Chapter 52 Population Ecology Overview 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 general area environmental factors and have a high likelihood of interacting with and breeding with one another among individuals and changing the frequencies of various traits over time Populations can evolve through natural selection acting on heritable variations Members of a population rely on the same resources are influenced by similar A population is a group of individuals of a single species that live in the same 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 the average 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 Individuals are trapped and captured marked with a tag recorded and then released recapture method 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 The mark recapture method assumes that each marked individual has the same probability of being From these data researchers estimate the total number of individuals in the population This may not be a safe assumption as trapped individuals may be more or less likely to be trapped a trapped as each unmarked individual second time 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 Density is not a static property of a population but is rather the result of the influx of new individuals from other areas movement of individuals out of a population The factors that remove individuals from a population are death mortality and emigration the 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 Some habitat patches are more suitable that others Social interactions between members of a population may maintain patterns of spacing environmental associations and social interactions of individuals within a population Environmental differences even at a local level contribute to variation in population density aggregated in patches Plants and fungi are often clumped where soil conditions favor germination and growth The most common pattern of dispersion is clumped with the individuals 1 Many animals spend much of their time in a particular microenvironment that satisfies their 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 requirements interactions between individuals in the population competitors For example some plants secrete chemicals that inhibit the germination and growth of nearby 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 change over time specifically females and death rates Demography Life Tables Demography is the study of the vital statistics of populations and how they Of particular interest are birth rates and how they vary among individuals 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 individuals of the same age from birth throughout their lifetimes until all are dead each age group and calculate the proportion of the cohort surviving from one age to the next available for only a limited number of species Cohort life tables are difficult to construct for wild animals and plants and are To build a life table we need to determine the number of individuals that die in The best way to construct a life table is to follow the fate of a cohort a group of Survivorship 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 drops steeply as death rates increase among older age groups Humans and many other large mammals exhibit Type I survivorship curves The Type II curve is intermediate with constant mortality over an organism s life span Many species of