Incorporation of coral and macroalgal dispersal adds novel stable states

Ariel Greiner, Marie-Josée Fortin, Emily S. Darling and Martin Krkosek, Ecology & Evolutionary Biology, University of Toronto, Toronto, ON, Canada, Emily S. Darling, Wildlife Conservation Society, Washington, DC

Background/Question/Methods
Coral reefs are thought to exhibit alternative stable states of coral versus macroalgal dominance. Previous studies have found that the stability of these states is linked with herbivore grazing rates--only the coral-dominated state is stable on reefs with high grazing rates, only the macroalgal-dominated state is stable on reefs with low grazing rates and at intermediate grazing rates both states are stable (bistable system) and separated by an unstable intermediate state. It is unclear if and how dispersal of coral or macroalgae among reefs might impact this bistability and the resilience of the coral-dominated state. Here, we design a mathematical model to investigate how the dynamics of coral and macroalgal cover are affected by (1) coral and macroalgal dispersal and (2) heterogeneous abundance of grazers.
Results/Conclusions
We find that at low levels of coral and macroalgal dispersal, a new stable state emerges at which both coral and macroalgae are present. Furthermore, we show that a reef abundant with coral and grazers can also support a coral-dominated stable state in a second reef depauperate of grazers via dispersal of coral and macroalgal larvae. These results help explain previous empirical findings on reefs that were once thought to be incongruent with traditional coral-macroalgal alternative stable states theory—such as intermediate coral and macroalgal biomass states and high macroalgae-high grazing and high coral-low grazing scenarios. Our findings indicate that changing dispersal levels (e.g. due to climate change, reef degradation) between reefs changes the possible stable states and the grazing rate at which the coral-dominated state is stable. Thus demonstrating the importance of considering dispersal among coral reefs or other bistable ecosystems when designing conservation management plans.