Contrasting trophic‐cascade effects driven by variation in morphology of the perches used by a larval damselfly

Abstract

Summary The presence of habitat structures (e.g. caves, ledges, branches) has well‐documented ecological effects. However, it remains largely unknown how variation in the morphology of particular habitat structures affects ecological interactions. Using an algae–cladoceran grazer–larval damselfly food chain as a model in a series of microcosm experiments, we manipulated food‐chain length and the length (long versus short) and diameter (thick versus thin) of vertically orientated damselfly perches (habitat structure) and examined the density of the grazers and algae. Because the larval damselflies are usually more flexible on thinner perches and have broader foraging domains on longer perches, we predicted that when on long and thin perches they would suppress grazer density more efficiently and hence confer a more positive trophic‐cascade effect on algal growth. As predicted, larval damselflies occupying long and thin perches most strongly reduced grazer density and increased algal density, illustrating a positive trophic cascade. In all other damselfly treatments, and despite reduced grazer density, algal density declined, showing a negative trophic cascade due to an elevation in grazer foraging efficiency under predation risk. This probably resulted from the increased activity of the grazers and their spatial shift to the lower water column where algal density was higher. In conclusion, perch morphology affected the direction and strength of the trophic cascade by altering both density‐mediated and behaviour‐mediated indirect interactions. Considering that anthropogenic disturbance is dramatically changing the morphological diversity of habitat structures, we call for more research into the ecological consequences of such physical diversity at community and ecosystem levels.