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Biomedical Engineering
BMEN 515
William Huddleston

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 the pop’n using the formula m /2 =2 /1 n 1# of marked in first sample n =total # of individuals in 2 sample 2 m 2# of marked in second sample N= estimated size of total pop’n  estimates of total pop’n size will be inaccurate if marked individuals learn to avoid traps or leave study area BIRTH AND DEATH RATES  ecologists use estimates of pop’n densities to estimate the rate at which births, deaths and movements take place in a pop’n and study how rates are influences by environmental factors  the number of individuals in a pop’n at given time is equal to the # present at some time in past plus the # born between then and now, minus the # tht died, plus the # that immigrated into teh pop’n, minus th
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