Communities worldwide are experiencing anthropogenic disturbances often causing shifts to degraded states. When a community’s resilience, defined as resistance to and recovery from disturbance, is exceeded it can shift to a degraded state. For effective management, it is critical to know whether these shifts are gradual, threshold responses, or bifurcations indicating alternative stable states (ASS). Overfishing is an anthropogenic disturbance that can contribute to shifts from healthy coral reefs to degraded algal-dominated states. While herbivorous fish are known to promote healthy reefs because they consume algae, the functional diversity of herbivorous fish is often overlooked in models testing for ASS. My objective was to incorporate herbivore and algal functional groups into a coral reef model to determine how this important ecological structure affects model predictions of the impact of fishing pressure on coral reef health and whether overfishing can cause ASS. I expanded a previous coral reef model to incorporate three herbivore (grazers, browsers, generalists) and two algal (turf, macroalgae) functional groups. I examined coral reef health (final coral cover) and resilience (bistability and hysteresis) under different levels of fishing, different initial reef states (coral vs. algal-dominated), and different herbivorous fish communities (dominated by grazers, browsers, or generalists).
Results/Conclusions
My model predicts that communities with functional complementarity support healthy coral reefs, but as fishing pressure increases, functional redundancy becomes crucial to maintaining reef health. On healthy reefs, algal community composition has little effect on reef health, regardless of fishing pressure. However, on degraded reefs (with high macroalgal cover and skewed herbivore community), reef recovery is affected by the abundance of different algal functional groups. Finally, herbivore community composition affects which fishing pressure conditions lead to bistability and hysteresis. Specifically, my model predicts a reef community can exhibit bistability and hysteresis even with no overfishing when the herbivore community is skewed towards grazers. Overall, incorporating functional groups into models provides insights about which functional groups are more important for healthy vs. degraded community states. Thus, it is important to incorporate functional groups into models when examining community responses to disturbance because ignoring functional groups can mask conditions resulting in bistability and hysteresis. To effectively manage communities in response to disturbance and to prevent shifts to degraded states, it is critical to know what conditions lead to ASS, which can be shaped by the functional roles of species present in the system.