Group Formation in Reef Fish

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Sometimes when life is hard you need a buddy. Fish are well known for their propensity to form groups (i.e. schools or shoals), and the formation of these groups can provide all sorts of anti-predator benefits including predator confusion, group vigilance (e.g. alarm calling or predator mobbing), and predator dilution. Of course, the benefits of group formation may or may not be outweighed by the negative effects of group living such as the aggregation of predators or increased resource competition among prey in areas of high density. Therefore group formation can lead to benefits and consequences for prey, but also can change how the overall foraging success of predators. The importance of prey density in modifying predator foraging behavior has been established for some time in Ecology for all sorts of processes including the stability of predator prey dynamics, the spatial distribution of predators, and the strength of species interactions in diverse food webs. However, the effects of prey density on predator foraging behavior can vary substantially with both  predator density (e.g. due to phenomena such as  intraguild predation, interference competition, or cooperative hunting) and the presence of multiple alternative prey species (e.g. due to competition within the prey community or aggregation of predators to areas of high prey density). Our empirical understanding of how different components of predator foraging behavior shift with predator density and the presence of multiple prey species remain limited.

We develop and apply an explicit statistical framework that combines predator functional response curves and short-term aggregative response of predators by estimating shifts in attack rate, handling time, and effective preda- tor density as alternative mechanistic models explaining variation in mortality rates of prey. For each experiment, we take both a ‘predator-centric’ (examining feeding rate of a single predator, as a function of prey density) and ‘prey-centric’ approach (examining per capita prey mortal- ity rate, as a function of prey density).

Title: Predator and Competitor Dependent Benefits of Group Formation in a Coral Reef Fish

Authors: Adrian C. Stier, Shane W. Geange, Benjamin M. Bolker

Abstract: Studies of predator foraging behavior typically focus on single prey species and fixed predator densities, ignoring the potential importance of complexities such as predator dilution; predator-mediated effects of alternative prey; heterospecific competition; or predator–predator interactions. Neglecting the effects of prey density is particularly problematic for prey species that live in mixed species groups, where the beneficial effects of predator dilution may swamp the negative effects of heterospecific competition. Here we use field experiments to investigate how the mortality rates of a shoaling coral reef fish (a wrasse: Thalassoma amblycephalum), change as a result of variation in: 1) conspecific density, 2) density of a preda- tor (a hawkfish: Paracirrhites arcatus), and 3) presence of an alternative prey species that competes for space (a damselfish: Pomacentrus pavo). We quantify changes in prey mortality rates from the predator’s perspective, examining the effects of added predators or a second prey species on the predator’s functional response. Our analysis highlights a model-fitting approach that discriminates amongst multiple hypotheses about predator foraging in a community context. Wrasse mortal- ity decreased with increasing conspecific density (i.e. mortality was inversely density-dependent). The addition of a second predator doubled prey mortality rates, without significantly changing attack rate or handling time – i.e. there was no evidence for predator interference. The presence of a second prey species increased wrasse mortality by 95%; we attribute this increase either to short-term apparent competition (predator aggregation) or to a decrease in handling time of the predator (e.g. through decreased wrasse vigilance). In this system, 1) prey benefit from intraspecific group living though a reduced predation risk, and 2) the benefit of group living is reduced in the presence of an alternative prey species.
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Stier, A. C., S. W. Geange, and B. M. Bolker. 2012. Predator density and competition modify the benefits of group formation in a shoaling reef fish. Oikos:online early.

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