CHAPTER OVERVIEW 9: The Ecology of Populations Learning Objectives Define what ecologists mean by a population of organisms and the focus and scope of the science of population ecology. Introduce the key features of populations studied by ecologists, wildlife biologists, and conservation biologists -- such as size, density, and range -- and the field methods used to study them. Outline common patterns of population change, such as growth, extirpation, extinction, and cycles. Introduce the concepts of population regulation, density independence, and density dependence. 9.1: What is population ecology? 9.2: Population Ecology Research Methods 9.3: Population Dynamics and Regulation 9.4: Scientist Spotlight - Jessie Isabelle Price Summary Populations are one of the major levels of biological organization, and "population thinking" has played a key role in ecology and evolutionary biology since Darwin. Currently, many researchers and natural resource managers are trained as population ecologists, from academics who study basic questions in evolutionary ecology to government scientists who determine catch limits for fisheries. In this chapter we define what populations are, their key features which are considered by basic and applied ecologists, and the techniques used out in the field to study populations. We'll also introduce key ecological concepts which will be elaborated on in the next chapter, including population regulation, density independence, and density dependence. 9: The Ecology of Populations is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
9.1: What is population ecology? What is population ecology? Thousands of bird species breed and reproduce in North America. Some, like the American Robin (Turdus migratorius), are widespread, and can be found building nests and raising their young in every state of the USA, several Canadian provinces, and many locations in Mexico. Others, like Kirtland's Warbler (Setophaga kirtlandii) breed almost entirely within a single state (Fig. 9.1.1); a few, like the Cozumel thrasher (Toxostoma guttatum) and Socorro mockingbird (Mimus graysoni) are found only on single small islands. Population ecologists study what determines the occurrence and abundance of species in space and time: their geographic ranges, population sizes and densities, and what factors result in them being so rare or common. Population ecology is the science of population dynamics in space and time Ecology is often defined as the study of the distribution and abundance of organisms. Population ecology is the branch of ecology that works to understand the patterns and processes of change over time or space for populations of a single species. A species is typically defined as a group of organisms capable of interbreeding. For some species, all of the members of the species occur in the same geographic area and could potentially meet and interbreed during their lifetimes. Most species, however, can be divided into geographically separate populations. Individuals within a single population are likely to interact and perhaps interbreed, while those from different populations will only come into contact if there is long-range movement between the populations (dispersal). Populations can be described by their size, density, or spatial extent One species that currently consists of a single population is the Kirtland's warbler (Setophaga kirtlandii), a North American songbird. Almost all members of this species occur in the northern part of the state of Michigan in the United States (Figure ). In contrast, the Spotted Owl (Strix occidentalis) is a species with many distinct populations throughout the western United States, southern Canada, and central Mexico.
: Left: "A male Kirtland's Warbler Setophaga kirtlandii in a forest in Michigan, USA" by Jeol Trick is licensed under CC BY 2.0. Right: Core habitat area of Kirtland's Warbler. Source: Google Earth: https://bit.ly/3ofgDK1.
For both species and populations, patterns of distribution and abundance can be considered in several ways. These include: 1. Size: How many total individuals there are? 2. Density: How many individuals per unit of area? 3. Dispersion: How are individuals in a population arranged spatially relative to another? Do they occur in clumps or are they evenly spread apart? 4. Occupancy: Does a species or member of a population occur in a given habitat, or is it absent? 5. Population distribution: Where does a population occur in space? 6. Geographic range: What are the furthest geographic limits of where a species occurs?
In addition to static characteristics of size and distribution, populations are dynamic and fluctuate based on a number of factors: seasonal and yearly changes in the environment, natural disasters such as forest fires and volcanic eruptions, and competition for resources between and within species. To study these many facets of a population's biology, ecologists use both systematic field observations to determine its current status, and mathematical tools to characterize how it responds to changes in the biotic and abiotic environments. Population size is the number of individuals in a population Population size is the actual number of organisms in a population. This is often of great interest to biologists - especially those working in forestry, wildlife management and conservation - and most of our basic population models work with population sizes. A complete census is one way to determine population size and entails counting each individual present within the population. This occurs in some well-studied populations, such as the Kasekela population of chimpanzees in Gombe National Park, Tanzania (Pusey et al. 2008), and the Seychelles Warbler on islands in the Indian Ocean off the coast of East Africa (Burt et al. 2016). Although it is the most accurate methodology, counting every individual in a population can be difficult, if not impossible. In most cases ecologists can only attempt to estimate the population size (N) by using well-designed field studies and statistics. Indeed, some population ecologists specialize in developing mathematical and statistical models to accurately estimate population size, such as mark-recapture models and camera-trapping methods (detailed below). Often, however, we do not have good estimates of the size of a population itself, but factors that should be correlated with the population size, such as the number of animals harvested by hunters or trapped by ecologists or the density of dung found during a survey. Data that we think correlates with actual abundance constitutes a population index. Index data are cheaper to collect than the data needed for formal estimates of population size such as the mark-recapture methods discussed below, but can be biased and provide an inaccurate sense of the status of a population (Stephens et al. 2015). Ideally, an index should be validated by checking its correlation with rigorous estimates of population size. For example, the abundance of large mammals such as lions, elephants and tigers is frequently indexed by the frequency of their tracks or scat. To determine the reliability of an indirect measure of population size, Belant et al. (2019) compared an index based on lion tracks to a formal estimate of population size. Unfortunately, the commonly used index of lion abundance based on their tracks overestimated abundance. When species become endangered researchers often try to determine - or at least estimate - the number of individuals surviving. For example, with only approximately 4000 individuals, Kirtland's Warbler is the rarest species breeding in the continental United States and was considered critically endangered throughout most of the 20th century. Researchers therefore worked each spring to determine as best as possible how many male warblers had established territories and were trying to attract mates. Species that are economically important or are central players in ecosystem functioning are also often monitored intensively. Since the middle of the 20th century the abundance of Wildebeest (Connochaetes taurinus) in the Serengeti ecosystem of East Africa has been intensively monitored by aircraft (Figure ). The population was considered to be small in the 1960s when it numbered around 250,000, but by the 1990s had grown to over 1 million (Mduma et al. 1999). A photo of an expansive flat field shows hundred of large, dark brown wildebeests grazing.
Figure : Wildebeest in Maasai Mara. Photo by Bjørn Christian Tørrissen, http://bjornfree.com/galleries.html.
Population density is the relative abundance of an organism While population size is a total count of individuals, population density is how many individuals occur in a given area of space. It is therefore a measure of relative abundance. For animals and trees, this is often the estimated number of animals per hectare (a hectare is 100 m by 100 m, or 2.47 acres). For plants, insects, and other smaller organisms this is often the number per square meter. Kirtland's Warbler is a habitat specialist and only nests in forests dominated by a single conifer, the Jack Pine (Pinus banksiana). Moreover, it only nests in Jack Pines of a certain age (5-20 years) and density (>3000 pines per hectare; Donner et al. 2018). When conditions are optimal, there is usually one breeding pair of warblers per 70 hectares, or 1.4 pairs per 100 hectares (Densities are usually reported for standard areas such as 1 square meter, 100 hectares, etc.). Estimates of population density are often much easier to obtain than estimates of total population size. Population density can be converted to a rough estimate population size through simple multiplication. If there are 1.4 pairs per hectare of good habitat, and
there are 2800 hectares of habitat, we can calculate the number of pairs as 1.4 x 2000 = 2800 pairs. Two things are key to this calculation, however: first, the estimate of density is accurate, and second, the estimate of the amount of good habitat is accurate. Recently, great progress in estimating animal density has been made using camera traps (Fig. ). These are especially useful for studying rare and nocturnal animals, such as predators. For example, lions (Panthera leo) are considered vulnerable to extinction and are most threatened in West Africa, where they are restricted to a few small national parks. In western-most West Africa lions occur only in Niokolo-Koba National Park in south-eastern Senegal (Henschel et al. 2014). Mamadou Kane used camera traps to estimate the density of lions in Niokolo-Koba (Kane et al. 2015). While the entire park is 9130 km2, Kane sampled an area of approximately 285.4 square kilometers within the highest quality lion habitat of the park. Kane estimated that there are about 3 lions per 100 square kilometers in this high-quality habitat (100 square kilometers is an area 10 km by 10 km). Three lions per 100 km2 equals 0.03 per km2. We can therefore estimate the total abundance in the study area as 0.03 x 285.4 = 8.6 lions.
Figure : "A camera trap, for taking pictures of game on trails" by Hustvedt is licensed under CC BY-SA 3.0. This model is for hunting. Motion detector on top, lens in the middle, flash on the bottom, with a little LCD for showing how many pictures taken/left on the left.
Occupancy reports the presence or absence of a population Occupancy is simply whether a given species or member of a population occurs in a habitat patch, fragment, or area. Often it can be very difficult to count the number of individuals or determine their density; instead of asking a question like "How many owls are there in this forest?" it's relatively easier to ask "Are there any owls present in this forest?" It can be very time-consuming to do counts or determine density, which can limit researchers' ability to study multiple sites or locations. Focusing on determining just the presence or absence of a population in a habitat often allows researchers to study a larger area.
: Frequency of Kirtland's warbler singing males counted per township from 2000-2011. Source: U.S. Fish and Wildlife Service.
Kirtland's Warbler is currently increasing its population size and is being found in stands of Jack Pine outside of the state of Michigan (Figure ). Since the warbler's population is increasing, researchers are more interested in doing occupancy surveys to determine which patches of forest warblers are occurring in rather than determining exactly how many warblers are in these forests (Richards 2008). Camera traps and occupancy studies can be used to determine if a species has become locally extinct. Giodano, Tumenta and Iongh (2017) carried out a camera-trap based occupancy study of Waza National Park in Cameroon. While they confirmed the occupancy of lions in the park, no leopards (Panthera pardus) were photographed. Since lions and leopards are typically similarly difficult to detect and lions were detected, they concluded that there were no longer any leopards occupying the park. Individuals within a population can have characteristics patterns of dispersion In addition to measuring simple density, further information about a population can be obtained by looking at the distribution of the individuals. Species dispersion patterns (or distribution patterns) summarize the spatial relationship between members of a population within a habitat at a particular point in time. In other words, they show whether members of the population live close together or far apart, and what patterns are evident when they are spaced apart. Individuals in a population can be more or less equally spaced apart, dispersed randomly with no predictable pattern, or clustered in groups. These are known as uniform, random, and clumped dispersion patterns, respectively (Figure ). Uniform dispersion can occur in plants and is thought to result from competition for below-ground resources such as water, or secretion of substances inhibiting the growth of nearby individuals, a phenomenon called allelopathy. In animals like penguins that nest in large colonies, uniform dispersion can occur due to territorial behavior. An example of random dispersion occurs with dandelion and other plants that have wind-dispersed seeds that germinate wherever they happen to fall in a favorable environment. A clumped dispersion may be seen in plants that drop their seeds straight to the ground, such as oak trees, or animals that live in groups (schools of fish or herds of elephants). Clumped dispersions may also be a function of habitat heterogeneity. Thus, the dispersion pattern of the individuals within a population provides more information about how they interact with each other than does a simple density measurement. Just as lower density species might have more difficulty finding a mate, solitary species with a random distribution might have a similar difficulty when compared to social species clumped together in groups.
Figure : Species may have uniform, random, or clumped distribution. Territorial birds such as penguins tend to have uniform distribution. Plants such as dandelions with wind-dispersed seeds tend to be randomly distributed. Animals such as elephants that
travel in groups exhibit clumped distribution (credit a: modification of work by Ben Tubby; credit b: modification of work by Rosendahl; credit c: modification of work by Rebecca Wood). Populations distributions are limited to suitable habitats In ecology, a niche is the match of a species to a specific environmental condition. It describes how an organism or population responds to the distribution of environmental resources (abiotic components of the environment) and predators, pathogens, and
competitors (biotic components of the environment). Habitat refers to the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. The geographic range of a species can be viewed as a spatial reflection of its niche, along with characteristics of the geographic template and the species that influence its potential to colonize. The fundamental geographic range of a species is the area it occupies in which environmental conditions are favorable, without restriction from barriers to disperse or colonize (Lomolino et al 2009). A species will be confined to its realized geographic range when confronting biotic interactions or abiotic barriers that limit dispersal, a more narrow subset of its larger fundamental geographic range. An early study on ecological niches conducted by Joseph H. Connell analyzed the environmental factors that limit the range of a barnacle (Chthamalus stellatus) on Scotland's Isle of Cumbrae (Connell 1961) (Figure . In his experiments, Connell described the dominant features of C. stellatus niches and provided explanation for their distribution on intertidal zone of the rocky coast of the Isle. Connell described the upper portion of C. stellatus's range as limited by the barnacle's ability to resist dehydration during periods of low tide. The lower portion of the range was limited by interspecific interactions, namely competition with a cohabiting barnacle species and predation by a snail (Connell 1961). By removing the competing B. balanoides, Connell showed that C. stellatus was able to extend the lower edge of its realized niche in the absence of competition. These experiments demonstrate how biotic and abiotic factors limit the distribution of an organism.
: Chthamalus stellatus, photographed near the upper shoreline, Lundy Island, UK. Max shell length about 10mm. Source: MichaelMaggs and is published with a CC-BY-SA 3.0 license.
The area occupied by a species can be considered at different scales Population distribution is the geographic area where a particular population of a species occurs. This area of occupancy is determined by the local availability of appropriate habitat. Some species are habitat generalists (species that can thrive in a wide variety of environmental conditions; see Chapter 13) and their populations spread out almost continuously across a landscape. American Robins, for example, can breed in parks, urbanized areas, farms, and the edges of forest. Other species are habitat specialists and only occur in specific places where a certain type of ecosystem occurs. Aquatic species also often occur in isolated populations because their habitat is necessarily bounded by the extent of the lake, wetland, or waterway they occur in. The Northern Spotted Owl is a subspecies of spotted owl that is a habitat specialist that only occurs in old-growth forests in the Pacific Northwest of North America (British Columbia, Washington, Oregon, California). While much of the Pacific Northwest is still forested, most forests are less than 100 years old and don't have the large, old trees that the owls nest in. The extent of each population of owls is therefore limited by the size of each patch of old growth forest. A species' geographic range is the total geographic area occupied by a species. Bald Eagles can be found breeding in almost every state of the USA and most provinces of Canada. Its geographic range therefore encompasses almost all of North America north of Mexico. In contrast, Golden Eagles breed almost exclusively in western Canada, the western United States, and Mexico. The Golden Eagle's range therefore is restricted to the western part of the continent and Mexico. As noted above, Kirtland's warblers are habitat specialists, which means that the species only reside in the Jack Pine Forests of northern Michigan and a few adjacent states and Canadian provinces. Subsequently, this phenomenon is referred to by biologists as a restricted range. A restricted range species refers to a species in which the range is so small, there is basically a single population. This results in the population distribution and geographic range is essentially the same.
Vocab Alert! A term that can sometimes be confused with geographic range is home range. Geographic range refers to the entire spatial area a species can be found in, while a home range refers to an area utilized and perhaps defended by a single organism.
A species' current geographic range is often very different from its historical range. Factors such as habitat loss, hunting, and climate change can all reduce the distribution of populations and the species' overall geographic range. For example, lions previously ranged broadly over Africa, Western Asia, the Middle East and India (Figure ).
: Historical (red) and present (blue) distribution of lions (Panthera leo). Source: Tommyknocker (Wikipedia), based on a map created by 'The African Lion Environmental Research Trust (ALERT).
View this video for a case study example of population ecology. Can you identify the key concepts and vocabulary terms you learned about for the Texas Mosquito Mystery? Population Ecology: The Texas Mosqu CrashCourse
Contributors and Attributions This chapter was written by N. Brouwer with text taken from the following CC-BY resources: Ecological niche by Wikipedia, the free encyclopedia OpenStax Biology 2e section 45.1 Population Demography 9.1: What is population ecology? is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
9.2: Population Ecology Research Methods Population ecology research methods Quadrat-based methods are frequently used by plant ecologists The most accurate way to determine population size is to simply count all of the individuals within the habitat. However, this method is often not logistically or economically feasible, especially when studying large habitats. Thus, scientists usually study populations by sampling a representative portion of each habitat and using this data to make inferences about the habitat as a whole. A variety of methods can be used to sample populations to determine their size and density. For immobile organisms such as plants, or for very small and slow-moving organisms, a quadrat may be used (Figure ). A quadrat is a way of marking off square areas within a habitat, either by staking out an area with sticks and string, or by the use of a wood, plastic, or metal square placed on the ground. After setting the quadrats, researchers then count the number of individuals that lie within their boundaries. Multiple quadrat samples are performed throughout the habitat at several random locations. All of this data can then be used to estimate the population size and population density within the entire habitat. The number and size of quadrat samples depends on the type of organisms under study and other factors, including the density of the organism. For example, if sampling daffodils, a 1 m2 quadrat might be used whereas with giant redwoods, which are larger and live much further apart from each other, a larger quadrat of 100 m2 might be employed. This ensures that enough individuals of the species are counted to get an accurate sample that correlates with the habitat, including areas not sampled.
Figure : A scientist uses a quadrat to measure population size and density. Source: NPS Sonoran Desert Network.
Distance sampling methods estimate the density of organisms Other techniques for sampling populations to estimate density or abundance are distance sampling methods. The main methods are based on line transects or point transects (Buckland et al 1993, Buckland et al 2003). In this method of sampling, the data collected are the distances of the objects being surveyed from these randomly placed lines or points, and the objective is to estimate the average density of the objects within a region (Everitt 2002). When using line transects, the observer randomly places a straight line or follows some pre-planned route through the habitat of the population of interest, then walks the line or route. Whenever the researcher observes an object of interest (e.g., an animal of the type being surveyed), they record the observation, the distance from their current position to the object (r), as well as the angle of the detection to the transect line (). The distance of the object to the transect can then be calculated using trigonometry as x = r * sin(). These distances x are the detection distances that will be analyzed in further modeling aspects of the population (e.g., population distribution in space).
Figure : Basic distance sampling survey approach using line transects. A field observer detects an object and records distance r and angle to the transect line. This allows the calculation of object distance to the transect (x). All x from the survey are used to model how detectability decreases with distance from the transect, which allows estimation of total population density in the surveyed area. (Elmidae)
Distance-based methods are frequently used for animals with observers making their observations while walking, riding in trucks, or in low-flying aircraft. They can also be used for tasks such as surveying for very rare trees (Kissa and Sheil 2012) or plants. Mark-recapture methods estimate the abundance of animals For mobile organisms, such as mammals, birds, or fish, a technique called mark and recapture is often used. This method involves marking a sample of captured animals in some way (such as tags, bands, paint, or other body markings), and then releasing them back into the environment to allow them to mix with the rest of the population; later, a new sample is collected, including some individuals that are marked (recaptures) and some individuals that are unmarked (Figure ).
Figure : Mark and recapture is used to measure the population size of mobile animals such as (a) bighorn sheep, (b) the California condor, and (c) salmon (credit a: modification of work by Neal Herbert, NPS; credit b: modification of work by Pacific
Southwest Region USFWS; credit c: modification of work by Ingrid Taylar). Using the ratio of marked and unmarked individuals, scientists determine how many individuals are in the sample. From this, calculations are used to estimate the total population size. This method assumes that the larger the population, the lower the percentage of tagged organisms that will be recaptured since they will have mixed with more untagged individuals. For example, if 80 deer are captured, tagged, and released into the forest, and later 100 deer are captured and 20 of them are already marked, we can determine the population size (N) using the following equation: Using our example data, Therefore, there are an estimated 400 total individuals in the original population.
A common issue with mark-recapture methods is that the process of capturing and marking the animals changes their behavior. This is known generally as a trap response. Some animals from the first catch may learn to avoid capture in the second round, thus inflating population estimates; this is known as trap shyness. Alternatively, animals may preferentially become trap-happy and be more likely to be re-trapped (especially if a food reward is offered), resulting in an underestimate of population size. In some cases, individuals may be harmed by the capture and marking technique, reducing their survival. Advanced mathematical techniques exist for dealing with trap shyness and trap happiness; if mark-recapture methods cause harm to animals, however, the method should not be used. A variety of other techniques have been developed to collect mark-recapture and similar data, including the electronic tracking of animals tagged with radio or GPS transmitters. Techniques also exist for using trapping-only data such as hunting or commercial fishing operations to estimate the size and health of populations and communities. Mark-Recapture Practice Exercise 1) You marked 50 whales at the beginning of the breeding season in 2022 and, during a re-sighting survey at the end of the breeding season, counted 100 whales, 2 of which were tagged. What is your estimate of the total breeding population size in 2022? 2) Do you have any concerns about the validity of your answer? 3) After conducting your mark-recapture study, you find out that female gray whales migrate before male gray whales do, so not all whales would have returned to the breeding grounds when you marked individuals at the start of the season. Using the mark-recapture equation, briefly describe how this impacts the accuracy of your population estimate. Answer 1) N = (M * S)/R = (50*100)/2 = 2500 whales estimated as the total breeding population size in 2022 2) Two is a very small sample size of re-sightings, making it difficult to ensure accuracy. Far more whales were counted at the end, suggesting that the effort may have been varied throughout the study. Also, sighting whales is difficult due to long diving periods, potentially leading to some whales not being counted, despite being present. 3) M/N = R/S but in this scenario, because N at the time of marking was not the actual size of the population, the R/S ratio at the end of the season will be smaller than predicted (since S will be larger than expected because not all of those individuals were there at the start of the season). As a result, M/N and N will be underestimated. Contributors and Attributions This chapter was written by N. Brouwer with text taken from the following CC-BY resources: Distance sampling by Wikipedia, the free encyclopedia OpenStax Biology 2e Section 45.1 Population Demography 9.2: Population Ecology Research Methods is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
9.3: Population Dynamics and Regulation How do populations change? Changes in population size over time and the processes that cause these to occur are called population dynamics. How populations change in abundance over time is a major concern of population ecology, wildlife ecology, and conservation biology, and is related to questions asked in evolutionary biology. The processes and mechanisms that drive population change are varied and include intraspecific competition with members of the same population, interspecific competition between species, the availability of food or other resources, extreme weather, inbreeding, predators or parasites. Populations are dynamic and frequently change size, density, or spatial extent We can consider changes in populations from multiple angles. For example, Kirtland's Warbler (Setophaga kirtlandii) in North America is currently: 1. Increasing in the overall number of individuals (population size). 2. Increasing in the number of occupied habitat patches (occupancy). 3. Increasing in the geographic area it occurs in (population distribution and species range). Importantly, since the warbler prefers a certain density of Jack Pine, its density within an occupied habitat also changes. Jack Pine stands are naturally prone to burning in forest fires, and are also logged for timber. As the density of trees changes due to these disturbances, the density of warblers changes. After a fire or logging there are few if any mature pine trees and therefore few warblers. Approximately five years after seedlings have sprouted and grown up to be the proper size, the density of warblers can increase. When pine forests get too old habitat conditions are not ideal for the warbler and their abundance declines. Many studies of population growth focus on changes in population size Though there are many dimensions to spatial and temporal population dynamics, discussions of population dynamics often center on changes in population size over time. Changes in population size are often displayed in a time series graph with time on the xaxis (usually in years) and population size (N) on the y-axis. General patterns of population dynamics in terms of population size include: 1. Growth: Growing larger than the current size (Snail kites: Figure Panel A) 2. Decline: Decreasing in abundance (Elk: Figure Panel B) 3. Stability: Staying approximately the same size over time (Wolves: Figure Panel C) 4. Recovery: Stability or growth following a period of decline. (Impala: Figure Panel D). 5. Extirpation (local extinction): Decline of one or more populations of a species to 0 (Kirtland's Warbler, Figure Panel A). 6. Extinction: Decline of all members of a species to 0 (Northern White Rhino, Figure Panel B). 7. Cycles: repeated patterns of growth followed by decline (Lynx: Figure Panel C)
Figure : Common patterns of population change. The x-axis in all panels is the year and the y-axis is the number of individuals. a) Growth in a Florida Snail Kite (Rostrhamus sociabilis) population from 1970s to 1980s (Sykes 1983); b) Decline of the Gallatin, Montana herd of elk (Cervus canadensis) from the 1920s to 1960s (Peek et al. 1967); c) Stability of the Isle Royale, Michigan pack of wolves (Canis lupus) in the 1980s and 1990s (Peterson et al. 1998); d) Recovery after population crashes in the
Lake Manyara National Park, Tanzania herd of impala (Prins and Weyerhaeuser 1987).
Figure : Common patterns of population change. The x-axis in all panels is the year. a) Decline to extirpation (local extinction) of Kirtland's Warbler in two populations (Probst 1986). The y-axis is the number of singing males; b) Decline to global extinction of the Northern White Rhinoceros (Ceratotherium simum cottoni), one of two subspecies of White Rhinos (Smith 2001,
Emslie 2012). The y-axis is the total number of rhinos in the wild. c) Repeated cycling of the Canada lynx (Lynx canadensis; Campbell and Walker 1977). The y-axis is the number of lynx trapped, an index of population size. Over the course of many years, a single population can display many of these dynamics. For example, Kirtland's Warbler populations were monitored by determining the number of males defending territories in their summer breeding habitat in the Great Lakes region North America, primarily Michigan. There were about 500 males with territories in the 1950s (Figure ). The following changes occurred over the next 50 years after the species began being protected by the Endangered Species Act (Kepler et al. 1996): 1. Decline over the course of the 1960s to ~200 territories. 2. A period of stability at ~200 territories from 1975 to 1990. 3. Steady growth to >2500 from 1990 through 2020.
Figure : Number of singing Kirtland's Warbler (Setophaga kirtlandii) males, 1950 to 2020.
Models can be used to understand and predict population dynamics Researchers who study population dynamics often use mathematical models to describe and predict population dynamics and understand what factors are driving those changes. For example, if there are 2500 Kirtland's Warblers in Michigan this year, can we predict how many will be around next year, or 10 years from now? Due to its small population size the Kirtland's Warbler was listed as an Endangered Species in 1967. In 2019 it was de-listed and now is considered "Near-threatened." Ecologists are very interested in using models to predict how large the Kirtland's Warbler population will be in the future, and what factors cause it to increase and decrease (Brown et al. 2019). In the next chapter we will explore the conceptual and mathematical tools ecologists use to understand population dynamics and predict their future trajectories.
Biotic interactions and abiotic conditions limit the sizes of populations Population dynamics can be regulated in a variety of ways. These are grouped into density-dependent factors, in which the density of the population at a given time affects growth rate and mortality, and density-independent factors, which influence mortality in a population regardless of population density. Note that in the former, the effect of the factor on the population depends on the density of the population at onset. Conservation biologists want to understand both types because this helps them manage populations and prevent extinction or overpopulation.
Most density-dependent factors are biological in nature (biotic), and include predation, inter- and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites. Usually, the denser a population is, the greater its mortality rate. For example, during intra- and interspecific competition, the reproductive rates of the individuals will usually be lower, reducing their population's rate of growth. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source.
An example of density-dependent regulation is shown in Figure
with results from a study focusing on the giant intestinal
roundworm (Ascaris lumbricoides), a parasite of humans and other mammals (Croll et al. 1982). Denser populations of the parasite
exhibited lower fecundity: they contained fewer eggs. One possible explanation for this is that females would be smaller in more
dense populations (due to limited resources) and that smaller females would have fewer eggs. This hypothesis was tested and
disproved in a 2009 study which showed that female weight had no influence (Walker et al. 2009). The actual cause of the density-
dependence of fecundity in this organism is still unclear and awaiting further investigation.
: In this population of roundworms Ascaris lumbricoides, fecundity (number of eggs) decreases with population density (Croll et al. 1982).
Density-Independent Regulation and Interaction with Density-Dependent Factors Many factors, typically physical or chemical in nature (abiotic), influence the mortality of a population regardless of its density, including weather, natural disasters, and pollution. An individual deer may be killed in a forest fire regardless of how many deer happen to be in that area. Its chances of survival are the same whether the population density is high or low. The same holds true for cold winter weather. In real-life situations, population regulation is very complicated and density-dependent and independent factors can interact. A dense population that is reduced in a density-independent manner by some environmental factor(s) will be able to recover differently than a sparse population. For example, a population of deer affected by a harsh winter will recover faster if there are more deer remaining to reproduce.
Contributors and Attributions This chapter was written by N. Brouwer with text taken from the following CC-BY resources: OpenStax Biology 2e section 45.4 Population Dynamics and Regulation 9.3: Population Dynamics and Regulation is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
9.4: Scientist Spotlight - Jessie Isabelle Price In November 2021, Oxford University Press (OUP) made headlines when the company announced its 2021 Word of the Year - Vax. Given the coronavirus pandemic, words like "vax" and "vaccine" have dominated our language and everyday lives, so this comes as no surprise. In a report on the language of vaccines, OUP states that the origin of "vaccine" relates to the pioneering work of 18th century British physician Edward Jenner. Vacca is the Latin word for cow, and through his work with inoculation, "Jenner conceived the Latin expression variolae vaccinae as a name for cowpox" (OUP 2021). At first, the word "vaccine" only described inoculation against cowpox, but its usage was expanded to include multiple variations of the word in reference to immunization against other diseases. But surely, one Englishman could not be the end-all-be-all, sole champion of vaccination? Cue Jessie Isabelle Price. Born in Montrose, Pennsylvania in 1930, Price was raised by a single mother and attended predominantly white public schools. After graduating from high school and being accepted into Cornell, she deferred for a year to prepare herself academically (Gillmer 2018). Price wanted to become a physician, but could not afford additional costs that the program required. Instead, she graduated with a Bachelors of Science in microbiology in 1953 and by 1959, Price had earned a Masters and PhD in bacteriology, pathology, and parasitology. Work from her dissertation, in which she studied Pasteurella anatipestifer infection in Pekin ducklings, was published in the journal Avian Diseases. Following her graduate studies, Price remained at Cornell as a research scientist focusing on "the identification and control of bacterial diseases in commercial white Pekin Ducklings." (Gillmer 2018) Although her career stretched beyond her time at the Cornell Duck Disease Research Laboratory, some of her most impactful work originated from her time there. For example, farmers internationally were losing ducklings due to respiratory disease, and Price discovered that they were dying of duck hepatitis, Pasteurella multocida, and Escherichia coli (Warren 1999). Not only did she uncover the source of this avian mortality, Price developed two vaccines that saved the poultry industry money, but more importantly prevented potential disease outbreaks in other bird species. Despite this monumental achievement, if asked about the history of vaccines, Jenner (or Fauci) is typically the only name that comes to mind.
Figure : "Cornell University's Duck Research Laboratory" provided by Cornell University is licensed under CC0 1.0.
References "Doctor to Long Island Ducks." Ebony Magazine. (September 1964). pp. 76-82. <https://books.google.co.uk/books? id=JaT6tBKGK3sC&lpg=PA1&pg=PA76#v=onepage&q&f=false>. Accessed December 28, 2021. Gilllmer, Sophia. (2018). Jessie Isabelle Price (1930-2015). BlackPast. <https://www.blackpast.org/african-american-history/pricejessie-isabelle-1930-2015/>. Accessed November 9, 2021. Oxford University Press. (2021). Word of the Year 2021: Vax. <https://languages.oup.com/word-of-the-year/2021/>. Accessed December 28, 2021. Warren, Wini. (1999). Jessie Isabelle Price. Black Women Scientists in the United States. Indiana University Press, Bloomington, Indiana, USA, pp. 237-243.
9.4: Scientist Spotlight - Jessie Isabelle Price is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.