Group Size and Predation Risk: A Field Analysis of Encounter and Dilution Effects

We propose a general fixed-constant linear-regression model that can be used to assess empirically the potential benefits and disadvantages of group living from the separate and combined effects of predator encounter and numerical dilution. Using this model, we assessed group-size-related predation risk in pupae of the stream-dwelling trichopteran Rhyacophila vao from the planarian predator Polycelis coronata. When considered on its own, Rhyacophila pupal aggregation conferred an apparent disadvantage from encounter-related effects, since both predator-encounter probabilities and local densities increased in a density-dependent manner with pupal group size. In contrast, dilution effects related to the functional response of Polycelis yielded group-size-related benefits. When their combined (i.e., attack-abatement) effect was considered, aggregation was found to confer a net fitness advantage to pupae by decreasing predation hazard, primarily from the benefits of predator dilution not being entirely swamped by the potentially deleterious encounter effects. Furthermore, assessment of these relationships at different spatial scales helped elucidate some of the underlying proximal biological mechanisms involved. This study shows how the separate consideration of predator-encounter and dilution effects can provide an incomplete depiction of the effectiveness of grouping as an antipredator defense and emphasizes the importance of assessing their combined, attack-abatement effect.