Showing papers by "Göran Arnqvist published in 1991"

Disruptive Selection on Prey Group Size: A Case for Parasitoids?

Abstract: The prevalence of group living in animals has generated several hypotheses about the potential benefits of gregariousness. Although some of these hypotheses involve specific behavioral interactions among group members (Bertram 1978; Pulliam and Caraco 1984), others concern the purely statistical effects of gregariousness in decreasing predation risk (Treisman 1975; Turner and Pitcher 1986). Sillen-Tullberg and Leimar (1988) modeled the costs and benefits of gregariousness in unpalatable and aposematically colored insects. Their analysis primarily concerned group formation in insect eggs, larvae, and adults, whose predators are often larger and more mobile than their prey. In such cases, predators can consume a large proportion of the prey group (no dilution), and gregariousness would not be expected to evolve. Nevertheless, in the case of unpalatable and aposematic prey, Sillen-Tullberg and Leimar showed that disruptive selection on prey group size may arise, because of the predator-satiating effect of unpalatability and aposematism. In this study, we demonstrate that disruptive selection on prey group size generally arises if a predator consumes a certain number of individuals (exceeding one) in a group rather than a fixed proportion of a group. This occurs whenever some type of predator-satiating mechanism is acting-for example, when parasitoids infest insect eggs, larvae, or pupae. In such situations, unpalatability and aposematism need not be invoked to explain bimodal distributions of prey group sizes.