Exploitation Ecosystems in Seasonal Environments

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Abstract
The model of exploitation ecosystems by Oksanen et al. contains a tacit assumption of constancy of the physical environment. Seaonality is a source of time delay in density dependent processes and thus potentially destabilizing. An analysis of grazing chain dynamics in seasonal environments where primary production is concentrated to a short and intense pulse (spring-summer or wet season) show that the strength of this destabilizing force depends on reproductive potential and generation time. If annual reproductive output is low and juveniles are relatively vulnerable to moderate shortage of winter resources, then the model of Oksanen et al. applies to seasonal ecosystem, too. However, for rapidly reproducing herbivores with short generation times, seasonality is a strong destabilizing force. This destabilizing potential can be contained by generalist predators in environments where switching between small and medium-size prey is possible (temperate areas, continental taigas with soft snow), but should create sustained population fluctuations in environments with converse conditions (thick, hard and long-lasting snow cover). If the productivity of the area is low, the fluctuations should be chaotic and devastation of the vegetation should be the proximate cause of population declines in herbivores. In productive environments, the fluctuations should be regularly cyclic, and population declines of herbivores should be caused by the combined action of social regulation and predation. Available data conform to these predictions.

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