Variation in the trophic ecology of reef herbivores in protected and unprotected areas of the Florida Keys National Marine Sanctuary

Background/Question/Methods

Marine Protected Areas (MPAs), established to combat coral reef decline, are documented to increase abundances of targeted species. Yet, unintended indirect effects on food web interactions may occur as targeted species, many of which are apex predators, recover within MPAs. Recovery of these predators could potentially alter grazing behavior of herbivorous fishes, ultimately changing how important grazers impact the health and persistence of corals in these protected zones. To evaluate the role predators have in shaping the trophic ecology of their prey, I used stable isotopes of carbon and nitrogen to derive a representation of trophic niche for reef herbivores (Sparisoma aurofrenatum and Acanthurus coerulus) in protected and unprotected zones of the Florida Keys National Marine Sanctuary (FKNMS). In this region, herbivores are not targeted by fishing activity and they are prey to predators (e.g., black grouper, mutton snapper, etc.), whose populations have been recovering inside protected zones of the FKNMS since 1997. Understanding how MPAs affect the trophic role of non-target herbivorous fishes, important for their role in removing macroalgae that can overgrow and kill living coral, is essential to predict ecosystem consequences of MPAs and to guide future conservation efforts.

Results/Conclusions

Fish length was positively correlated with δ¹⁵N and δ¹³C values for both herbivore species (S. aurofrenatum: δ¹³C, p<0.001, R²=0.12 / δ¹⁵N, p<0.001, R²=0.36 & A. coerulus: δ¹³C, p<0.001, R²=0.09/ δ¹⁵N, p<0.001, R²=0.52). However, when examined at the site level, there was no relationship between δ¹³C and total length for fish at protected sites only. This suggests that diets are constrained as the result of predatory fish populations increasing inside of reserves. To further explain the variation in δ¹³C, regression models with predictors such as site characteristics, the availability of resources and fish biomass were ranked using Akaike’s Information Criterion. Top models included the terms, competitor and predator biomass and resource availability, adding support to my hypothesis. To examine the trophic niche of each species, I calculated niche metrics using δ¹⁵N and δ¹³C values, which represent different aspects of the degree of niche diversification within a population. There were significant differences in many of these values between protected and unprotected sites and the direction and magnitude of these differences were consistent within species but varied between species. These results suggest species-specific behavioral differences in foraging strategies and/or diet choice develop in response to resource availability, competition and predation risk.