Crowding

Crowding affects insect tendency to disperse, and in some cases it may stimulate morphological or physiological transformations that facilitate dispersal. Survival and fecundity are often density dependent (i.e., inversely related to population density). Therefore, dispersing individuals may achieve higher fitnesses than do nondispersing individuals at high population densities (Price 1997). For example, some bark beetle species oviposit their full complement of eggs in one tree under low-density conditions, but only a portion of their eggs in one tree under high-

density conditions, leaving that tree and depositing remaining eggs in other trees (T. Wagner et al. 1981). If all eggs were laid in the first tree under crowded conditions, the large number of offspring could deplete resources before completing development.

Crowding has been shown to stimulate feeding and developmental rates. Under crowded conditions, some insects spend more time eating and less time resting (R. Chapman 1982). Crowding may increase the incidence of cannibalism in many species (Fox 1975a, b), encouraging dispersal. In addition, crowding can induce morphological changes that promote dispersal. Uncrowded desert locusts tend to repel one another and feed quietly on clumps of vegetation, whereas crowded locusts are more active, attract one another, and march en masse, spending little time on vegetation (Matthews and Matthews 1978). Accompanying physiological changes in color, wing length, and ability to feed on a wider variety of plants facilitate migration and the chances of finding suitable resources.

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