Are detritus feeders more sensitive to predator threat?

Posted on December 16, 2014

In studies of predator-prey interactions, most attention has been focussed on the lethal impacts of predators. However, the mere threat of predators, as advertised by their chemical signatures, can have non-lethal effects on prey behaviour, morphology and life history. Such changes have the potential to modify the structure of aquatic communities, but little is known about this because most studies have been carried out over very short time scales with highly simplified systems – often just one species of predator and one species of prey. In a joint Canadian-Australian study, these problems were overcome by using as a model system a community of freshwater algae, protozoa, rotifers and small invertebrates with short generation times. Replicate assemblages were maintained in 60 microcosms (covered glass jars), which were sampled 11, 22 and 33 days after the experiment began. The detritus-based food web included rotifers, Amoeba, and bacteria-feeding and predatory ciliates, while the algal-based food web included flagellates and their predators. All these life forms were potential prey for the top predator, a flatworm, Stenostomum virginianum. Because the main aim of the study was to see if the non-lethal impacts of predators were different for photosynthetic and detrital food webs, half of the mesocosms were kept in continuous dark (where there was total dependence on the detrital food web), and the other mesocosms were maintained on a 12h :12h light:dark regime (where there was significant ecological emphasis on the algal food web). Some mesocosms contained flatworm predators, some contained predator cues (freeze-killed flatworms) only, and others had no predators or predator cues. Analysis showed that both light and the type of predator treatment had a significant influence on the structure of the community. In the light, “predator cue” and “no predator” assemblages were similar, but both were significantly different from “predator” assemblages, which implied that the main effects of predators were lethal rather than non-lethal. In contrast, in the dark, assemblage similarities changed over time. Early in the experiment, the pattern was the same as that seen in the light, but by its end, “predator” and “predator cue” assemblages were similar and both were significantly different from the “no predator” assemblages. This change was interpreted partly as a result of the build-up of predator cue chemicals over time. It was suggested that species that dominate in the dark, which rely on active detrital-feeding rather than the products of photosynthesis, tend to be relatively hard-hit by the need to slow their activity in order to reduce contact with predators. As a result, their competitive ability is decreased and the community changes.

Reference: Forbes, C. & Hammill, E. 2013. Fear in the dark? Community-level effects of non-lethal predators change with light regime. Oikos 122, 1662–1668.