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Spatial structure and population dynamics in an insect epidemic ecosystem

University of British Columbia

A major theme in contemporary ecology is how, and whether, spatial structure considerations such as dispersal and environmental heterogeneity must be invoked for satisfactory explanations of observed population dynamics. This study shows how behavioral and environmental components of spatial structure shape emergent population dynamics in one relatively simple and well studied natural ecosystem: the spruce budworm (Choristoneura fumiferana, Lepidoptera: Tortricidae), and the boreal forests of eastern Canada which it periodically defoliates. The study begins by partitioning spatial structure considerations into components of local structure, dispersal processes, and local uniqueness. Existing data on the budworm-forest ecosystem are then synthesized into an array of mathematical models reflecting possible relationships among those components. Numerical simulation techniques are used to explore these models. Subsequent qualitative analyses simplify the simulations, focussing on equilibrium isoclines for population growth and the associated phase portraits of system dynamics. The principle methodological contribution of the study is its explicit inclusion of dispersal effects in these qualitative analyses. Model predictions are compared with a variety of real world experiments and observations to assess the limits of present understanding. The study concludes that spatial population dynamics in the budworm ecosystem can be explained largely in terms of local structure relationships, modified to account for the local effects of insect immigration and emigration. Detailed knowledge of dispersal distance or direction distributions is unnecessary. Population dynamics are shown to be sensitive to (a) the proportion of moths dispersing, and its modification by habitat conditions or moth density; (b.) the survival of passively dispersing young larvae, and its dependence on forest structure; (c) the relationship between foliage quality/quantity and budworm feeding; and (d) predation by vertebrates on larvae at low insect densities. Priorities for future research are identified, and some retrospective doubts concerning the utility of the population dynamics perspective in ecology are expressed.

Text

2010-03-11

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http://hdl.handle.net/2429/21803

Zoology

University of British Columbia

Graduate