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Biology 4218A

How Do Ecologists Study Populations? • A population consists of the individuals of species within a given area. • The members of a population are distributed over space, and they differ in age and size. • The age distribution of individuals are spread over the environment describe its population structure. • The number of individuals of a population per unit of area is its population density. • The structure of a population changes continually because demographic events are common occurrences. Population densities can be estimated from samples • Estimating population densities is easiest for sedentary organisms. • Counting mobile organisms is much more difficult because individuals move into and out of census area. (write equation here). Birth and death rates can be estimated from population density data • (Write equation here). • Individuals born at the same time cohort. • The number of a population still alive at a given period of time in the future  survivorship. How Do Ecological Conditions Affect Life Histories? • An organism’s life history describes how it allocates its time and energy among the various activities that occupy its life. • Ecological interactions influence the evolution of life histories. What factors Influence Population Densities? All populations have the potential for exponential growth • All populations have the potential for explosive growth. As the number of individuals in a population increase, the number of new individuals added per unit of time accelerates (even if the rate of increase is expressed on a per individual basis remains constant). • If birth and deaths occur continuously and at constant rates, a graph of the population size over time forms a continuous upward curve  exponential growth. • (Write equation here). • The difference between the average per capita birth rate and the average per capita death rate of a population produces the net productive rate of a population. • The highest possible value for the net productive rate is called r max, or the intrinsic rate of increase, expressed: (write equation here). • For very short periods, some populations may grow at rates close to the intrinsic rate of increase. Population growth is limited by resources and biotic interactions • No real population can maintain exponential growth for very long. As a population increases in density, environmental limits cause birth rates to drop and death rates to rise. • An environment can support no more than a certain number of individuals of any particular species per unit of area  environmental carrying capacity, K. It is determined by the availability of resources as well as by diseases, predators and sometimes social interactions. • Growth of a population typically slows down as its density approaches the environmental carry capacity b/c resource limitations and the activities of predators and pathogens lower birth rates and increase death rates  logistic growth. Population densities influence birth and death rates • Because each additional individual typically makes things worse for other members of the population in an environment with limited resources, per capita birth and death rates usually change together with changes in population density  density dependent. • Birth rates and death rates may be density dependent for several reasons: o As a species increases in abundance, it may deplete its food supply. o Predators may be attracted to areas with high densities of their prey. o Diseases can spread more easily. • Factors that change per capita birth and death rates in a population independently of its density are said to be density independent (e.g. drought). • All populations fluctuate less than the theoretical maximum, but the sizes of some populations fluctuate remarkably little. • Stable populations are seen in species with long lived individuals that have low reproductive rates. • Small, short lived individuals are generally more vulnerable to envi
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