Dynamical Behavior of Stage Structured Populations

Extremely high fecundity and ecological complexity resulting from the multistage life cycle result in stage-structured populations showing a greater range of population dynamics than age-class-structured populations. Although generally much more density independent than age-class-structured populations, population dynamics of any life stage can range from density independent to intensely density dependent (e.g., amphibian larvae in ephemeral pools). At the population level, dynamics similarly can range from density independent to populations that show multiple stable states or equilibrium points. At the extreme, high fecundity and extremely differing life stages can result in chaotic behavior in population dynamics.

For example, populations of spruce budworms and other insect 'pests' may show multiple stable equilibrium points. Usually, the lower is associated with habitat conditions of low quality (such as immature balsam fir (Abies balsamea) and white spruce (Picea glauca) forests in the case of spruce budworms) in combination with predation and parasitism on both the six larval instars (caterpillar) and adult (moth) stage. As habitat quality increases (forests mature), budworm populations can achieve a 'critical mass' that allows them to irrupt to population levels orders of magnitude greater than the low density equilibrium. The irruption is facilitated by the highly mobile adult (moth) stage, which can disperse hundreds of kilometers in a single generation. Because of high mortality of host trees associated with large-scale outbreaks, conditions necessary for outbreaks may occur approximately only every 30 years. The time span between irruptions allows affected forests to regenerate and adjacent forests to attain a suitable mature forest structure, highlighting both the temporal and spatial variability in suitable habitats for this stage-structured species.

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