The unique traits of large animals often allow them to fulfill functional roles in ecosystems that small animals cannot. However, large animals are also at greater risk from human activities. Thus, it is critical to understand how losing large animals impacts ecosystem function. In the oceans, selective fishing for large animals alters the demographics and size-structure of numerous species. While the community-wide impacts of losing large animals is a major theme in terrestrial research, the ecological consequences of removing large animals from marine ecosystems remain understudied. Here, we combine survey data from 282 sites across the Caribbean with a field experiment to investigate how altering the size-structure of parrotfish populations impacts coral reef communities. We show that Caribbean-wide, parrotfish populations are skewed towards smaller individuals, with fishes <11 cm in length comprising nearly 70% of the population in the most heavily fished locations versus ~25% at minimally fished sites. Despite these differences in size-structure, sites had similar overall parrotfish biomass. As a result, algal cover was unrelated to parrotfish biomass and instead, was negatively correlated with the density of large parrotfishes. To mechanistically explore how large parrotfishes shape benthic communities, we manipulated fishes’ access to the benthos to create three distinct fish communities with different size-structure. We found that excluding large or large and medium-sized parrotfishes did not alter overall parrotfish grazing rates but caused respective 4- and 10-fold increases in algal biomass. Unexpectedly, branching corals benefited from excluding large parrotfishes whereas the growth of mounding coral species was impaired. Similarly, removing large parrotfishes led to unexpected increases in coral recruitment that were absent when both large and medium bodied fishes were excluded. Our data highlight the unique roles of large parrotfishes in driving benthic dynamics on coral reefs and suggests that diversity of size is an important component of how herbivore diversity impacts ecosystem function on reefs. This study adds to a growing body of literature revealing the ecological ramifications of removing large animals from ecosystems and sheds new light on how fishing down the size-structure of parrotfish populations alters functional diversity to reshape benthic reef communities.

Data collected from manipulative experiments conducted in the Florida Keys. Includes data on algal diversity in fish exclosure plots, reported as total number of unique macroalgal species in each plot. Coral growth data, measured over the course of 14 months. Benthic cover, calculated as the percent cover of canopy and benthos for specific algal groups, and feeding data, presented as the sum of bites taken by scarids and acnathurids of different size classes in exlcosure treatments throughout the experiment.