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The total effect of predators on prey is a combination of direct consumption, and non-consumptive effects (NCEs), such as predator-induced changes to prey morphology, behaviour and life history. Past research into NCEs has tended to focus on pair-wise interactions between predators and prey, while in natural ecosystems, species exist in complex communities with several trophic levels made up of multiple autotrophic and heterotropic species. To address how predator NCEs alter the photosynthetic and heterotrophic components of communities, we exposed microbial microcosms to one of three predator treatments: live predators (full predator effect), freeze-killed predators (NCEs only) or no predators (control), and incubated them under either 12 h:12 h light:dark conditions or continual darkness. Under 12 h:12 h light:dark conditions, NCEs-only communities never differed from predator-free communities, but differed from live predator communities. Under conditions of continual darkness, the structure of NCEs-only communities differed from predator-free controls, but not from live predator communities, suggesting NCEs can be strong enough to structure communities. Predation threat may cause certain prey to induce defences, such as reductions in movement, which make them less competitive in a community setting. This reduction in competitive ability could lead to these species being driven to extinction through interspecific competition, resulting in similar communities to those in which live predators are present. Heterotrophic species whose rates of resource acquisition depend on movement rates may be affected to a greater extent than autotrophs by predator-induced reductions in movement, accounting for our observed differences in predator NCEs in ‘dark’ and ‘light’ communities. Our results suggest that the community-level consequences of fear are greater in the dark.

Synthesis

Predators affect prey through consumptive and non-consumptive effects (NCEs) such as alterations to prey behaviour, morphology, and life history. However, predators and prey do not exist in isolated pairs, but in complex communities where they interact with many other species. Using a long term study (>10 predator generations), we show that predator NCEs alone can alter community structure under conditions of darkness, but not in a 12h:12h light:dark cycle. Our results demonstrate for the first time that although the community-level consequences of predator NCEs may be dramatic, they depend upon the abiotic conditions of the ecosystem.