‘Take‐away’ foraging spatially uncouples predator and prey‐attack distributions

Abstract

Summary1. Ideal-free distribution theory assumes that in a patchy environment foragers maximize fitness and hence their feeding rate by balancing gains from more food against losses from more competition. Ultimately, individuals cannot increase their feeding rate by moving to another patch and they distribute themselves over patches in proportion to prey density per patch. 2. In our experiments with shore crabs Carcinus maenas foraging on two adjacent patches with mussels Mytilus edulis, the implicit assumption of ideal-free distribution theory that catch should match time spent in a prey patch is not met, however. Despite aggregating their attack where it is most profitable shore crabs distributed themselves homogeneously across mussel patches: they 'take away' the prey caught and consume it elsewhere to reduce interference. 3. Thus, predator distributions can be quite different from prey-attack distributions. This is important because the latter is shown to be decisive for persistence of predator and prey populations in ecological models.

Aggregated prey and predation rates: Juvenile shore crabs (Carcinus maenas) foraging on post-larval cockles (Cerastoderma edule)

“…This further supports that interference competition cannot be generalised as prey dependent (only prey availability is important), or ratio dependent (ratio of predator to prey) as discussed by DeLong and Vasseur (2011). Predators will often show complex behaviours in response to, or even to mitigate against as suggested in this study and by Smallegange et al (2010), interference competition. Interference competition occurs on small spatial scales (Thrush, 1999), and so the findings of this study are valuable in understanding how patchiness in prey distributions can affect prey survival.…”

“…This behaviour may reduce the effect of the prey resource being aggregated. This behaviour will decouple the crab distribution from its prey-attack distribution, and therefore does not fit the assumption of the 'ideal free distribution' where predator distributions reflect prey profitability (Fretwell and Lucas, 1969;Smallegange et al, 2010). In this way predatory crabs may reduce the risk of kleptoparasitism, injury through fights and cannibalism during a particularly vulnerable period while handling food (McDonald et al, 2001;Moksnes, 2004a).…”

“…There are two possible reasons for this. Firstly crabs avoided interactions when handling prey by 'take-away foraging', a term coined by Smallegange et al (2010) to describe the observation that crabs carried prey items away from optimal prey patches to distance themselves from another crab. This behaviour may reduce the effect of the prey resource being aggregated.…”

“…Smallegange and van der Meer (2007) found that foraging was greatly reduced when conspecifics were of an equal size and they also observed that crabs mostly avoided competitors when they were handling prey. Moksnes (2004b) and later Smallegange et al (2010) demonstrated the importance of mutual interactions amongst juvenile C. maenas in controlling their distribution. They observed both in field and laboratory experiments that juvenile crabs actively migrate across open sand from patches of shelter and available food to avoid interference competition with conspecifics.…”

Potential kleptoparasitism-scavenging interactions between crabs and shell-boring gastropods feeding on bivalve prey: a global survey

“…This survey (and most previous studies reporting crabgastropod K-S interactions; Table 1) considered the case in which a crab kleptoparasitizes or scavenges prey from a gastropod, which is usually more likely than the reverse order of events (i.e., a gastropod steals or scavenges prey initially attacked by a crab) because crabs tend to be larger, more aggressive, and more rapidly feeding than snails, can prey on gastropods, and also leave little prey tissue behind after feeding to be scavenged (Smallegange et al 2010, Quinn et al 2012, Wong et al 2012, Boudreau et al 2013. However, in cases involving relatively large and fast-moving and -feeding gastropods and/or smaller/slower crabs, then the reverse order of events may be probable (Morissette & Himmelman 2000, Morton & Yuen 2000, Fodrie et al 2012, Walsh 2016.…”