Indirect interactions in seagrasses: fish herbivores increase predation risk to sea urchins by modifying plant traits

Summary The dominant paradigm of top‐down control of ecological communities through direct consumption pathways is giving way to a more nuanced understanding of trophic interactions with the recognition that subtler indirect effects can often play an important role in structuring communities. Direct and indirect trophic and non‐trophic processes could well be acting simultaneously within the same food web, and their overall effects may even interact with each other. We studied indirect interactions in a relatively simple trophic system comprising a single producer, two herbivores and a guild of predators. In particular, we assessed whether (i) the principal herbivore fish, S arpa salpa, is capable of modifying a seagrass trait, canopy height, by grazing and (ii) whether grazing‐induced habitat alteration can trigger an environment‐mediated modification of the interaction between herbivorous sea urchins P aracentrotus lividus and their predators. We tested these hypotheses with field experiments including fish herbivore‐exclusion experiments (to examine the ability of S . salpa to modify seagrass habitats) and predation experiments using tethered sea urchins in a meadow with varying canopy heights (to test whether habitat modification can mediate urchin predation risk). The effect of S . salpa herbivory was highly significant. Canopy height in herbivore‐excluded plots was more than 3·5 times higher than in uncaged control plots. In addition, adult sea urchin predation risk in the most highly grazed plots was 4–5 times higher than plots with higher canopy heights. In contrast, predation risk on juvenile urchins was not influenced by canopy height. Our results show that predation pressure on a key herbivore can be modified both by the environmental context within which it finds itself and by the actions of another herbivore that modifies the plant traits that create this environmental context. These act as two discrete pathways that interact in potentially nonlinear ways, mediating top‐down control in these ecosystems. Herbivores, particularly when acting as ecosystem engineers, may have the potential to mediate and increase predation risk, as they substantially modify habitat structure, with consequences for refuge availability, among others. Assessing these indirect interactions is not just important to understand the functioning of the system itself but may have important management and conservation consequences.