BIOL 240 Lecture Notes - Lecture 6: Homo Sapiens, Metapopulation, Introduced Species

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26 Jan 2013
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WHAT FACTORS INFLUENCE POPULATION DENSITIES
as the # of individuals in a pop’n increases, the # of new individuals added per unit of time
accelerates, even if the rate of increase expressed on a per individual basis-per capita growth
rate-remains constant- exponential growth
difference between the average per capita birth rate in a pop’n and its avg per capita death rate
is the net reproductive rate
intrinsic rate of increase: highest possible value for the net reproductive-the rate at which the
pop’n would grow under optimal conditions
as pop’n increases in density, environmental limits cause birth rates to drop and death rates to
rise
environmental carrying capacity: environment can support no more than a certain # of
individuals of any particular species per unit of area-determined by the availability of resources
growth of a pop’n typically slows down as its density approaches the environmental carrying
capacity because resource limitations and the activities of predators lower birth rates and
increase death rates
s-shaped, logistic equation-each individual added to teh pop’n depresses the pop’n growth by
an equal amount; pop’n growth stops when N=K
per capita birth and death rates usually change together with changes in pop’n density –density-
dependent- reasons:
o species increases in abundance, depletes food supply, increase death, decrease birth
o if predators capture a larger proportions of the prey than they did when the prey was
scarce, per capita death rate of prey rises
o diseases can spread easily in dense pop’n than sparse pop’n
density-independent: factors that change per capita and death rates in a pop’n independently
of its density
fluctuations in the density if a pop’n are determined by all of the density-dependent and
density-independent factors acting on it
song sparrows, over 12 yrs, # of sparrows fluctuated 4 and 72 breeding females and 9 and 100
territorial males; death rates were high during particularly winter; # of breeding males-limited
by larger the number of males, teh larger the # that failed to gain a territory and lived as
“floaters” with little chance of reproducing; larger # of breeding females, fewer offspring each
female fledged
population densities increase following years of good reproductive success, but they decrease
following yrs of poor reproduction
densities of pop’n of species that depend on a single or just a few resources are likely to
fluctuate more than those of species that use a greater variety of resources.
Factors that determine why typical pop’n densities vary so greatly aming species, but four of
them-resources abundance, the size of individuals, the length of time a species has lived in an
area and social organization
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