Feeding in fear: Indirect effects of predatory fish on macrophyte communities mediated by altered crayfish foraging behaviour

The threat of predation influences decisions made by prey animals and alters the way that prey interact with other species in their environment. Animals often alter their foraging behaviour in response to a predatory threat by increasing or decreasing food consumption or by changing their feeding preferences to focus on foods that limit their exposure to the threat. Changes in grazing pressure and diet selectivity can have significant impacts on the abundance and diversity of the prey's food resources. In this study, we examined the indirect effects of a predatory fish on macrophyte biomass, mediated by crayfish responses to predator odours. Largemouth bass (Micropterus salmoides, Centrarchidae) were housed in flow-through stream mesocosms to produce odour cues that initiated a nonconsumptive effect on rusty crayfish (Faxonius rusticus, Cambaridae) held in an adjacent section of the stream. Crayfish were offered three species of macrophytes (Myriophyllum exalbescens, Haloragaceae; Elodea canadensis, Hydrocharitaceae; Chara spp., Characeae) and were allowed to graze for 48 hr. Consumption of each plant species was measured by weighing the plant samples before and after every trial. The plant samples were further analysed for total phenolic content using the Folin-Ciocalteu method. Foraging and shelter use behaviours were quantified by analysis of video recordings for time spent foraging in the mesocosm and time spent in the shelter zone of the mesocosm. Crayfish consumed greater quantities of macrophyte tissue when predator cues were present, and this result was unexpected and is contrary to most observations in the literature. When threatened, the crayfish also showed a preference for two of the macrophyte species, which did not exist in trials without bass odour. This could be due to differences in total phenolic content among the three plant species tested. The results provide evidence for a strong indirect effect of predatory fish on the macrophyte community mediated by changes in crayfish behaviour under threat. Crayfish act as keystone species in stream ecosystems; thus, changes in their foraging behaviour and dietary preferences caused by exposure to predator odour cues could have important effects on macrophyte communities in lotic systems.