Reassessing the role of herbivores on urban coral reefs: A case study from a heavily impacted reef near Cartagena Bay, Colombia

Coral reefs are beginning to experience conditions unlike any in recent history. Understanding ecosystem function on future reefs will require reassessing ecological processes under novel environmental regimes. For many coastal reefs, severely degraded water quality will be a hallmark of these novel regimes. While herbivory has traditionally been considered essential for maintaining coral dominance, recent evidence from urban reefs suggests this pattern may be changing. Here, we reexamined the impacts of herbivores on a shallow, turbid reef exposed to extensive coastal development. We found that although herbivore biomass, size-structure, and grazing rates were significantly reduced relative to a nearby protected reef, coral cover on this shallow urban reef remained > 45%. In contrast, coral cover at the nearby protected site was roughly 50% lower. Differences in coral cover between the sites were due to greater cover of two groups of corals at the urban site: depth-generalist Orbicella spp., particularly O. faveolata, and Agaricia spp. with weedy life-history characteristics. Both groups are tolerant of low light but susceptible to coral bleaching. Our results suggest that diminished top-down pressure did not promote algal dominance. Instead, turbidity-induced reductions in available light drove community structure, leading to dominance of coral and algae species able to acclimate to low-light. Our study demonstrates how environmental context can alter the importance of critical processes on coral reefs and highlights the need to reexamine traditional paradigms in reef ecology to understand ecosystem function on future reefs.

Automated summary

Coral reefs are facing unprecedented conditions, with degraded water quality becoming a common characteristic. Recent evidence from urban reefs suggests that the role of herbivores in maintaining coral dominance may be changing. A study on a turbid urban reef found that although herbivore biomass, size-structure, and grazing rates were reduced, coral cover remained high compared to a nearby protected reef. This was due to the dominance of corals tolerant of low light conditions. The study highlights the importance of considering the environmental context in understanding ecosystem function on future reefs.