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population densities.doc

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SOCY 227
Richard Ascough

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 2n 2n /1 n1=# of marked in first sample nd n2=total # of individuals in 2 sample 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 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 • survivorship curves fall into 3 patterns: o most individuals survive for most of their potential life span and die about the same age o probability of surviving to the next yr is abt the same over most of the life span once individuals are a few months old o produce a large # of offspring, each of which receives little investment of energy • high death rates of young individuals are followed by high survival rates 54.2 HOW DO ECOLOGICAL CONDITIONS AFFECT LIFE HISTORIES • life history: describes how organism allocates its time and energy among the variance activities that occupy its life • female rockfish continue to grow throughout their lives; larger females are much more productive than smaller females b/c the # of eggs a female produces is proportional to her size; older, larger females produce eggs with larger oil droplets-provide energy to newly hatched-grow faster and survive better • guppies living in predator-free areas upstream from those waterfalls have low mortality rates; those that live below the falls have high mortality rates; david reznick reared 240 guppies from high-predation and low predation sites; supplied some with plentiful food and others with limited; where no predators were present, guppies from high-predation sites matured earlier, reproduced more 54.3 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 depr
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