BIO 311D 1st Edition Lecture 34Outline of Last Lecture I. CompetitionII. Ecological Niches and Natural SelectionIII. Character DisplacementIV. PredationV. Species DiversityVI. Diversity and Community StabilityVII. Tropic StructureVIII. Food WebsOutline of Current Lecture I. Population EcologyII. Density and DispersionIII. Patterns of DispersionIV. DemographicsV. Survivorship CurvesVI. Reproductive RatesVII. Per Capita Rate IncreasesVIII. Exponential GrowthCurrent Lecture• Population ecology is the study of populations in relation to their environment, including environmental influences on density and distribution, age structure, and population sizeDynamic 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 sizeDensity 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 populationDensity: 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 populationPatterns 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 repulsionsDemographics• 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 demographersSurvivorship 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 amongolder 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 curvesReproductive 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 populationThe 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 growthPer 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 bannedThe 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
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