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John F. Bruno

John F. Bruno

Posted: February 16, 2011 04:26 PM

One of the many factors causing the global loss of reef building corals is anthropogenic climate change, which is slowly warming the world's oceans. When summertime temperatures are warmer than usual, corals can die from "bleaching" and disease outbreaks. This in turn is devastating for the countless organisms that inhabit coral reefs.

A new paper (Graham et al 2011) by an international team of marine scientists describes a framework to predict and measure the impacts of coral loss on fish populations. They created an "extinction risk index" for reef fish, based on the habitat requirements, feeding ecology and other traits of each species. For example, obligate corallivores, fish that feed on corals such as butterflyfish, were scored as highly sensitive. In all, 56 of the 134 coral fish species studied were found to be at risk from loss of their habitat, shelter and food sources caused by climate change. Those most in jeopardy were the smaller fishes with specialized eating and sheltering habits. One of the interesting findings from this exercise is that fish predicted to be vulnerable to climate change are not vulnerable to over-harvesting, and vice versa.

The team then tested their model by comparing its predictions to documented fish extinctions caused by a massive coral bleaching event that occurred in 1998 in the Indian ocean in which whole landscapes of corals died off. The fish species predicted to be the most sensitive to such habitat degradation experienced the greatest population losses.

Taken broadly, their results indicate that more than a third of coral reef fish species are in jeopardy of local extinction from the impacts of climate change.
"The loss of particular species can have a critical effect on the stability of an entire ecosystem - and our ability to look after coral reefs depends on being able to predict which species or groups of fish are most at risk," explains lead author Dr. Nick Graham of the ARC Centre of Excellence in Coral Reef Studies and James Cook University. "Until now, the ability to do this has been fairly weak."

"Where there is a widespread death of corals from a climate-driven event such as bleaching, the fish most affected are the ones that feed or shelter almost exclusively on coral."
The study does, however, offer encouragement by showing that the fish most at risk from climate change are seldom those most at risk from overfishing or other direct human impacts, pointing to scope to manage reef systems and fishing effort in ways that will protect a desirable mix of fish species that promote ecosystem stability.  

Dr. Tim McClanahan of the Wildlife Conservation Society explains:

Critically, the species of fish that are important in controlling seaweeds and outbreaks of deleterious invertebrate species are more vulnerable to fishing than they are to climate change disturbances on coral reefs. This is encouraging, since local and regional commitment to fisheries management action can promote coral recovery between disturbances such as storms and coral bleaching events.

One thing I really like about the paper is that it focuses on the impacts of habitat loss on reef inhabitants. Corals are dying in droves from climate change and local impacts like runoff from coastal deforestation, but reef scientists are not worried about coral extinctions. It is reef ecosystems that we are going to lose along with all the ecological splendor and economic value that is based on them.
Their paper, "Extinction vulnerability of coral reef fishes," by Nicholas A. J. Graham, Pascale Chabanet, Richard D. Evans, Simon Jennings , Yves Letourneur, M. Aaron MacNeil, Tim R. McClanahan, Marcus C. Öhman, Nicholas V. C. Polunin and Shaun K. Wilson, appears in the latest issue of the journal Ecology Letters.