New research has revealed that parrotfish may be the key to maintaining the stability of coral reef habitats and preventing them from transforming into a stable macroalgal-dominated condition following disturbance.
The results are published in the most recent issue of the journal Nature by Peter Mumby, Alan Hastings and Helen Edwards.
The authors used a simulation model to predict the effects of various perturbations to the system, including the effects of hurricanes, as well as the removal of herbivorous sea urchin on coral cover and macroalgal abundance on the complex forereef habitats of the Caribbean.
Macroalgae (seaweed) proliferate if dead coral is not sufficiently grazed and are capable of either arresting coral growth or overgrowing living coral.
The authors found that the macroalgae-dominated community is an alternative stable state of the ecosystem and one that is not readily reversible.
The simulation models showed that the Caribbean reefs became susceptible to this alternative stable state once the sea urchins were removed (as what actually happened in 1983, when a mass disease-induced mortality event led to the removal of the herbivorous urchin Diadema antillarum from the ecosystem), as parrotfishes became the sole grazers.
The role of parrotfishes in maintaining reef stability has implications for their conservation (parrotfishes are often fished for food).
According to the authors, An unexploited community of parrotfishes can maintain approximately 40% of the reef in a permanently grazed state but overfishing reduces this capacity to about 5%...
For more information, see the paper: Mumby, PJ, A Hastings and HJ Edwards (2007) Thresholds and the resilience of Caribbean coral reefs. Nature 450, pp. 98"101.