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Population dynamics of forest insects

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  1. Introduction.
  2. Detecting patterns and identifying processes.
  3. Top-down versus bottom-up.
  4. Multitrophic interactions.
  5. Population cycles.
  6. Conclusion.

Population dynamics is the study of changes in the number of organisms in populations and the factors in?uencing these changes. It thus, by necessity, includes the study of the rates of loss and replacement of individuals and of those regulatory processes that can prevent excessive changes in those numbers. A wide variety of factors can affect the population dynamics of a particular species. These can be divided roughly into two categories. First, the extrinsic or environmental in?uences on populations, such as temperature, weather, food supply, competitors, natural enemies, diseases, and all possible combinations of the preceding; and second, the interactions between members of the same populations, be these direct or indirect, e.g., intraspeci?c competition, behavioral processes, and aggregation.

[...] There are, however, ways in which the host plant can in?uence the development of population cycles in forest insects. First, even if the nutritional quality of the host plant remained unchanged, the build-up of the herbivore population on the host plant can result in competition for resources, either through depletion of the food source or by the increase in the number of larvae feeding on a ?nite host plant. Second, the physiological state of trees (and other plants) is not static, and their susceptibility/suitability as food plants both within and between years can be changed. [...]

[...] Yet another effect of sublethal plant defenses is that the insect herbivore, feeding as it does on a suboptimal diet, is more likely to become stressed and more susceptible to infection by pathogens, e.g., fungal and viral diseases, although in some cases it is possible that the insect is able to sequester plant chemicals that inhibit virus infection. Population Cycles So how do these top-down and bottom-up forces interact with the insect herbivore to produce the population cycles seen in so many forest Lepidoptera? Populations that cycle are characterized by highs (peaks) and lows (troughs) in abundance. As foresters usually ?rst become aware of defoliating insects when they outbreak, it is appropriate to start our consideration on a peak, when the population is at its maximum. [...]

[...] The herbivore population is at its peak, and the trees are likely to be showing marked signs of defoliation, either totally stripped or at least half their foliage removed. The nutritional quality of the plants for the insect is at its lowest, either because of a scarcity of foliage and/or because of induced defenses. Interspeci?c competition between the insects is markedly higher than before and the caterpillars are small and stressed. Their growth rates will be low and this will make them susceptible to natural enemies. [...]

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