Ecological effects of digging behavior by red grouper (Epinephelus morio) in Florida Bay

Background/Question/Methods

Red grouper (Epinephelus morio) is an ecological engineer: it excavates benthic habitat which creates shelter for a suite of organisms, including valuable fishery species (e.g. black grouper, spiny lobster). In Florida Bay red grouper remove sediment and detritus from existing limestone solution holes, which function as important habitats for juvenile reef fishes and macroinvertebrates. Engineer species are expected to interact with other species in multiple ways, but the primary focus of previous research has been on the indirect interactions that result from engineering activities. To date few studies have looked at the direct interactions between engineer species and cohabitants. Here I report the results of a multi-year study done to determine how red grouper interacts both directly and indirectly with the species that colonize excavated solution holes in Florida Bay. From 2010 – 2012 divers conducted an annual census of the fish and motile invertebrate communities associated with solution holes to determine the rate of occupancy by red grouper and the variability in solution-hole-associated communities. In 2011 and 2012 red grouper were removed from a subset of these holes and the communities were observed for 1-month to determine short-term changes to these communities attributable to the presence of red grouper.   

Results/Conclusions

Inter-annual observations showed that for a given year, 56.8% of the solution holes in the study area were occupied by red grouper. On average, communities associated with occupied holes had significantly more species (9.66 ± 0.223 versus 5.28 ± 0.248) and more individuals (75.9 ± 4.62 versus 22.5 ± 2.47) compared to communities associated with unoccupied holes. Short-term removal experiments suggest that the presence of the red grouper affects only part of the total species pool. The largest effect of removing a red grouper was seen in the juvenile reef fish complex (9 species, 50.9% reduction in recruitment), cleaning shrimp (2 species, 45.8% increase), clinging crabs (4 species, 71.3% increase), and the Caribbean spiny lobster (Panulirus argus, 34.2% decrease). These four groups, representing 34% of the total species pool, accounted for over 90% of the total measured change in individual species abundance over the course of the experiment. Combining long-term observations and short-term manipulative experiments will ultimately lead to a better understanding of these communities and their relationship to the engineer species.