40 80 120 160 Number of adults in autumn
20 40 60 80 Total number of males
20 40 60 80 Number of breeding females
40 80 120 160 Number of adults in autumn term implies that each individual added to the population depresses population growth by an equal amount:
Population growth stops when N = K because then (K - N) = 0, so (K - N)/K = 0, and thus AN/At = 0.
Population densities influence birth and death rates
Because each additional individual typically makes things worse for other members of the population in a limited environment, per capita birth and death rates usually change in response to population density; that is, they are density-dependent. Birth and death rates may be density-dependent for several reasons. First, as a species increases in abundance, it may deplete its food supply, reducing the amount of food available to each individual. Poorer nutrition may increase death rates and decrease birth rates. Second, predators may be attracted to areas with high densities of their prey. If predators capture a larger proportion of the prey than they did when the prey were scarce, the per capita death rate of the prey rises. Third, diseases spread more easily in dense populations than in sparse populations.
However, not all factors affecting population size act in a density-dependent way. A cold spell in winter or a hurricane that blows down most of the trees in its path may kill a large proportion of the individuals in a population regardless of its density. Factors that change per capita birth and death rates in a population independently of its density are said to be density-independent.
Fluctuations in the density of a population are determined by all the density-dependent and density-independent factors acting on it. The combined action of these factors is shown by the dynamics of a population of song sparrows on Mandarte Island, off the coast of British Columbia, Canada.
54.8 Regulation of an Island Population of Song Sparrows
The size of the population of song sparrows (Melospiza melodia) on Mandarte Island, British Columbia, is determined in part by the severity of winter weather. In addition, the population is regulated by (a) the territorial behavior of males, (b) the reproductive success of females, and (c) the survival ofjuveniles in relation to population density.
Over a period of 12 years, the number of song sparrows fluctuated between 4 and 72 breeding females and between 9 and 100 territorial males. Death rates are high during particularly cold, snowy winters, regardless of the density of the population. Several density-dependent factors also contribute to fluctuations in the density of the population. The number of breeding males, for example, is limited by territorial behavior: The larger the number of males, the larger the number that fail to gain territories and must live as "floaters" with little chance of reproducing (Figure 54.8a). Also, the larger the number of breeding females, the fewer offspring each female fledges (raises to the age when it can leave the nest) (Figure 54.8b). And the more birds alive in autumn, the poorer the chances of juveniles born in that year surviving the winter (Figure 54.8c). Thus, the number of males and females breeding each year is influenced by both density-independent and density-dependent factors.
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