Ecological Effects of Red Grouper (Epinephelus Morio) in Florida Bay

Ecosystem engineers can have complex effects on communities through a variety of direct and indirect pathways. Describing these effects is a necessary step in understanding and predicting the effects of engineer species. Red Grouper (Epinephelus morio) manipulate habitats by excavating sediment and detritus from karst solution holes in Florida Bay; they are also predators that consume a variety of crustaceans and benthic fish prey. As both a habitat manipulator and predator, the effects of Red Grouper on communities associated with the habitats they modify will likely be complex and difficult to predict. Here I present the results of observational and experimental work investigating the community and species-level effects of Red Grouper on faunal communities associated with Florida Bay solution holes. By measuring solution holes and conducting diver surveys of faunal communities associated with a fixed set of solution holes over time, I was able to show how the communities and the solution holes themselves were different in the presence and absence of Red Grouper. In general, solution holes were deeper, and communities were both more abundant and diverse when Red Grouper were present. However, while many species changed in abundance and occurrence from year-to-year, short term experiments, on the order of weeks, indicated that these effects were primarily driven by a small set of species that interacted strongly with the Red Grouper. Caribbean Spiny Lobster (Panulirus argus) was a common and abundant member of the solution-hole communities, and were more abundant in solution holes with Red Grouper, despite the fact that Red Grouper are known lobster predators. Investigating the Red Grouper-lobster interaction across lobster ontogeny indicated that Red Grouper positively affected the abundance of large lobsters, but negatively affected the abundance of smaller lobsters. To determine if this result was due to predation or avoidance behavior by juvenile lobsters, I conducted an experiment to test relative predation risk of lobsters adjacent to solution holes with Red Grouper, and another experiment at artificial dens to test lobster avoidance in the presence and absence of Red Grouper. The results of both experiments confirmed the hypothesis that size-selective predation by Red Grouper maintains the observed distribution of lobsters in solution holes, and not avoidance behaviors by lobsters. The community level analysis also showed a strong positive effect of Red Grouper on the abundance of juvenile coral reef fishes. I hypothesized that this effect was caused by a behaviorally-mediated indirect interaction (BMII) between Red Grouper and juvenile reef fishes via solution-hole associated piscivores. A recent addition to the suite of solution-hole associated piscivores is the Indo-Pacific lionfish (Pterois volitans and P. miles). As resident piscivores with high site fidelity in relatively high abundance in the bay, lionfish served as a focal species to test the BMII hypothesis. Juvenile reef fishes were most abundant at solution holes with Red Grouper present, least abundant in solution holes with lionfish present, and at intermediate abundances when both predators were present. These results confirmed the strong negative effects that lionfish have on native reef fish communities, and also suggest that Red Grouper can ameliorate some of these negative effects through a BMII that results in higher native fish abundance. Overall, Red Grouper had largely positive effects on the abundance and diversity of the fish and crustacean communities found inside and around solution holes in Florida Bay. The results presented here suggest that Red Grouper have strong effects on a wide range of species which together shape the ecosystems they manipulate.