Overfishing and the ecological impacts of extirpating large parrotfish from Caribbean coral reefs

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.