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Population Ecology What is a population? • All the members of a species living in the same place at the same time. • Organisms usually breed with members of their own population. How to describe populations • Density: number of individuals per unit area, or volume. Ex. Number of deer/km2. • Dispersion: The distribution or arrangement of individuals in space. • Dispersion may be clumped, even or random. Growth Rate • A change in the size of a population over a given period of time = growth rate. Growth rate = births – deaths • Populations usually stay about the same size from year to year as factors kill many individuals before they can reproduce. How fast can a population grow? • Reproductive potential: The maximum number of offspring that each member of a population can produce. • Some species have much higher reproductive potential than others. How many bunnies? • • • • Rabbits can begin reproducing at 4 - 6 months Gestation is 31 days Litters can be up to 14 Females can become pregnant within hours of giving birth • Lifespan is about 7 years • 95 billion rabbits in 7 years! Exponential growth • Occurs only when there is enough: Food Space Little or no competition No predators • Populations rarely grow at their reproductive potential Carrying Capacity • Carrying capacity is: the maximum population that an ecosystem can support indefinitely. • Ecosystems change so carrying capacity is difficult to predict or calculate precisely. Resource Limits • A species reaches its carrying capacity when it consumes a natural resource at the same rate at which the ecosystem produces the resource. • This resource is called the limiting resource. • Examples include: light, nutrients, water. Density Dependent Population Regulation • Deaths occur more quickly in a crowded population. • Limited resources, predation and disease result in higher death rates in dense populations. • Ex. Emerald Ash Borer possibly going to invade Iowa Density independent • A certain proportion of a population dies regardless of population density. Ex. Severe weather