Decomposing Population Dynamics

Analyses of time series of population fluctuations show large interspecific differences in population dynamics. As a first step, it is convenient to classify them into time series that show a long-term increase or decrease, and time series that show fluctuations around some mean population size. In the first group, there is no density dependence, and the changes from one year to another are determined by the population growth rate and stochastic variation in the growth rate. In the second group, density dependence is also present, which produces a characteristic return time to equilibrium.

Stochastic variation in the population growth rate is due to demographic and environmental stochasticity. Demographic stochasticity is caused by random variation among individuals in fitness contributions due to independent chance events of individual survival and reproduction. This produces random fluctuations in population size, with variance proportional to o2A/N. Environmental stochasticity affects the age-specific demographic rates of all individuals in the population similarly, producing a constant variance among years in the population growth rate ce, independent of population size. Thus, environmental stochasticity affects the population at all sizes, whereas demographic stochasticity most strongly influences the dynamics at smaller population sizes.

To understand the effects of generation time we must examine how it affects each of the components in the population dynamics.

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