it was not as severe as the decline off the Newfoundland coast, fishing continued, but the quota of cod that could be caught was reduced. By 2010, the U.S. government greatly reduced the amount of cod that could be caught by commercial anglers, in the hope that the population would rebound. An assessment in 2011 found that the cod population had been very slow to respond, and so limits were reduced even further for 2013 through 2016. U.S. cod fishermen lobbied for higher cod quotas so that they could continue to make a living. However, the government biologists argued that if the cod limits were not lowered substantially, there would soon be no cod left to
catch. This debate mirrored the Canadian experience of two decades earlier. Although reducing the harvest of overharvested species has real economic impacts on those employed in the industry, failure to restrict the harvest hastens the decline of the species to levels at which there are no individuals left to harvest. Unfortunately, cod fishing in New England has continued to decline through 2016, with record low numbers of cod caught by commercial anglers. There has been a steady increase in the percent of collapsed fisheries, and estimates are that approximately 14 percent of fisheries are now collapsed, as illustrated in Figure 23.15. Some regions, such as the eastern Bering Sea off the shores of Alaska, have very few collapsed fisheries. In contrast, collapsed fisheries occur in more than 25 percent of species assessed off the coast of the northeastern United States and more than 60 percent of species assessed off the coast of eastern Canada.
Figure 23.15 Collapsed fisheries. Over the past 60 years, there has been a steady increase in the percent of fish and other seafood that are categorized as collapsed. (a) Around the world, 14 percent of assessed species are considered collapsed. In different regions of North America, these percentages differ a great deal, including (b) a low percentage of collapsed fisheries in the eastern Bering Sea, (c) a modest percentage in the water off the coast of the northeastern United States, and (d) a high percentage off the coast of eastern Canada.
Introduced Species
Another cause of declines in biodiversity is the increasing number of species that are introduced from one region to another. Some of these introductions are intentional, such as when tropical plants are sold for houseplants in temperate parts of the world. Often, however, species are introduced by accident, as we saw in Chapter 15 with the many pathogens that have moved between continents and have caused emerging infectious diseases. Although only about 5 percent of introduced species become established in a new region, those that do can have a variety of effects. Some introduced species provide important benefits, such as the common honeybee that was introduced to North America from Europe in the 1600s. Other introduced species can have substantial negative effects on native species, like the brown tree snake introduced to the island of Guam, which we discussed in Chapter 14 (see Figure 14.2). In that case, the snake caused the decline or extinction of nine species of birds, three species of bats, and several species of lizards. In general, introduced species that compete with native species rarely cause extinction of native species, whereas introduced species that act as predators or pathogens on native species can cause large population declines and extinctions of native species. Some of the most complete data on introduced species exist for the Nordic countries of Sweden, Finland, Norway, Denmark, and Iceland. As you can see in Figure 23.16, the number of introduced species in this area has rapidly increased since 1900. Across terrestrial, freshwater, and marine ecosystems, there are currently more than 1,600 introduced species in the Nordic region. Similarly, during the past 200 years in North America, thousands of species have been introduced, many of which have spread rapidly and are considered invasive species. According to the Center for Invasive Species and Ecosystem Health, North America currently has a large number of invasive species that include nearly 200 pathogens, 300 vertebrates, 500 insects, and 1,600 plants.
Figure 23.16 Increases in introduced species over time. In the Nordic countries of northern Europe, the number of species introduced to terrestrial, freshwater, and marine ecosystems has increased over time. One of the highest-profile introduced species in the United States is the silver carp (Hypophthalmichthys molitrix), a species of fish that has been introduced around the world because it consumes excess algae in ponds used by water treatment plants and aquaculture operations. Brought to the United States in the 1970s, the carp escaped captivity in the 1980s when floodwaters washed them out of their ponds and into the Mississippi River. The species rapidly spread through the Mississippi River and its tributaries, including the Illinois River. The primary concern was that the Illinois River connects the Mississippi River to Lake Michigan and, thus, the carp had the potential to invade the entire Great Lakes ecosystem. In 2010, the carp’s DNA was detected in Lake Michigan, which suggests that the carp may be spreading throughout the Great Lakes. The silver carp is such a voracious consumer of algae that scientists worry it will compete with native consumers of algae, which serve as a key link in the food chain for many species of commercially important fish. The carp also has the unusual behavior of jumping out of the water when a boat passes by. Since the silver carp can reach a mass of 18 kg and jump up to 3 m out of the water, it poses a serious safety risk to boaters (Figure 23.17). It will take several years before we can assess the full impact of the silver carp on North American waters.
Figure 23.17 Silver carp. Silver carp jump into the air and pose a danger to boaters like this biologist on the Illinois River near Starved Rock State Park. One introduced species with largely unappreciated negative effects is the domestic house cat. In 2013, researchers examined predation by free-ranging domestic cats in the United States and found that the cats killed 1.4 to 3.7 billion birds and 7 to 21 billion mammals each year. Collectively, these data suggest that introduced species have the potential to cause widespread effects on native species and ecosystems. As the movement of people, cargo, and species becomes more common among the regions of the world, the unique species compositions originally found in different regions are slowly become more similar, a process known as biotic homogenization. Biotic homogenization The process by which unique species compositions originally found in different regions slowly become more similar due to the movement of people, cargo, and species.
Pollution
We have talked about many types of pollution throughout this book. For example, in Chapter 2, we discussed acid precipitation, and in Chapter 21, we looked at the effects of adding excess nutrients to bodies of water. Both examples demonstrate the harmful effects of pollutants on biodiversity. Pesticides are a common type of pollutant. They include insecticides that
kill insects and other invertebrate animals, herbicides that kill plants, and fungicides that kill fungi. These chemicals are designed to target a particular type of pest; ideally, they will not harm nonpests in the ecosystem. However, some pesticides do kill nonpests, either directly, by being toxic, or indirectly, by altering food webs, as we saw in the wetland study discussed at the end of
Chapter 18. In that study, a very small amount of an insecticide, while not directly toxic to tadpoles, was toxic to zooplankton. The death of zooplankton set off a chain of events in the food web that prevented the tadpoles from obtaining enough food to metamorphose before the pond dried. By indirect effect, the insecticide caused the death of approximately half of the tadpoles. A classic study on pesticides examined the role of the insecticide DDT on birds of prey. During the 1950s and 1960s in the United States, populations of many predatory birds, particularly the peregrine falcon (Falco peregrinus), bald eagle, osprey (Pandion haliaetus), and brown pelican (Pelecanus occidentalis), declined drastically. Several of these species disappeared from large areas and the peregrine falcon disappeared from the entire eastern United States. The causes of these population declines were traced to pollution of aquatic habitats by DDT, an insecticide that was widely used to control crop pests and mosquito vectors of malaria after World War II. This insecticide was favored both because it was effective at killing insect pests and because it persisted in the environment, allowing it to continue killing pest insects for a long time. When this pesticide enters a water body, it binds to particles, including algae, to become about 10 times more concentrated than it is in the water. When the algae are consumed by zooplankton, DDT accumulates in fat tissues until the zooplankton have a concentration that is about 800 times higher than in the water, as shown in Figure 23.18. The process of increasing the concentration of a contaminant as it moves up the food chain is known as biomagnification. When small fish eat the zooplankton and large fish eat the small fish, DDT is further concentrated about 30-fold. Finally, when a fisheating bird such as an osprey consumes a large fish, DDT becomes concentrated another 10-fold. In short, at the top of the food chain, the insecticide is 276,000 times more concentrated in the body of the fish-eating bird than it was in the water. Such a high concentration in predatory birds interferes with their physiology in a way that causes the eggs they lay to have very thin shells. When the parents sit on these thin-shelled eggs, the egg
shells break, and the embryos die. This eggshell thinning caused the populations of predatory birds to plummet during the 1960s.
Figure 23.18 Biomagnification of DDT. When DDT was widely used to control insects, it could be found in very low concentrations in the water. However, its concentration increases in particles with which it binds in the water, such as algae. At each higher trophic level, the insecticide becomes more concentrated.
Biomagnification The process by which the concentration of a contaminant increases as it moves up the food chain. Understanding the connection between DDT and declining bird populations led the U.S. government to ban the use of DDT and related pesticides in 1972, although it is still used in other parts of the world. Fortunately, alternative insecticides have since been developed that do not persist and are not stored in the fat of animals and, as a result, do not biomagnify through a food web. With the help of many biologists who handreared hundreds of predatory birds, populations of species such as the peregrine falcon and bald eagle have rebounded. DDT is still used in many tropical countries, although now in a limited manner, such as in houses to control the mosquitoes that carry malaria.
Global Climate Change
Changing global climates play both future and current roles in the biodiversity of species. In Chapter 5, we discussed the greenhouse effect and how gases such as water vapor, CO2, methane, and nitrous oxide allow our planet to be warmed naturally by absorbing infrared radiation emitted by Earth and then reradiating a portion back toward Earth. During the past century, human activities have increased the concentration of these greenhouse gases in the atmosphere (see “Ecology Today: Applying the Concepts” in Chapter 4), as well as the gases used as refrigerants known as chlorofluorocarbons (CFCs). One result of the increase in greenhouse gases has been an increase in the average temperature on Earth. From 1880, when the earliest measurements were taken, to 2013, the temperature on Earth has increased an average of 0.8 °C, as illustrated in Figure 23.19. While this is an average, some parts of the world such as northern Canada and Alaska have experienced temperature increases as high as 4 °C.
Figure 23.19 Global warming over time. Based on thousands of measurements around the world, scientists have observed a 0.8 °C increase in the average temperature of our planet. Because of year-to-year variation in temperatures, the pattern of increasing temperatures is clearer when we examine the 5-year-running mean temperatures. Temperature anomaly is a comparison of each year’s temperature compared to the mean temperature observed from 1951 to 1980. Global warming has caused a number of effects that can be readily observed. For example, we might expect warmer temperatures to cause more of the world’s ice to melt. As we noted at the end of Chapter 5, the massive polar ice cap in the Arctic has decreased in mass by 45 percent during the past 30 years. Similarly, the decline in ice from 2002 to 2016 has been more than 1,500 metric gigatons for Antarctica and more than 3,500 gigatons for Greenland. You can view these data in parts a, b, and c of Figure 23.20. All of this melting ice, combined with an overall warming of the oceans that causes water to expand, should cause a rise in sea levels. Researchers have examined water gauges for ocean tides since 1870 and, as expected, have observed a steady increase in sea level; today, the sea level is 0.2 m higher than it was in 1870. During the past 20 years, the sea level has been rising more than 3 mm per year, as shown in Figure 23.20d. At this rate, the sea will rise 0.3 m every 100 years, which is enough to dramatically affect habitats on islands and along coastlines.
Figure 23.20 Global warming and melting ice. In (a) the Arctic, (b) Antarctica, and (c) Greenland, the amount of ice has steadily declined from 2002 to 2010. Data are plotted relative to the mean ice mass throughout the entire time period. (d) Measurements of sea level from 1993 to 2013 demonstrate a continual increase in sea level compared to the sea level in 1993. As we have seen throughout this book, these changes in global temperatures are already affecting species. In Chapter 8, we discussed how many species of plants now flower earlier in the spring than they did in past decades and how some species of birds and amphibians now breed earlier in the year than they used to. In Chapter 11, we saw that warming ocean temperatures have caused a major shift in the fish species that live in the North Sea. Several of the species that historically lived in the North Sea have moved farther north to cooler waters, while a number of species that had lived in more southern waters moved up into the North Sea; this changed the species composition of the community. Global climate change has not yet led to the widespread extinction of species. However, even using computer models, it is difficult to predict how temperature and precipitation will change over future decades. One of the critical factors is how much we continue to increase the amount of CO2 in the atmosphere. As you can see in Figure 23.21, it is predicted that a small increase in CO2 will cause the northern latitudes to experience an additional 4 °C rise in average temperatures by the end of this century. A large increase in CO2 will raise temperatures by an additional 7 °C. Researchers predict that these
changes will cause extreme weather events such as hurricanes and droughts to occur more frequently. In addition, some regions of the world will receive more annual precipitation than is currently typical, while other regions will receive less. Although specific predictions vary with different models, the distributions of organisms in nature will probably shift as the Earth’s climates change over the next century.
Figure 23.21 Predicted changes in global temperatures. Depending on whether we assume (a) low or (b) high increases in CO2 in future years, the temperature is predicted to change. The changes in temperature are relative to the mean temperature observed from 1961 to 1990.
Analyzing Ecology
Contaminant Half-Lives As we have mentioned, an important factor in determining whether a contaminant such as a pesticide or radioactive compound affects the environment is the length of time the chemical persists in the environment. A helpful way to assess this persistence is to measure the time required for the chemical to break down to half of its original concentration, which is called its half-life. To calculate the half-life, we start by recognizing that most chemicals have a breakdown rate that
follows a negative exponential curve, as shown in the accompanying figure for a hypothetical chemical. If we were to start with 8 mg of the chemical and then ask how many days must pass until the chemical breaks down to only 4 mg, we see that it takes 10 days. To break down by half again, from 4 to 2 mg, it takes another 10 days. Half-life The time required for a chemical to break down to half of its original concentration. It is relatively easy to determine the half-life when we have a line graph. However, it is more difficult when we only have two data points: the amount of a chemical we start with and the amount of chemical remaining at some later point in time. In such cases, we can make use of an equation that calculates half-lives: t1/2=t ln(2)÷ln(N0÷Nt) where t1/2 is the half-life of the chemical, N0 is the initial amount of the chemical, and Nt is the amount of chemical after some amount of time (t) has elapsed. For example, using data from the figure, we begin with 8 mg of a chemical and after 30 days, we have 1 mg remaining. Using these data, we can calculate the half-life of the chemical:
t1/2=30 ln(2)÷ln(8÷1)t1/2=20.8÷2.08t1/2=10 Note that this is the same answer we obtained by estimating the half-life directly from the curve. YOUR TURN Modern pesticides are often designed to break down rapidly so their effects on nontarget species in the ecosystem are minimized. Imagine that you spray a wetland with an insecticide to kill mosquitoes that carry the West Nile virus, and this causes a concentration in the water of 50 parts per billion (ppb). Twenty-four hours later, you sample the water and you find that the water now contains 10 ppb of the chemical. Using these data, calculate the chemical’s half-life.
Concept Check
1. How has forested habitat been changing in North America? 2. What evidence indicates that overharvesting animals is not a recent problem? 3. Why do some pollutants biomagnify through a food web?
#### 23.4 Conservation Efforts Can Slow or Reverse Declines in Biodiversity
23.4 Conservation efforts can slow or reverse declines in biodiversity We have seen that human activities have caused a decline in the world’s biodiversity. We now look at what can be done to slow or even reverse these declines. Over the long term, we need to stabilize the size of the human population because human activities have caused most species declines during the past several centuries. Over the short term, we need to reduce human-caused sources of mortality and low reproduction in species so that these species can persist long into the future. During the past several decades, scientists have been developing effective strategies to preserve biodiversity, although these approaches are often neither easy nor inexpensive. In this section, we will focus on habitat protection, habitat management, reduced harvest, and species reintroductions.
Habitat Protection
Because the largest contributor to the loss of biodiversity is the loss of habitat, one of the major factors in conserving biodiversity has been habitat protection. The goal in protecting a habitat is commonly the preservation of a large enough area to support a minimum viable population (MVP), which is the smallest population size of a species that can persist in the face of environmental variation. The population must also be distributed widely enough to prevent local catastrophes, such as hurricanes and fires, from threatening the entire species. At the same time, some degree of population subdivision may help prevent the spread of disease from one subpopulation to another. Minimum viable population (MVP) The smallest population size of a species that can persist in the face of environmental variation. The task of protecting a suitable habitat becomes more complicated when a population’s habitat requirements change with the seasons or when it undertakes large-scale seasonal migrations. In the Serengeti ecosystem of East Africa, for example, patterns of rainfall distribution and plant growth vary seasonally. Huge populations of wildebeests, zebras, and gazelles
migrate seasonally in search of suitable grazing areas. Because migratory populations use the entire area of the Serengeti ecosystem over the course of a year, the preservation of only one section would not meet their needs. For similar reasons, the massive herds of bison, also known as buffalo, can never be fully restored to North American prairies because their seasonal migration routes are now blocked by miles of fencing and agricultural fields. Bison survive in a few small reserves in the American West—most notably in the Greater Yellowstone Ecosystem—but most of the lands they once occupied can never be recovered. This Greater Yellowstone Ecosystem, an 80,000-km2 area that is centered on Yellowstone National Park in Wyoming, is one of the best-known examples of preserving a large collection of habitat and includes federal, state, and private lands from three states, as illustrated in Figure 23.22. The management plan for the Greater Yellowstone Ecosystem seeks to maintain the region in a natural, self-sustaining condition. Natural forest fires should be allowed to burn, as they did in 1988 over half of Yellowstone National Park, and top predators, such as the grizzly bear and gray wolf, should be restored. As we will see in “Ecology Today: Applying the Concepts,” these top predators are natural controls over populations of large grazers, which has far-reaching effects on the ecosystem.
Figure 23.22 The Greater Yellowstone Ecosystem. To better preserve the species in the region, Yellowstone National Park and the many different types of land surrounding it are collectively managed as one large ecosystem. The need to preserve habitat is recognized around the world, and many countries are setting aside such areas. The amount of terrestrial habitat that is being protected has continually increased over the past 40 years, as shown in
Figure 23.23a. In fact, 57 percent of the world’s countries have protected at least one-tenth of their land. Though the need to protect marine habitats has been recognized much more recently, as you can see in Figure 23.23b, it has also increased over time.
Figure 23.23 Protected terrestrial and marine areas. Dots represent measured data, while the line represents the projected trajectory of future protection. (a) Terrestrial ecosystems have a longer history of being protected, and this level of protection has increased over the past 40 years. (b) Marine ecosystems began receiving protection much later, but the amount of protected marine habitats has also grown over time. A small amount of additional terrestrial and marine habitat has been protected, but the initial year of protection is unknown so it is not shown. Even when lands are set aside, many countries cannot afford to protect them from squatters and poachers, and often governments allow mining and
logging within protected lands. However, involving local people in the design and management of protected areas has been particularly effective since the benefits of conservation, which often include the income generated by ecotourism, become tangible and economically compelling to local people. The most successful efforts typically include large geographic areas, low human densities, a positive public attitude toward the preservation effort, and effective law enforcement to protect species from being poached.
Reduced Harvesting
When overharvesting is identified as a cause of decline in a species, reducing the harvest is an obvious approach to protection. However, this is complicated when it involves people’s livelihoods. For example, we saw that the collapse of the Atlantic cod fishery had a major negative economic impact on the fishing industry. Although some species can take a long time to recover their population size, reducing the harvest of a declining species often leads to a return to abundance. For instance, the northern elephant seal (Mirounga angustirostris) was hunted so much that by the late 1800s it was thought to be extinct (Figure 23.24). Small populations were subsequently discovered in the 1890s and the total population was estimated to be about 100 individuals. In the early 1900s, the seal was given protection status by the United States and Mexico. Once protected, the elephant seal population started to rebound rapidly, and today there are more than 150,000 individuals, distributed throughout much of the former range of this species in California and Mexico. Similar successes have occurred for the American crocodile (Crocodylus acutus), the whooping crane (Grus americana), and the bald eagle.
Figure 23.24 The northern elephant seal. Once hunted nearly to extinction, legal protection by Mexico and the United States has allowed the population to increase to more than 150,000 today. These seals are located on the coast of central California.
Species Reintroductions
Sometimes a species comes so close to extinction that it requires human intervention to bring it back from the brink. We saw an example of this when we discussed the recovery program for the black-footed ferret at the end of
Chapter 13. Such efforts can require a great deal of effort and money, and the recovery may take decades. A classic example of a species reintroduction is the case of the California condor, a large vulture that feeds on dead animals (Figure 23.25). During the latter half of the twentieth century, condor numbers dwindled due to a variety of causes, including illegal shooting and the illegal collection of eggs from condor nests. Moreover, some of the animals the condor scavenged and consumed contained lead bullet fragments or, in the case of coyotes and rodents, had been poisoned. When the condor population in southern California sank to fewer than 30 individuals in the late 1970s, biologists made the difficult decision to bring the entire population into captivity. From 1982 to 1987, the 22 remaining wild birds were captured and brought to special breeding facilities located at zoos in Los Angeles and San Diego, where they were protected from the threats that had taken such a toll in the wild.
Figure 23.25 The California condor. This large vulture was on the brink of extinction in the 1980s when only 22 birds remained in the wild. A captive breeding program, followed by reintroductions into the wild, has increased that number to 400 birds today living both in captivity and in the wild. Recovering the condor population is extremely challenging. Individuals take 6 to 8 years to reach maturity and in the wild typically lay only one egg per year. However, researchers discovered that if they removed the single egg, a female would lay another. In fact, a female condor could be tricked into laying up to three eggs per year. Biologists incubated the surplus eggs and reared the birds using puppets to feed the chicks to minimize human contact. As the chicks grew up, they were released back into the wild. At the same time, efforts were made to reduce the threats to the condors. These substantial efforts have paid off; in 2016, there were 446 condors alive both in captivity and in the wild following reintroductions into California, Arizona, and Mexico. In addition to increasing the condor population, this high-profile case heightened local residents’ awareness of conservation issues and resulted in the preservation of large tracts of habitat in mountainous regions of southern California. People have also come to understand that restoring the condor population can be compatible with other land uses. Recreation does not have to be banned from condor habitats as long as human access to nesting sites is restricted. Legal hunting doesn’t harm condors as long as steel rather than lead bullets are used. Finally, ranching does not threaten condor populations so long as coyote and rodent control programs are condor-safe.
Species reintroductions can cost millions of dollars and are usually directed toward species that appeal to the public, such as black-footed ferrets, condors, and wolves. Some people may question the wisdom of spending millions of dollars to save one species. However, efforts made to save a single species, such as setting aside habitat and reducing the prevalence of poisons, often have positive effects on a large number of species. We will see an excellent example of this in “Ecology Today: Applying the Concepts,” which details the ecosystem effects of reintroducing the gray wolf to Yellowstone National Park. Throughout this book, we have examined the factors that determine the distribution and abundance of species throughout the world. We have seen that there is an amazing diversity of species on Earth that impresses us with its beauty and plays essential roles in the ecosystems upon which we depend. The decline in the world’s biodiversity is at a critical stage because human activities threaten populations of many species. However, by appreciating the importance of the world’s species and understanding their ecology, we can take the steps necessary to slow population declines and species extinctions, and to find ways to coexist.
Concept Check
1. How has the amount of protected habitat around the world changed over recent decades? 2. How has the ban on harvesting marine mammals affected populations of northern elephant seals? 3. How can investment in a large, charismatic species also favor the conservation of other species?
Concepts
Returning Wolves to Yellowstone The wolves and elk of Yellowstone. Following the reintroduction of wolves, the elk population has declined and some regions of the ecosystem have changed dramatically. As we have seen in this chapter, restoring biodiversity can be a difficult challenge, and nowhere has this been more apparent than in the attempt to return the gray wolf to the Greater Yellowstone Ecosystem. The challenges in this case include ecological, economic, social, and political factors. After all, not everyone is excited about bringing back a top predator with a reputation for being a bloodthirsty killer. The gray wolf once roamed throughout most of North America, but fear of wolves and concerns about their preying on livestock led to government programs that eliminated the wolf from nearly all of the United States and southwestern Canada. These programs caused the wolf to be eradicated from all of the lower 48 states except for
northern Minnesota by 1925. In the 1940s, Aldo Leopold, a prominent professor of wildlife management, first proposed returning the wolves to Yellowstone National Park. By the 1960s, public attitudes toward wolves started to shift as many people began to see the intrinsic value of restoring ecosystems to a more natural state. In 1973, the Endangered Species Act was passed, and it classified gray wolves as an endangered species, which required the development of a plan that would increase their abundance. Because Yellowstone was a large protected area, it was an obvious place for a possible reintroduction. The plan to reintroduce wolves had supporters and detractors. Public opinion polls found that the general public and the tourists who visited the park favored the idea. However, local hunters worried that the wolves would reduce the populations of large game such as elk. Moreover, the local livestock ranchers worried about their herds because wolves sometimes kill cattle and sheep. For nearly 2 decades, the reintroduction of wolves was debated, studied, and pushed in different directions by politicians representing various constituents. Throughout the early 1990s, the U.S. Fish and Wildlife Service held 130 town meetings in the area and received thousands of written comments on a proposed reintroduction plan. They also estimated the impacts of a wolf reintroduction to Yellowstone. Their best estimate was that an eventual population of 100 wolves would kill about 20 cattle, 70 sheep, and 1,200 wild ungulates such as elk, deer, and antelope each year from a population of 95,000 wild ungulates. The economic benefit from increased tourism to the area because of the wolves was estimated at $23 million, which represented a substantial instrumental value.
After a series of court hearings and repeated judgments in favor of the reintroduction plan, 31 wolves were captured in Canada and released into Yellowstone in 1995 and 1996. Effects of wolves on the Yellowstone ecosystem. With the reduction of elk populations due to wolf introductions, aspen and cottonwood trees along some river banks have recovered, as seen in the foreground of the three photos. These photos were taken at
Soda Butte Creek in (a) 1997, (b) 2001, and (c) 2010. Nearly 2 decades later, the effects of the reintroduction have been felt throughout the ecosystem. When the wolves were eradicated in the early 1900s, the elk population rapidly increased and their feeding nearly eliminated aspen and cottonwood trees along rivers. The return of the wolves, and their rapid increase to a population of more than 220 individuals by 2001, reduced the elk population by half. As a result, aspen and cottonwood trees are now thriving in some areas of the park. Some researchers have argued that the effects on aspen growth are due to a combination of density- and trait-mediated effects (see
Chapter 18). It has been suggested that the trait-mediated effects result from wolves scaring the elk away from streams and rivers and into higher elevations, where the elks are safer from predation. However, others argue that there is insufficient evidence to suggest that such fearinduced habitat shifts have had a widespread effect on the growth of aspen and cottonwood seedlings. Because aspens and cottonwoods are a favorite food of beavers, the increase in these trees has attracted more beavers into the region, which has led to an increase in beaver dams that form large ponds. In addition, the wolf reintroduction caused a severe reduction in the population of coyotes since they are prey for wolves. This likely had a cascading effect on the many prey species that coyotes consume. The abundant carcasses of elk and other prey of the wolves also benefited populations of scavengers such as ravens and golden eagles, which are once again common in Yellowstone. In short, the reintroduction of the wolf caused the ecosystem to move back to more closely
resemble its earlier condition. In 2008, the Yellowstone wolf population was deemed to have recovered well enough to be removed from the federal list of endangered species. The wolves are now hunted in limited numbers and the wolf population has been maintained at about 100 individuals from 2013 to 2015. It will be interesting to see what continued effects the wolves will have on the ecosystem. The return of the wolf to the Greater Yellowstone Ecosystem demonstrates that restoring biodiversity is not a simple task. Restoration plans need to consider not only the ecology of the reintroduction but also related social, political, and economic factors. The plans must be evaluated by all interested parties and the process can often take a long time. With patience, however, species and ecosystems can eventually be restored. SOURCES: Fritts, S. H., et al. 1997. Planning and implementing a reintroduction of wolves to Yellowstone National Park and Central Idaho. Restoration Ecology 5: 7–27. Ripple, W. J., and R. L. Betscha. 2011. Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction. Biological Conservation 145: 205–213. Smith, D. W., et al. 2003. Yellowstone after wolves. BioScience 53: 330–340. Kaufmann, M. J., et al. 2010. Are wolves saving Yellowstone’s aspen? A landscape-level test of a behaviorally mediated trophic cascade. Ecology 91: 2742–2755. Beschta, R. L., and W. J. Ripple. 2013. Are wolves saving Yellowstone’s aspen? A landscape-level test of a behaviorally mediated trophic cascade: Comment. Ecology 94: 1420–1425.
Kaufmann, M. J., et al. 2013. Are wolves saving Yellowstone’s aspen? A landscape-level test of a behaviorally mediated trophic cascade: Reply. Ecology 94: 1425–1431. Beschta, R. L., and W. J. Ripple. 2015. Divergent patterns of riparian cottonwood recovery after the return of wolves in Yellowstone, USA. Ecohydrology 8: 58–66.
Summary of Learning Objectives
23.1 The value of biodiversity arises from social, economic, and ecological considerations. Instrumental values represent the material benefits that species and ecosystems provide to humans, including food, medicines, water filtration, and pollination. Intrinsic values recognize that species and ecosystems are valuable regardless of any benefit to humans. Key Terms: Instrumental value of biodiversity, Intrinsic value of biodiversity, Provisioning services, Regulating services, Cultural services, Supporting services
23.2 Although extinction is a natural process, its current rate is unprecedented. Historically, there have been five mass extinctions, each followed by continued speciation. The current rate of species extinction is higher than background rates, which suggests that we may be in the early stages of a sixth mass extinction. Many species are also experiencing a rapid decline in genetic diversity, although efforts are being made to preserve the genetic diversity of livestock and crops. Key Term: Mass extinction events
23.3 Human activities are causing the loss of biodiversity. One of the most important factors contributing to species declines is the loss of habitat. Other impacts include overharvesting, introducing exotic species, and polluting the environment with contaminants. Global warming and global climate change present a rising threat. Some changes have already occurred and species may not be able to accommodate more substantial climate shifts in the future. Key Terms: Collapsed fishery, Biotic homogenization, Biomagnifications, Half-life
23.4 Conservation efforts can slow or reverse declines in biodiversity. Major efforts are being made to respond to the decline in biodiversity. Increasing amounts of terrestrial and marine habitat are being protected, harvest regulations are being adjusted to slow population declines, and species approaching extinction are being reintroduced where high-quality habitat exists and the threats can be reduced. Key Terms: Minimum viable population (MVP)
Critical Thinking Questions
1. Why might different people or groups favor different criteria when prioritizing biodiversity hotspots? 2. Compare and contrast instrumental values versus intrinsic values of species and ecosystems. 3. How can economic benefits of biodiversity be used as an argument to protect biodiversity? 4. What steps can be taken to slow the sixth mass extinction? 5. Why should we be concerned with preserving both species diversity and genetic diversity? 6. Why have humans historically overharvested many species of animals? 7. Why might introduced competitors result in less extinction of native species than introduced predators? 8. Why do we need to consider the process of biomagnification when assessing the risk of a pesticide to wildlife? 9. Why is it difficult to predict which species will be driven extinct by global warming? 10. What are the ecological, economic, and social challenges that can arise when considering a species reintroduction?
GRAPHING THE DATA Stacked Bar Graphs When we want to compare the distribution of categories within different groups, such as the conservation status of species of various taxa, we can either examine percentages or absolute numbers. As part of our discussion of the conservation status of conifers, birds, mammals, and amphibians, Figure 23.5 used pie charts to show differences in the percentages of species that were categorized as of least concern, near-threatened, and threatened. However, we can obtain a somewhat different perspective if we graph the absolute number in each category using a stacked bar graph, similar to Figure 23.13. A stacked bar graph allows us to display data in a way that stacks one set of data on top of another set of data within a category. This permits us to observe each category in terms of its separate and combined components. Using the following data for the number of species of conifers, birds, mammals, and amphibians in each status category, create a stacked bar graph. Conservation Status Conifers Birds Mammals Amphibians
Threatened
1,313 1,139 1,933 Compare your graph to the pie charts presented in Figure 23.5. What different insights do you gain by plotting the absolute numbers of species using a stacked bar graph that you could not obtain from a pie chart of proportions of species in each category?