19 - Phase shifts and coral reef fishes

Summary

Phase shifts, defined as major changes in the composition of a community, have mainly been assessed for the benthos of coral reefs, with scant attention to the consequences for reef fish assemblages. A greater number of species of fish from more families feed on or shelter within live coral as compared to macroalgae (a common alternative benthic organism). Survey and experimental assessments have demonstrated a limited role of softcoral-dominated substrates for reef fishes. Furthermore, surveys have demonstrated more species of juvenile reef fishes, a greater density, diversity and abundance of most feeding groups of fishes and a more complex size structure of fish assemblages associated with live coral habitats than macroalgal habitats. Further, phase shifts in crypotobenthic reef fish assemblages have been observed to persist following reversal back to a coral-dominated benthos. The evidence suggests that coral-dominated reefs usually provide more diverse and economically valuable fisheries, even though some abundant fish species on macroalgal-dominated reefs have fast life histories and can sustain heavily exploited fisheries. Much more research is necessary to fully understand the stabilization of reef fish assemblages on reefs of different state, how ecosystem processes are affected, and how ecosystem services are likely to change.

PHASE SHIFTS ON CORAL REEFS

Phase shifts refer to a major change of composition in a community, such that a different suite of organisms come to dominate [2270]. Such shifts can be temporary, or relatively stable if a new set of feedbacks and attractors operate to maintain the system in a new configuration [1196]. Evidence for alternate attractors and thus long-term persistent phase shifts is the topic of some debate, with most evidence currently from Caribbean reefs [1784,2159]. Whether permanent or temporary, phase shifts have now been well documented in most reef regions of the world [e.g. 442,1439], and have been the topic of much research attention since the early 1990s following the publication of several key articles [701,1190,1359]. Indeed, coral reefs are used as an example ecosystem where such shifts are well documented [2270].