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Lecture 8

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Biology 3466B

How Do Ecologists Study Populations? • Population consists of the individuals of a species within a given area • any given time, an individual organism occupies only one spot in space- particular age and size • members are distributed over space, differ in age and size • age distribution of individuals in a pop’n and the way they are spread over the environment describe its population structure • population density: number of individuals of a pop’n per unit of area • structure of a pop’n changes continually b/c of demographic events- births, deaths, immigration, emigration • study of birth death and movement rates that create population dynamics changes in pop’n structure and density) is known as demography • ecologists determine how many individuals are found in an area and where they are located • individuals change their locations by migrating or dispersing • investigators need to be able to recognize and track individual animals • field studies of animals pop’ns require tagging or marking individuals in some way • birds are marked by coloured bands on legs; butterflies coloured spots on wings; bees-numbered tags on bodies; mammals-tags or dye on fur; plants- tags on branch or nearby ground • microchips and other forms of electronic tagging are used on organism of all sizes • molecular markers used to determine movement of individuals over long distances • one way – analyze the chemical composition of feathers that the birds molt as they migrate south-can determine where they molted by evaluating hydrogen isotopes in feathers b/c there is a strong latitudinal gradient in stable hydrogen isotopes in precipitation • Most individuals molted their feathers close to the breeding ground POPULATION DENSITIES • Ecologists usually measure the densities of organism in terrestrial environments as the number of individuals per unit of area • For species whose members differ markedly in size, such as plants and animals, the percentage of ground covered or the total mass of individuals may be more useful measures of density than the number of individuals • most accurate way to determine the density and structure of a pop’n is to count every individual and note its location • estimating pop’n densities is easiest for sedentary organisms • only need to count # of individuals in a sample of representative habitats and extrapolate the counts to entire ecosystem • counting mobile organisms is much more difficult b/c they move into and out of census areas • involves capturing, marking ,releasing-after having time to mix with the unmarked individuals, another sample is taken-proportion of individuals in new sample that is marked can be used to estimate the size of th
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