The Spatial Response of Chironomid Larvae to the Predatory Leech Nephelopsis obscura

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Abstract
Nephelopsis is a randomly distributed predator in Stephenson Pond (near Calgary, Alberta, Canada) and feeds on aggregated prey. The spatial heterogeneity of prey is highest when the leeches are present. The spatial heterogeneity of the chironomids decreased when they were released from predation, perhaps to minimize competition between the larvae. Intraspecific competition strongly affected chironomid growth rates during these experiments (Rasmussen 1985). Because the leeches consumed a lower proportion of larvae at higher larval densities (Rasmussen 1983), it is likely that predation rates would be higher, and larval survival lower, if larvae did not aggregate when exposed to predation. Differences in the spatial distribution of prey were not detectable by 3 days after but appeared before 28 days had elapsed. Prey organisms were sufficiently mobile to avoid positive correlations with predators, and the predators were sufficiently tenacious to prevent negative correlations with prey. The plasticity of spatial behavior of the chironomid larvae probably contributes to maintaining this dynamic balance.