Abstract
AbstractGroup formation reduces individual predation risk when the proportion of prey taken per predator encounter declines faster than the increase in group encounter rate (attack‐abatement). Despite attack‐abatement being an important component of group formation ecology, several key aspects have not been empirically studied, that is, interactions with the hunting mode of the predator and how these relationships are modified by local habitat quality. In 79 cage trials, we examined individual egg predation risk in different‐sized egg clutches from the blue willow beetle Phratora vulgatissima for two predators with different hunting modes (consumption of full group [Orthotylus marginalis] vs. part group [Anthocoris nemorum]). Because these predators also take nutrients from plant sap, we could examine how the quality of alternative food sources (high‐ vs. low‐quality host plant sap) influenced attack‐abatement patterns in the presence of different hunting strategies. For the O. marginalis predator, individual egg predation risk was largely independent of group size. For A. nemorum, egg predation risk clearly declined with increasing group size. However, approximately one‐third of the grouping benefit was lost to an increase in group detectability. There were clear differences in attack‐abatement patterns between plants with high‐ vs. low‐quality sap. When O. marginalis was the predator, there was no clear change in attack‐abatement in relation to host plant quality. However, for A. nemorum there was a clear reduction in overall predation risk and a stronger attack‐abatement pattern with increasing group size when plant sap quality increased. This implies that the relative benefits of prey grouping behavior for any species might show diurnal or seasonal changes as other aspects of resource/habitat quality change for the focal predator. Modulation of attack‐abatement by bottom‐up effects such as plant‐based food resources is yet to be incorporated into general theory, despite the ubiquity of omnivorous predators and with omnivory being important for shaping food webs, ecosystem functions, and in biological control. Thus, ongoing refinement of attack‐abatement theory by focusing on bottom‐up vs. top‐down processes could have significant impacts on many important contemporary fields of study.
Highlights
Does egg predation risk relative to group size follow expectations from attack-a batement theory as the dilution effect varies from being complete (A. nemorum) to zero (O. marginalis) for a single prey species? Second, to what extent does group size influence the group encounter rate for an egg predator, and is the effect similar between group size categories? we investigate how these patterns of egg predation risk relative to group size vary on host plants differing in food quality and architecture
Because each group size represented a threefold increase in clutch size (5, 15, and 45), we modeled the generalized linear mixed model (GLMM) using group size coded as a continuous variable (0, 1, 2) to compare between-predator differences in intercepts and slopes
The decline in predation risk relative to group size was largely similar on both Salix species, there was a clear difference in the absolute predation risk between S. dasyclados and S. viminalis
Summary
Prey species have evolved many behaviors to reduce predation risk (Hendrichs et al 1991, Cocroft 1999), with one of the most important being group living (Ebensperger 2001, Krause and Ruxton 2002, Pollard and Blumstein 2008).A primary benefit of prey grouping behavior results from group-size-r elated changes between the predator encounter rate and the proportion of the group preyed upon during each encounter (i.e., numerical “dilution”): the so-c alled “attack- abatement” (Turner and Pitcher 1986; see Fig. 1 and Appendix S1 for an expanded definition).v www.esajournals.orgNovember 2016 v Volume 7(11) v Article e01541 Individual predation risk probabilityC full E and D partial E or D no E or DAttack-abatement effectDespite the central role attack-a batement has in explaining the advantages of prey group living (Krause and Ruxton 2002, Davies et al 2012), few studies have successfully disentangled the relative contribution of encounter and dilution (but see Foster and Treherne 1981, Wrona and Dixon 1991). The attack-a batement model is often illustrated using a single idealized prey species under the full range of possible predatory conditions of encounter and dilution (Turner and Pitcher 1986, Inman and Krebs 1987), empirical support comes instead from comparisons of different prey species operating under specific fixed predator conditions In such cases, the proportion of prey taken universally declines with increasing group size (Calvert et al 1979, Foster and Treherne 1981, Wrona and Dixon 1991, Uetz and Hieber 1994). This is because the dilution effect in these studies is always complete (Foster and Treherne 1981) or partial (Wrona and Dixon 1991), meaning that the relationship between group size and predation risk under some conditions predicted by theory have never been tested—when the dilution effect is zero because all prey in a group are consumed upon encounter (Fig. 1)
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