The impact of size-selective herbivore removal on a Caribbean coral reef benthic community

Background/Question/Methods: Animal size can be important in structuring consumer-resource interactions. Both within- and between species, size can shape species’ access to resources, nutritional requirements, and functional role in the environment. However, manipulative studies exploring the impacts of herbivores on coral reef communities tend to treat herbivory as a binary process in which herbivores are either present or absent when in reality, large gradients in grazing pressure exist. Because top-down pressure from herbivores is vital for shaping coral reef communities, understanding how the size-structure of herbivorous fish populations impacts benthic reef communities is essential for appropriately managing reef fish fisheries. To help answer this question, we experimentally manipulated size-based access to the benthos on a reef in the Florida Keys for 14 months. To do so, we deployed large cages with openings of varied sizes cut into their mesh to create three treatments: (i) Full exclosures that only admitted fishes whose body height did not exceed the mesh size; (ii) Partial exclosures with 7.5 cm diameter holes to exclude large fishes only; and (iii) Control plots with 50cm x 50cm panels removed.

Results/Conclusions: Our treatments successfully simulated three distinct, size-structured parrotfish communities. Grazing within Control plots did not differ from other areas on the reef, whereas no parrotfishes >25cm length fed in our Partial Exclosures and no parrotfishes >15cm fed in our Full exclosures. While overall bite rates did not differ between the three treatments, significant differences in consumer size led to the development of distinct benthic communities under each grazing regime. Algal biomass increased by ~4 and ~10-fold in Partial and Full exclosures relative to Control plots. The percent cover of algae followed a similar pattern but the diversity and dominance of species differed. Partial exclosures were dominated by articulated calcareous algae whereas Full exclosures were dominated by large filamentous and foliose species. Unexpectedly, coral recruitment was highest in our Partial exclosures while coral growth and mortality depended on an interaction between colony morphology and the size of parrotfishes that could access the cages. Overall, our study reveals the loss of herbivore functional diversity as grazer size is limited. Because fishing is a highly selective process, our study will help managers decide on gear and size-based fishing restrictions that allow maximum harvest without impairing the top-down control of reefs.