Reducing consumer pressure increases community dissimilarity leading to coral- or algal-dominance on a coral reef

In many ecosystems, anthropogenic influences are altering the strength of top-down (e.g., herbivory and predation by consumers) and bottom-up (e.g., nutrients) processes through the exploitation of consumers and increasing anthropogenic nutrient inputs. Altered top-down and bottom-up processes may significantly change ecological communities, resulting in shifts in the abundance or composition of primary producers, or shifts in ecosystem states. An often-overlooked consequence of altered top-down and bottom-up process is how these forces shape community variability, especially following disturbances when alterations to top-down and bottom-up processes may have particularly strong effects during community reassembly. Here, we experimentally test the relative importance of top-down and bottom-up processes in determining community development after a simulated disturbance on a coral reef in Moorea, French Polynesia. We established eight 30-m² plots in which corals and macroalgae were removed to mimic the effects of a severe cyclone. Within each plot, we nested four consumer exclosures (~1.25 m² each) with different size openings that allowed different size fishes access to the benthos, creating four levels of consumer pressure (Very Low, Low, Medium, High). Each plot was then assigned to either Ambient or Enriched nutrient conditions. Consumer pressure strongly affected community development, whereas nutrients had a weaker, but significant, effect. After 4 years, consumer pressure was inversely related to beta diversity, where plots with High consumer pressure had highly similar communities, and plots with reduced consumer pressure had dissimilar communities. This dissimilarity in community structure under reduced consumer pressure was the result of plots becoming rapidly colonized by either coral or macroalgae. By contrast, community development in plots with High consumer pressure was slow but predictable, with all plots slowly increasing in coral cover and remaining relatively free of macroalgae. Our results suggest that reduced top-down pressure by consumers can result in alternate community paths that lead to either coral- or algal-dominance under similar environmental conditions. We conclude that consumer pressure influences both the rate and predictability of community development in these ecosystems and that human stressors that alter consumer pressure may result in ecological surprises on coral reefs.