Posted by: oikosasa | May 17, 2013

Habitat complexity, preys and predators

In the new Early View paper “Trait-mediated indirect interactions in a marine intertidal system as quantified by functional responses”, Mhairi E. Alexander and co-workers, have studied how factors as habitat compelxity affect predators and how the predators effect prey populations. Here’s their own summary:

It is well known that predation is important in community structure and functioning. It is also understood that the impact of a predator can be influenced by a number of biotic and abiotic factors. For example, the presence of higher-order predators can influence behaviours of intermediate species that can affect their consumption of prey through trait-mediated effects. Habitat complexity can also be an important mediating influence that in turn can influence the numbers of prey that are consumed. What is less well understood however is how these factors interact and contribute to prey population stability. In this study we address this by detecting and quantifying such trait-mediated indirect interactions (TMIIs) using functional responses, which consider a predator’s consumption over a range of prey densities, to investigate the implications for prey population regulation and stability,

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We conducted several experiments to investigate how the influence of a higher-order fish predator combined with habitat complexity affects the behaviour of an intermediate amphipod predator from marine intertidal habitats. We first tested whether amphipods were able to determine higher-order predator presence. We found that amphipods demonstrated anti-predatory behaviour via a reduction in activity with the addition of cue that was seawater mixed with crushed conspecifics as well as seawater from tanks holding fish fed conspecifics and also those fed bloodworm. Interestingly, there was no reaction to fish fed an algal diet or those that had been starved. As we didn’t find any response of the basal prey, a commonly occurring isopod, to these cues, we went on to investigate how the presence of the cue in combination with habitat complexity affected the amphipods predation rates and whether the observed reduced activity translated into reduced foraging.

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We observed that when there was no habitat or fish cue, amphipods showed what are considered to be potentially destabilising predatory responses towards the isopod prey. With the addition of habitat, however, the response was found to become stabilising as a result of a reduction in consumption of prey at low densities.  When habitat complexity was not included, the presence of fish cue was found to reduce the magnitude of the predator’s consumption of prey at higher densities, as would be expected with reduced activity in the presence of a predator. However when habitat was present in combination with fish cue, although reduced consumption occurred at low densities, at high prey densities it was increased in comparison to predation with habitat complexity and no cue. This seemed to occur as the fish cues drove the amphipods into habitat with more prey and thus actually enhanced predation of the basal prey.

The results from this study demonstrate the utility of functional responses in addressing questions of prey population stability. In addition we have further highlighted how complex predator-prey interactions can be, as well as exploring the relevance of environmental and biological cues that can result in unexpected and complex outcomes.

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