Chapter 52: Population EcologyOverview: Earth’s Fluctuating Populations- No population, including humans, can continue to grow indefinitely. - Population ecology is the study of populations in relation to the environment, including environmental influences on population density and distribution, age structure, and variations in population size.Concept 52.1: Dynamic biological processes influence population density, dispersion, and demography- A population is a group of individuals of a single species that live in the same general area.- 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.- Populations can evolve through natural selection acting on heritable variations among individuals and changing the frequencies of various traits over time.Density and Dispersion- Every population has a specific size and specific geographical boundaries.- Once defined, the population can be described in terms of its density and its dispersion. Density is the number of individuals per unit area or volume Dispersion is the pattern of spacing among individuals within the boundaries of the population.Density: A Dynamic Perspective- Measuring density of populations is a difficult task. We can count individuals, but we usually estimate population numbers. 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.◦ For example, they might count the number of individuals in a series of randomly located plots, calculate theaverage density in the samples, and extrapolate to estimate the population size in the entire area. Such estimates are accurate when there are many sample plots and a homogeneous habitat. A sampling technique that researchers commonly use to estimate wildlife populations is the mark-recapture method.◦ Individuals are trapped and captured, marked with a tag, recorded, and then released.◦ After a period of time has elapsed, traps are set again, and individuals are captured and identified.◦ The second capture yields both marked and unmarked individuals.◦ From these data, researchers estimate the total number of individuals in the population.◦ The mark-recapture method assumes that each marked individual has the same probability of being trapped as each unmarked individual.◦ This may not be a safe assumption, as trapped individuals may be more or less likely to be trapped a second time.- Density is not a static property of a population, but is rather the result of dynamic interplay between processes that add individuals to a population and those that remove individuals from it. Additions to a population occur through birth (including all forms of reproduction) and immigration (the influx of new individuals from other areas). The factors that remove individuals from a population are death (mortality) and emigration (the movement of individuals out of a population). Immigration and emigration may represent biologically significant exchanges between populations.Patterns of Dispersion- Within a population’s geographic range, local densities may vary substantially. Variations in local density are important population characteristics, providing insight into the environmental associations and social interactions of individuals within a population.- Environmental differences-even at a local level-contribute to variation in population density ◦ Some habitat patches are more suitable that others.◦ Social interactions between members of a population may maintain patterns of spacing.- The most common pattern of dispersion is clumped, with the individuals aggregated in patches. Plants and fungi are often clumped where soil conditions favor germination and growth.1 Many animals spend much of their time in a particular microenvironment that satisfies their requirements. Clumping of animals may also be associated with mating behavior Group living may increase the effectiveness of certain predators, such as a wolf pack.- A uniform, or evenly spaced, pattern of dispersion may result from direct interactions between individuals in the population. For example, some plants secrete chemicals that inhibit the germination and growth of nearby competitors. Animals often exhibit uniform dispersion as a result of antagonistic social interactions, such as territoriality, the defense of a bounded space against encroachment by others.- Uniform patterns are not as common in populations as clumped patterns- Random dispersion (unpredictable spacing) occurs in the absence of strong attractions or repulsions among individuals of a population or where key physical or chemical factors are relatively homogeneous across the study area The position of each individual is independent of other individuals For example, plants may grow where windblown seeds land. Random patterns are not common in nature; most populations show at least a tendency toward a clumped distribution Demography- Demography is the study of the vital statistics of populations and how they change over time.- Of particular interest are birth rates and how they vary among individuals (specifically females), and death rates.Life Tables- A life table is an age-specific summary of the survival pattern of a population.- Population ecologists adapted this approach to the study of nonhuman populations and developed quantities demography as a branch of population ecology- The best way to construct a life table is to follow the fate of a cohort, a group ofindividuals of the same age, from birth throughout their lifetimes until all are dead.- 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.- Cohort life tables are difficult to construct for wild animals and plants and are available for only a limited number of speciesSurvivorship Curves- A graphic way of representing the data in a life table is a survivorship curve, a plot of the numbers or proportion of individuals in a cohort still alive at each age. (1000 individuals, generally) - Survivor curves can be classified into three general types: A Type I curve is relatively flat at the start, reflecting a low death rate in early and middle life, and dropssteeply as death