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BMEN 515 Lecture Notes - Guppy

Biomedical Engineering
Course Code
BMEN 515
William Huddleston

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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
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
m2/n2=n1/N n1=# of marked in first sample
n2=total # of individuals in 2nd sample
m2=# 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
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 the # that emigrated
life table-created by tracking a group of individuals born at the same time (cohort) and
determining the # that are still alive at later dates (survivorship)
used to predict future trends in pop’n
study of the seed-eating cactus finch-210 birds that hatched in 1978, followed until 1991,
only 3 alive; mortality rates high during first yr then dropped for several yrs, then increase
later; survival of birds depends on seed production which is related to rainfall
graphs are helpful for highlighting important changes in pop’ns
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