Summary of Chapter Concepts

Populations fluctuate naturally over time. This occurs because density-dependent and density-independent factors can change from year to year and from place to place. The magnitudes of the fluctuations are often related to the ability of a species to resist changes in the environment and to differences in their life histories, including reproductive rates and life span. In some species, the population can overshoot its carrying capacity and then experience a rapid die-off. In populations that have an age structure, fluctuations in population size over time can be detected by disproportionate numbers of individuals in particular age classes. Many species experience cyclic fluctuations in population size.

Density dependence with time delays can cause populations to be inherently cyclic. Delayed density dependence allows populations to fluctuate above and below their carrying capacity. Delayed density dependence can be incorporated into our population models by having the population’s growth rate depend on the population density that occurred at some time in the past. Using these models, we find that the magnitude of the fluctuations depends on the product of the intrinsic growth rate (r) and the time delay (τ). Increasing values of this product causes the population to shift from experiencing no oscillations to damped oscillations to a stable limit cycle. Experiments have confirmed that time delays due to energy reserves or development times between life stages can cause cyclic fluctuations.

Chance events can cause small populations to go extinct. Smaller populations are more likely to go extinct than large populations. This occurs due to demographic and environmental stochasticity.

Metapopulations are composed of subpopulations that can experience independent population dynamics across space. Metapopulations exist when a habitat exists in small fragments, either naturally or from human activities. The basic model of metapopulation dynamics informs us that metapopulations persist due to a balance between extinctions in some habitat patches and colonizations that occur in other patches. Although the basic metapopulation model assumes that all patches are equal, in reality, larger patches generally contain larger subpopulations and patches that are less isolated are more likely to be occupied as a result of both the rescue effect and higher rates of recolonization.

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