Trophic cascades in seagrass meadows depend on mesograzer variation in feeding rates, predation susceptibility, and abundance

Seagrasses provide important habitat for fishes and invertebrates but are declining around the globe, often due to overgrowth by algae. One hypothesis for this overgrowth is that overfishing of top consumers has led to greater numbers of small predatory fishes that reduce the abundance of mesograzers. This trophic cascade hypothesis requires that the same species that control algal biomass are also susceptible to fish predation. While mesograzers are known to vary in their feeding rates on algae and seagrasses, much less is known about variation in predation susceptibility and how this is related to grazing abilities. For 6 common mesograzers from Bodega Harbor, California, USA, we assessed feeding rates on macroalgae (Ulva spp.), epiphytic microalgae, and eelgrass. We then assessed predation susceptibility using juvenile cabezon Scorpaenichthys marmoratus in tanks of eelgrass habitat with and without Ulva. We found that the fastest consumers of all 3 primary producers were the least susceptible to predation. This appeared to be due to predator avoidance strategies; fish consumed visible caprellids at a higher rate than the larger consumers, which were either better camouflaged or able to avoid predation by building tubes within the macroalgae. Using our feeding and predation rates, along with relative abundances from field surveys, we calculated the expected trophic cascade effect with and without grazer species differences. Because fish predation was skewed towards the most abundant but least important (per capita) grazers, incorporating trait variation led to a 50 to 80% reduction in expected trophic cascade effects. Examining other seagrass communities for either similar grazer species or a similar mismatch between feeding rates and predation susceptibility may improve our understanding of the variation in trophic cascade effects across systems.