Population Ecology

1.         Population ecology is the study of populations in relation to the environment, including environmental influences on population density and distribution, age structure, and population size.

            a.         A population is a group of individuals of a single species that live in the same general area.

            b.         Members of a population rely on the same resources, are influenced by similar environmental factors, and have a high likelihood of interacting with and breeding with one another.

            c.         Populations can evolve through natural selection acting on heritable variations among individuals and changing the frequencies of various traits over time.

            d.         Two important characteristics of any population are density and the spacing of individuals. Every population has a specific size and specific geographical boundaries.

                        i.         The density of a population is measured as the number of individuals per unit area or volume.

                        ii.        The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries.

2.         Measuring density of populations is a difficult task.

            a.         It is almost always impractical to count all individuals in a population. Instead, ecologists use a variety of sampling techniques to estimate densities and total population sizes.

                        i.         For example, they might count the number of individuals in a series of randomly located plots, calculate the average density in the samples, and extrapolate to estimate the population size in the entire area.

                        ii.        A sampling technique that researchers commonly use to estimate wildlife populations is the mark-recapture method.

                                    (1)       Individuals are trapped and captured, marked with a tag, recorded, and then released.

                                    (2)       After a period of time has elapsed, traps are set again, and individuals are captured and identified.

                                    (3)       The second capture yields both marked and unmarked individuals.

                                    (4)       From these data, researchers estimate the total number of individuals in the population.

                                    (5)       The mark-recapture method assumes that each marked individual has the same probability of being trapped as each unmarked individual.

                                    (6)       This may not be a safe assumption, as trapped individuals may be more or less likely to be trapped a second time.

3.         Density results from dynamic interplay between processes that add individuals to a population and those that remove individuals from it.

            a.         Additions to a population occur through birth (including all forms of reproduction) and immigration (the influx of new individuals from other areas).

            b.         The factors that remove individuals from a population are death (mortality) and emigration (the movement of individuals out of a population).

4.         Within a population’s geographic range, local densities may vary substantially

            a.         Some habitat patches are more suitable that others.

            b.         Social interactions between members of a population may maintain patterns of spacing.

                        i.         Dispersion is clumped when individuals aggregate in patches.

                                    (1)       Plants and fungi are often clumped where soil conditions favor germination and growth.

                                    (2)       Animals may clump in favorable microenvironments (such as isopods under a fallen log) or to facilitate mating interactions.

                                    (3)       Group living may increase the effectiveness of certain predators, such as a wolf pack.

                        ii.        Dispersion is uniform when individuals are evenly spaced.

                                    (1)       For example, some plants secrete chemicals that inhibit the germination and growth of nearby competitors.

                                    (2)       Animals often exhibit uniform dispersion as a result of territoriality, the defense of a bounded space against encroachment by others.

                        iii.       In random dispersion, the position of each individual is independent of the others, and spacing is unpredictable.

                                    (1)       Random dispersion occurs in the absence of strong attraction or repulsion among individuals in a population, or when key physical or chemical factors are relatively homogeneously distributed.

                                    (2)       For example, plants may grow where windblown seeds land.

5.         Demography is the study of factors that affect population density and dispersion patterns and how they change over time.

            a.         A life table is an age-specific summary of the survival pattern of a population.

                        i.         The best way to construct a life table is to follow the fate of a cohort, a group of individuals of the same age, from birth throughout their lifetimes until all are dead.

                        ii.        To build a life table, we need to determine the number of individuals that die in each age group and calculate the proportion of the cohort surviving from one age to the next.

            b.         A graphic way of representing the data in a life table is a survivorship curve.

                        i.         This is a plot of the numbers or proportion of individuals in a cohort of 1,000 individuals still alive at each age. There are several patterns of survivorship exhibited by natural populations.

                                    (1)       A Type I curve is relatively flat at the start, reflecting a low death rate in early and middle life, and drops steeply as death rates increase among older age groups.

                                                (a)       Humans and many other large mammals exhibit Type I survivorship curves.

                                    (2)       The Type II curve is intermediate, with constant mortality over an organism’s life span.

                                                (a)       Many species of rodent, various invertebrates, and some annual plants show Type II survivorship curves.

                                    (3)       A Type III curve drops slowly at the start, reflecting very high death rates early in life, then flattens out as death rates decline for the few individuals that survive to a critical age.

                                                (a)       Type III survivorship curves are associated with organisms that produce large numbers of offspring but provide little or no parental care.

                                                (b)       Examples are many fishes, long-lived plants, and marine invertebrates.

                                    (4)       Many species fall somewhere between these basic types of survivorship curves or show more complex curves.

            c.         A reproductive table is an age-specific summary of the reproductive rates in a population.

                        i.         The best way to construct a reproductive table is to measure the reproductive output of a cohort from birth until death.

                        ii.        For sexual species, the table tallies the number of female offspring produced by each age group.

                        iii.       Reproductive output for sexual species is the product of the proportion of females of a given age that are breeding and the number of female offspring of those breeding females.

6.         The traits that affect an organism’s schedule of reproduction and survival make up its life history. Life histories entail three basic variables: when reproduction begins, how often the organism reproduces, and how many offspring are produced during each reproductive episode.

            a.         Semelparity (or big-bang reproduction) is when an individual produces a large number of offspring and then dies.

                        i.         When the survival of offspring is low, as in highly variable or unpredictable environments, big-bang reproduction (semelparity) is favored.

            b.         Iteroparity is the production of a few offspring during repeated reproductive episodes.

                        i.         Repeated reproduction (iteroparity) is favored in dependable environments where competition for resources is intense. In these cases, a few, well-provisioned offspring have a better chance of surviving to reproductive age.

            c.         When resources are limited, organisms have to compromise between investment in reproduction and survival.

            d.         Selective pressures also influence the trade-off between number and size of offspring.

                        i.         Plants and animals whose young are subject to high mortality rates often produce large numbers of relatively small offspring.

                                    (1)       Plants that colonize disturbed environments usually produce many small seeds, only a few of which reach suitable habitat.

                        ii.        In other organisms, extra investment on the part of the parent greatly increases the offspring’s chances of survival.

                                    (1)       Oak, walnut, and coconut trees all have large seeds with a large store of energy and nutrients to help the seedlings become established.

                                    (2)       In animals, parental care does not always end after incubation or gestation.

7.         The exponential model of population describes population growth in an idealized, unlimited environment.

            a.         All populations have a tremendous capacity for growth, however, unlimited population increase does not occur indefinitely for any species, either in the laboratory or in nature.

            b.         Imagine a hypothetical population living in an ideal, unlimited environment.

            c.         To simplify, let’s ignore immigration and emigration and define a change in population size during a fixed time interval based on the equation:

Change in population size      =         Births                          -          Deaths

during time interval                            during time interval                during time interval

or, in mathematical notation:

ΔN = B - D                  where  N represents population size

Δt                                             t represents time

B is the number of births

D is the number of deaths

            d.         The per capita birth rate is the number of offspring produced per unit time by an average member of the population.

                        i.         e.g., If there are 34 births per year in a population of 1,000 individuals, the annual per capita birth rate is 34/1000, or 0.034.

                        ii.        If we know the annual per capita birth rate (expressed as b), we can use the formula B = bN to calculate the expected number of births per year in a population of any size.

            e.         Similarly, the per capita death rate (symbolized by m for mortality) allows us to calculate the expected number of deaths per unit time for a population of any size.

            f.         Now we will revise the population growth equation, using per capita birth and death rates: DN/Dt = bN - mN

            g.         Population ecologists are most interested in the differences between the per capita birth rate and the per capita death rate, or per capita rate of increase (r) which equals b - m.

                        i.         The value of r indicates whether a population is growing (r > 0) or declining (r < 0).

                        ii.        If r = 0, then there is zero population growth (ZPG). Births and deaths still occur, but they balance exactly.

            h.         Using the per capita rate of increase, we rewrite the equation for change in population size as: DN/Dt = rN

            i.         Population growth under ideal conditions is called exponential population growth.

                        i.         Under these conditions, we may assume the maximum growth rate for the population (r_max), called the intrinsic rate of increase.

                        ii.        The equation for exponential population growth is: DN/Dt = r_maxN

            j.         The size of a population that is growing exponentially increases at a constant rate, resulting in a J-shaped growth curve when the population size is plotted over time.

            k.         J-shaped curves are characteristic of populations that are introduced into a new or unfilled environment or whose numbers have been drastically reduced by a catastrophic event and are rebounding.

8.         The logistic model of population growth incorporates the concept of carrying capacity.

            a.         Typically, resources are limited. As population density increases, each individual has access to an increasingly smaller share of available resources.

            b.         Ultimately, there is a limit to the number of individuals that can occupy a habitat. Ecologists define carrying capacity (K) as the maximum stable population size that a particular environment can support.

                        i.         Carrying capacity is not fixed but varies with the abundance of limiting resources.

                        ii.        Energy limitation often determines carrying capacity, although other factors, such as shelters, refuges from predators, soil nutrients, water, and suitable nesting sites can be limiting.

                        iii.       If individuals cannot obtain sufficient resources to reproduce, the per capita birth rate b will decline.

                        iv.       If they cannot find and consume enough energy to maintain themselves, the per capita death rate m may increase.

                        v.         A decrease in b or an increase in m results in a lower per capita rate of increase r.

            c.         In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached.

                        i.         If the maximum sustainable population size (carrying capacity) is K, then K - N is the number of additional individuals the environment can accommodate and (K - N)/K is the fraction of K that is still available for population growth.

                        ii.        By multiplying the intrinsic rate of increase r_max by (K - N)/K, we modify the growth rate of the population as N increases. DN/Dt = r_maxN((K - N)/K

                                    (1)       When N is small compared to K, the term (K - N)/K is large and the per capita rate of increase is close to the intrinsic rate of increase.

                                    (2)       When N is large and approaches K, resources are limiting.

                                    (3)       In this case, the term (K - N)/K is small and so is the rate of population growth.

                                    (4)       Population growth is greatest when the population is approximately half of the carrying capacity.

                                    (5)       At this population size, there are many reproducing individuals, and the per capita rate of increase remains relatively high.

            d.         The logistic model of population growth produces a sigmoid (S-shaped) growth curve when N is plotted over time.

                        i.         New individuals are added to the population most rapidly at intermediate population sizes, when there is not only a breeding population of substantial size, but also lots of available space and other resources in the population.

                        ii.        Population growth rate slows dramatically as N approaches K.

            e.         The logistic model predicts different per capita growth rates for populations of low or high density relative to carrying capacity of the environment.

                        i.         At high population density, selection favors adaptations that enable organisms to survive and reproduce with few resources.

                                    (1)       Competitive ability and efficient use of resources should be favored in populations that are at or near their carrying capacity.

                                    (2)       These are traits associated with iteroparity.

                        ii.        At low population density, adaptations that promote rapid reproduction, such as the production of numerous, small offspring, should be favored.

                                    (1)       These are traits associated with semelparity.

                        iii.       Ecologists have attempted to connect these differences in favored traits at different population densities with the logistic model of population growth.

                                    (1)       Selection for life history traits that are sensitive to population density is known as K-selection, or density-dependent selection.

                                                (a)       K-selection tends to maximize population size and operates in populations living at a density near K.

                                    (2)       Selection for life history traits that maximize reproductive success at low densities is known as r-selection, or density-independent selection.

                                                (a)       r-selection tends to maximize r, the rate of increase, and occurs in environments in which population densities fluctuate well below K, or when individuals face little competition.

9.         Population-Limiting Factors

            a.         Density-dependent factors have an increased effect on a population as population density increases. This is a type of negative feedback.

            b.         Density-independent factors are unrelated to population density.

            c.         A variety of factors can cause negative feedback, preventing unlimited population growth.

                        i.         Resource limitation in crowded populations can reduce population growth by reducing reproductive output.

                        ii.        In animal populations, territoriality may limit density.

                        iii.       Population density can also influence the health and thus the survival of organisms.

                                    (1)       As crowding increases, the transmission rate of a disease may increase.

                        iv.       Predation may be an important cause of density-dependent mortality for a prey species if a predator encounters and captures more food as the population density of the prey increases.

                                    (1)       As a prey population builds up, predators may feed preferentially on that species, consuming a higher percentage of individuals.

                        v.         The accumulation of toxic wastes can contribute to density-dependent regulation of population size.

                        vi.       For some animal species, intrinsic factors appear to regulate population size.

                                    (1)       White-footed mice individuals become more aggressive as population size increases, even when food and shelter are provided in abundance.