BIO 311D 1st Edition Lecture 34 Outline of Last Lecture I Competition II Ecological Niches and Natural Selection III Character Displacement IV Predation V Species Diversity VI Diversity and Community Stability VII Tropic Structure VIII Food Webs Outline of Current Lecture I Population Ecology II Density and Dispersion III Patterns of Dispersion IV Demographics V Survivorship Curves VI Reproductive Rates VII Per Capita Rate Increases VIII Exponential Growth Current Lecture Population ecology is the study of populations in relation to their environment including environmental influences on density and distribution age structure and population size Dynamic biological processes influence population density dispersion and demographics A population is a group of individuals of a single species living in the same general area Populations are described by their boundaries and size Density and 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 In most cases it is impractical or impossible to count all individuals in a population Sampling techniques can be used to estimate densities and total population sizes Population size can be estimated by either extrapolation from small samples an index of population size e g number of nests or the mark recapture method Mark recapture method Scientists capture tag and release a random sample of individuals s in a population Marked individuals are given time to mix back into the population Scientists capture a second sample of individuals n and note how many of them are marked x Population size N is estimated by Density is the result of an interplay between processes that add individuals to a population and those that remove individuals Immigration is the influx of new individuals from other areas Emigration is the movement of individuals out of a population Patterns of Dispersion Environmental and social factors influence the spacing of individuals in a population In a clumped dispersion individuals aggregate in patches A clumped dispersion may be influenced by resource availability and behavior A uniform dispersion is one in which individuals are evenly distributed It may be influenced by social interactions such as territoriality the defense of a bounded space against other individuals In a random dispersion the position of each individual is independent of other individuals It occurs in the absence of strong attractions or repulsions Demographics Demography is the study of the vital statistics of a population and how they change over time Death rates and birth rates are of particular interest to demographers Survivorship Curves A survivorship curve is a graphic way of representing the data in a life table The survivorship curve for Belding s ground squirrels shows a relatively constant death rate Survivorship curves can be classified into three general types Type I low death rates during early and middle life and an increase in death rates among older age groups Type II a constant death rate over the organism s life span Type III high death rates for the young and a lower death rate for survivors Many species are intermediate to these curves Reproductive Rates For species with sexual reproduction demographers often concentrate on females in a population A reproductive table or fertility schedule is an age specific summary of the reproductive rates in a population It describes the reproductive patterns of a population The exponential model describes population growth in an idealized unlimited environment It is useful to study population growth in an idealized situation Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth Per Capita Rate of Increase If immigration and emigration are ignored a population s growth rate per capita increase equals birth rate minus death rate where DN is the change in population size Dt is the time interval B is the number of births and D is the number of deaths The population growth equation can be revised Zero population growth ZPG occurs when the birth rate equals the death rate r 0 Exponential Growth Exponential population growth is population increase under idealized conditions Under these conditions the rate of increase is at its maximum denoted as rmax The equation of exponential population growth is Exponential population growth results in a J shaped curve The J shaped curve of exponential growth characterizes some rebounding populations For example the elephant population in Kruger National Park South Africa grew exponentially after hunting was banned The logistic model describes how a population grows more slowly as it nears its carrying capacity Exponential growth cannot bote sustained for long in any population A more realistic population model limits growth by incorporating carrying capacity Carrying capacity K is the maximum population size the environment can support Carrying capacity varies with the abundance of limiting resources
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