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Lecture 6

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Biology 3466B

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 o Species that use abundant resources generally reach higher population densities than species that use scarce resources o Species with small body sizes generally reach higher pop
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