42 Populations

42.1 Populations Are Patchy in Space and Dynamic over Time Chapter 42 Populations Key Concepts 42.1 Populations Are Patchy in Space and Dynamic over Time 42.2 Births Increase and Deaths Decrease Population Size 42.3 Life Histories Determine Population Growth Rates

42.4 Populations Grow Multiplicatively, but the Multiplier Can Change Chapter 42 Populations Key Concepts 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change 42.5 Immigration and Emigration Affect Population Dynamics 42.6 Ecology Provides Tools for Conserving and Managing Populations

Chapter 42 Opening Question How does understanding the population ecology of disease vectors help us combat infectious diseases?

Concept 42.1 Populations Are Patchy in Space and Dynamic over Time Populations: groups of individuals of the same species Humans have long been interested in understanding species abundance: To increase populations of species that provide resources and food To decrease abundance of crop pests, pathogens, etc.

Concept 42.1 Populations Are Patchy in Space and Dynamic over Time Population density—number of individuals per unit of area or volume Population size—total number of individuals in a population Counting all individuals is usually not feasible; ecologists often measure density, then multiply by the area occupied by the population to get population size.

Concept 42.1 Populations Are Patchy in Space and Dynamic over Time Abundance varies on several spatial scales. Geographic range—region in which a species is found Within the range, species may be restricted to specific environments or habitats. Habitat patches are “islands” of suitable habitat separated by areas of unsuitable habitat.

Figure 42.1 Species Are Patchily Distributed on Several Spatial Scales Figure 42.1 Species Are Patchily Distributed on Several Spatial Scales (A) The geographic range of Edith’s checkerspot butterfly (Euphydryas editha) extends from British Columbia and Alberta to Baja California. (B) Within the geographic range, the butterflies occur in patches of suitable habitat which, in the San Francisco Bay area, are on serpentine soils. Arrows indicate colonization events that are discussed in Concept 42.5.

Concept 42.1 Populations Are Patchy in Space and Dynamic over Time Population densities are dynamic—they change over time. Density of one species population may be related to density of other species populations.

Figure 42.2 Population Densities Are Dynamic Figure 42.2 Population Densities Are Dynamic Densities of acorns, rodents, and nymphal black-legged ticks varied over time in an oak forest near Millbrook, New York. (Notice that the population density of each organism is measured on a spatial scale appropriate for that organism’s average density; see Appendix C for definitions of units of area.)

Concept 42.2 Births Increase and Deaths Decrease Population Size Change in population size depends on the number of births and deaths over a given length of time. “Birth–death” or BD model of population change:

Concept 42.2 Births Increase and Deaths Decrease Population Size Population growth rate (change in size over one time interval):

Concept 42.2 Births Increase and Deaths Decrease Population Size Change in population size can be measured only for very small populations that can be counted, such as zoo animals. To estimate growth rates, ecologists keep track of a sample of individuals over time.

Concept 42.2 Births Increase and Deaths Decrease Population Size Per capita birth rate (b)—number of offspring an average individual produces Per capita death rate (d)—average individual’s chance of dying Per capita growth rate (r) = (b – d) = average individual’s contribution to total population growth rate

Concept 42.2 Births Increase and Deaths Decrease Population Size If b > d, then r > 0, and the population grows. If b < d, then r < 0, and the population shrinks. If b = d, then r = 0, and population size does not change.

Concept 42.3 Life Histories Determine Population Growth Rates Demography: study of processes influencing birth, death, and population growth rates Life history: timing of key events such as growth and development, reproduction, and death during an average individual’s life Example: Life cycle of the black-legged tick

Figure 42.3 Life History of the Black-Legged Tick Figure 42.3 Life History of the Black-Legged Tick The seasonality of the ticks’ environment and the difficulty of obtaining the blood meal necessary to grow, molt, and reproduce have produced a 2-year life history that includes two long periods of winter domancy.

Concept 42.3 Life Histories Determine Population Growth Rates A life history shows the ages at which individuals make life cycle transitions and how many individuals do so successfully: Survivorship—fraction of individuals that survive from birth to different life stages or ages Fecundity—average number of offspring each individual produces at different life stages or ages

Table 42.1

Concept 42.3 Life Histories Determine Population Growth Rates Survivorship can also be expressed as mortality: the fraction of individuals that do not survive from birth to a given stage or age. Mortality = 1 – survivorship

Concept 42.3 Life Histories Determine Population Growth Rates Survivorship and fecundity affect r. The higher the fecundity rate and survivorship, the higher r will be. If reproduction shifts to earlier ages, r will increase as well.

Concept 42.3 Life Histories Determine Population Growth Rates Life histories vary among species: how many and what types of developmental stages, age of first reproduction, frequency of reproduction, how many offspring they produce, and how long they live. Life histories can vary within a species. For example, different human populations have different life expectancies and age of sexual maturity.

Concept 42.3 Life Histories Determine Population Growth Rates Individual organisms require resources (materials and energy) and physical conditions they can tolerate. The rate at which an organism can acquire a resource increases with the availability of the resource. Examples: Photosynthetic rate increases with sunlight intensity; an animal’s rate of food intake increases with the density of food

Figure 42.4 Resource Acquisition Increases with Resource Availability—Up to a Point Figure 42.4 Resource Acquisition Increases with Resource Availability—Up to a Point (A) A plant’s photosynthetic rate increases with light intensity. (B) Kangaroo rats (Dipodomys spp.) are able to harvest seeds faster as seed density increases. (Note the cheek pouch bulging with seeds.) Error bars indicate 95 percent confidence intervals around the mean. See Appendix B for discussion of statistical concepts.

Concept 42.3 Life Histories Determine Population Growth Rates Principle of allocation Once an organism has acquired a unit of some resource, it can be used for only one function at a time, such as maintenance, growth, defense, or reproduction. In stressful conditions, more resources go to maintaining homeostasis. Once an organism has more resources than it needs for maintenance, it can allocate the excess to other functions.

Figure 42.5 The Principle of Allocation Figure 42.5 The Principle of Allocation Organisms use resources (energy and time) to acquire more resources. The organism must then allocate acquired resources among competing life functions. In general, an organism’s first priority is to maintain the integrity of its body. This figure assumes that the same amount of energy or time is allocated to resource acquisition in all three conditions.

Concept 42.3 Life Histories Determine Population Growth Rates In general, as average individuals in a population acquire more resources, the average fecundity, survivorship, and per capita growth rate increase.

Concept 42.3 Life Histories Determine Population Growth Rates Life-history tradeoffs—negative relationships among growth, reproduction, and survival Example: A species that invests heavily in growth early in life cannot simultaneously invest heavily in defense. Environment is also a factor: if high mortality rates are likely, it makes sense to invest in early reproduction.

Concept 42.3 Life Histories Determine Population Growth Rates Species’ distributions reflect the effects of environment on per capita growth rates. A study of temperature change in a lizard’s environment, combined with knowledge of its physiology and behavior, led to conclusions about how climate change may affect survivorship, fecundity, and distribution of these lizards.

Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 1) Figure 42.6 Climate Warming Stresses Spiny Lizards Barry Sinervo and colleagues wanted to know whether climate warming will stress Mexican blue spiny lizards (Sceloporus serrifer) by reducing the number of hours they can remain outside their burrows without overheating. These lizards feed only during daytime and retreat to their cool burrows when their body temperature gets too high (see Figure 29.8). In 2008, the researchers conducted experiments in four locations in Mexico where these lizards had been found in 1975. Climate records indicated that the four locations did not warm at the same rate between 1975 and 2008, perhaps because they differ in such things as proximity to the ocean, elevation above sea level, and latitude.a [aB. Sinervo et al. 2010. Science 328: 895–899.]

Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 2) Figure 42.6 Climate Warming Stresses Spiny Lizards Barry Sinervo and colleagues wanted to know whether climate warming will stress Mexican blue spiny lizards (Sceloporus serrifer) by reducing the number of hours they can remain outside their burrows without overheating. These lizards feed only during daytime and retreat to their cool burrows when their body temperature gets too high (see Figure 29.8). In 2008, the researchers conducted experiments in four locations in Mexico where these lizards had been found in 1975. Climate records indicated that the four locations did not warm at the same rate between 1975 and 2008, perhaps because they differ in such things as proximity to the ocean, elevation above sea level, and latitude.a [aB. Sinervo et al. 2010. Science 328: 895–899.]

Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 3) Figure 42.6 Climate Warming Stresses Spiny Lizards Barry Sinervo and colleagues wanted to know whether climate warming will stress Mexican blue spiny lizards (Sceloporus serrifer) by reducing the number of hours they can remain outside their burrows without overheating. These lizards feed only during daytime and retreat to their cool burrows when their body temperature gets too high (see Figure 29.8). In 2008, the researchers conducted experiments in four locations in Mexico where these lizards had been found in 1975. Climate records indicated that the four locations did not warm at the same rate between 1975 and 2008, perhaps because they differ in such things as proximity to the ocean, elevation above sea level, and latitude.a [aB. Sinervo et al. 2010. Science 328: 895–899.]

Concept 42.3 Life Histories Determine Population Growth Rates Laboratory experiments have also shown the links between environmental conditions, life histories, and species distributions. Example: Quantifying life history traits of two species of grain beetles in different temperature and humidity conditions explained distributions of these species in Australia.

Figure 42.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions Figure 42.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions Charles Birch’s laboratory research revealed that R. dominica populations performed better under warmer conditions than did S. oryzae populations. The lines marked r = 0 indicate the limit of the conditions under which populations of each species can grow; within the hatched areas, per capita growth rates are positive for the species.

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change Population growth is multiplicative—an ever- larger number of individuals is added in each successive time period. In additive growth, a constant number (rather than a constant multiple) is added in each time period.

In-Text Art, Chapter 42, p. 873 (2)

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change Charles Darwin was aware of the power of multiplicative growth: “As more individuals are produced than can possibly survive, there must in every case be a struggle for existence.” This ecological struggle for existence, fueled by multiplicative growth, drives natural selection and adaptation.

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change Multiplicative growth with a constant r has a constant doubling time. The time it takes a population to double in size can be calculated if r is known.

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change Populations do not grow multiplicatively for very long. Growth slows and reaches a more or less steady size:

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change r decreases as the population becomes more crowded; r is density dependent. As the population grows and becomes more crowded, birth rates tend to decrease and death rates tend to increase. When r = 0, the population size stops changing—it reaches an equilibrium size called carrying capacity, or K.

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change K can be thought of as the number of individuals that a given environment can support indefinitely. When population density reaches K, an average individual has just the amount of resources it needs to exactly replace itself. When density <K, an average individual can more than replace itself; when density >K, the average individual has fewer resources than it needs to replace itself.

Figure 42.8 Per Capita Growth Rate Decreases with Population Density Figure 42.8 Per Capita Growth Rate Decreases with Population Density (A) The decline in r as population density increases, as measured in a population of a small crustacean, Daphnia pulex (see Figure 42.10). (B) Schematic representation of density-dependent growth, in which r declines linearly with population density. This graph is an idealization—few species follow it exactly. For example, close inspection of (A) suggests that the decline in r for Daphnia pulex is not precisely linear.

Example: the rodents and ticks in Millbrook, New York Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change Spatial variation in environmental factors can result in variation of carrying capacity. Temporal variation in environmental conditions may cause the population to fluctuate above and below the current carrying capacity. Example: the rodents and ticks in Millbrook, New York

Figure 42.2 Population Densities Are Dynamic Figure 42.2 Population Densities Are Dynamic Densities of acorns, rodents, and nymphal black-legged ticks varied over time in an oak forest near Millbrook, New York. (Notice that the population density of each organism is measured on a spatial scale appropriate for that organism’s average density; see Appendix C for definitions of units of area.)

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change Environmental changes affected fecundity of the Galápagos cactus ground finches: When females were 7 and 8 years old, they produced no surviving young, and survivorship dropped. Low food availability during these years resulted from a severe drought in 1985. When the females were 5, a wet year produced abundant food and high fecundity.

Table 42.1

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change The human population is unique. It has grown at an ever-faster per capita rate, as indicated by steadily decreasing doubling times. Technological advances have raised carrying capacity by increasing food production and improving health.

Figure 42.9 Human Population Growth Figure 42.9 Human Population Growth Advances in technology have allowed the human population to grow at an ever-faster per capita rate.

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change In 1798 Thomas Malthus pointed out that the human population was growing multiplicatively, but food supply was growing additively, and predicted that food shortages would limit human population growth. His essay provided Charles Darwin with a critical insight for the mechanism of natural selection. Malthus could not have predicted the effects of technology such as medical advances and the Green Revolution.

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change Many believe that the human population has now overshot its carrying capacity for two reasons: Technological advances and agriculture have depended on fossil fuels—a finite resource. Climate change and ecosystem degradation have been a consequence of 20th century population expansion.

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change If the human population has indeed exceeded carrying capacity, ultimately it will decrease. We can bring this about voluntarily if we continue to reduce per capita birth rate.

Concept 42.5 Immigration and Emigration Affect Population Dynamics Many species occupy habitat patches separated from other patches by unsuitable environments. Each patch is occupied by a subpopulation, the set of subpopulations in a region is a metapopulation. Individuals may move in or out of subpopulations.

Figure 42.10 A Metapopulation Has Many Subpopulations Figure 42.10 A Metapopulation Has Many Subpopulations The coast of Scandinavia is characterized by natural freshwater rock pools, which constitute suitable habitat for small crustaceans of the genus Daphnia that feed by filtering algae from the water. Each rock pool that is occupied by Daphnia constitutes a subpopulation within a larger metapopulation.

Concept 42.5 Immigration and Emigration Affect Population Dynamics The BIDE model of population growth adds the number of immigrants (I) and emigrants (E) to the BD growth model.

Concept 42.5 Immigration and Emigration Affect Population Dynamics In the BD model, populations are considered closed systems—no immigration or emigration. In the BIDE model, subpopulations are considered open systems—individuals can move among them.

Concept 42.5 Immigration and Emigration Affect Population Dynamics Small subpopulations in habitat patches are vulnerable to environmental disturbances and chance events and may go extinct. If dispersal is possible, individuals from other subpopulations can recolonize the patch and “rescue” the subpopulation from extinction.

Concept 42.5 Immigration and Emigration Affect Population Dynamics Immigrants also contribute to genetic diversity within subpopulations. This gene flow combats the genetic drift that can occur in a small population that reduces a species’ evolutionary potential.

Concept 42.5 Immigration and Emigration Affect Population Dynamics In the metapopulation of Edith’s checkerspot butterfly, all but the largest subpopulation went extinct during a severe drought between 1975 and 1977. In 1986, nine habitat patches were recolonized from the Morgan Hill subpopulation. Patches closest to Morgan Hill were most likely to be recolonized because adult butterflies do not fly very far.

Figure 42.1 Species Are Patchily Distributed on Several Spatial Scales Figure 42.1 Species Are Patchily Distributed on Several Spatial Scales (A) The geographic range of Edith’s checkerspot butterfly (Euphydryas editha) extends from British Columbia and Alberta to Baja California. (B) Within the geographic range, the butterflies occur in patches of suitable habitat which, in the San Francisco Bay area, are on serpentine soils. Arrows indicate colonization events that are discussed in Concept 42.5.

Conserving endangered species Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations Understanding life history strategies can be useful in managing other species. Conserving endangered species Larvae of the endangered Edith’s checkerspot butterfly feed on two plant species found only on serpentine soils. The two plant species are being suppressed by invasive non-native grasses. Grazing by cattle can control the invasive grasses.

Black rockfish grow throughout their life. Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations Fisheries Black rockfish grow throughout their life. Older, larger females produce more eggs, and the eggs have larger oil droplets, which give the larvae a head start on growth. Because fishermen prefer to catch big fish, intense fishing reduced the average age of female rockfish from 9.5 to 6.5 years.

Population density rapidly declined. Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations These younger females were smaller, produced fewer eggs, and the larvae did not survive as well. Population density rapidly declined. Management may require no-fishing zones where some females can mature and reproduce.

Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations Reducing disease risk The black-legged tick’s life history indicates that success of larvae in getting a blood meal has greatest impact on the abundance of nymphs. Thus, controlling the abundance of rodents that are hosts for the larvae is more effective in reducing tick populations than controlling the abundance of deer, the hosts for adults.

Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations Conservation plans begin with inventories of habitat and potential risks to the habitat. Largest patches can potentially have the largest populations and genetic diversity and are given priority. Quality of the patches is evaluated; ways to restore or maintain quality are developed. Ability of the organism to disperse between patches is evaluated.

Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction (Part 1) Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction Data from the experiments summarized here suggest that corridors between patches of habitat for small arthropod species increase the chances of dispersal, and thus of subpopulation persistence.a [aA. Gonzalez and E. J. Chaneton. 2002. Journal of Animal Ecology 71: 594–602.]

Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction (Part 2) Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction Data from the experiments summarized here suggest that corridors between patches of habitat for small arthropod species increase the chances of dispersal, and thus of subpopulation persistence.a [aA. Gonzalez and E. J. Chaneton. 2002. Journal of Animal Ecology 71: 594–602.]

Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction (Part 3) Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction Data from the experiments summarized here suggest that corridors between patches of habitat for small arthropod species increase the chances of dispersal, and thus of subpopulation persistence.a [aA. Gonzalez and E. J. Chaneton. 2002. Journal of Animal Ecology 71: 594–602.]

Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations For some species, a continuous corridor of habitat is needed to connect subpopulations and allow dispersal. Dispersal corridors can be created by maintaining vegetation along roadsides, fence lines, or streams, or building bridges or underpasses that allow individuals to avoid roads or other barriers.

Figure 42.12 A Corridor for Large Mammals Figure 42.12 A Corridor for Large Mammals An overpass above the Trans-Canada Highway in Banff National Park, Alberta. Bears, elk, deer, and other large mammals use such overpasses as corridors to move between areas that would otherwise be effectively isolated from one another by the highway with its speeding traffic.

Answer to Opening Question By understanding the factors that control abundance and distribution of pathogens and their vectors, we can devise ways to control their abundance or avoid contact. Black-legged ticks are vectors for the bacterium that causes Lyme disease. For these ticks, abundance of hosts for larvae (rodents) determines tick abundance.

Answer to Opening Question Rodent abundance depends on acorn availability. Acorn production can be used to predict areas that are likely to become infested with ticks, and measures can be taken to minimize human contact.