Reef building corals rely on herbivore animals to continuously remove unwanted algae growth from them, since algae compete with the corals for both sunlight and nutrients. Without regular cleaning, corals eventually die and the reef becomes overgrown by various types of algae. A report scheduled to be published this week in the early edition of the journal Proceedings of the National Academy of Sciences now suggests that having herbivore animals present on the reef isn’t enough; there must also be a proper balance between the various species. This conclusion results from a long-term study on coral reef recovery and seaweed[1] carried out by Dr. Mark Hay, the Harry and Linda Teasley Professor of Biology at the Georgia Institute of Technology, and his co-author Dr. Deron Burkepile who is now Assistant Professor at the Florida International University’s Marine Science Program.

Different fish feed on different algae and maintaining a proper balance may therefore be critical. “Of the many different fish that are part of coral ecosystems, there may be a small number of species that are really critical for keeping big seaweeds from over-growing and killing corals,” says Hay. “Our study shows that in addition to having enough herbivores, coral ecosystems also need the right mix of species to overcome the different defensive tactics of the seaweeds. This could offer one more approach to resource managers. If ecosystems were managed for critical mixes of herbivorous species, we might see more rapid recovery of the reefs.”

Coral reef

The 10 month long study was carried out 18 metres (60 feet) below the surface off the coast of Florida, where Hay and Burkepile placed 32 cages on a coral reef in November 2003. At this point, the coral reef area chosen by the researchers had only four to five percent live coral coverage. Each cage was roughly two metres square and one metre tall (1 metre = 3.3 feet) and the mesh was fine enough to prevent large fish from entering or leaving the cage. The scientists then carefully selected the number and type of fish to place in each cage, using the four following combinations:

· Two fish capable of eating hard, calcified plants

· Two fish capable of eating eat soft plants that defends themselves chemically

· Both types of fish.

· No fish at all

The two species used for the experiement where the Redband parrotfish (Sparisoma aurofrenatum) and the Ocean surgeonfish (Acanthurus bahianus).

As suspected, the type of fish turned out to play a key role in the growth of algae and seaweed on the reef.

“For the cages in which we mixed the two species of herbivores, the fish were able to remove much more of the upright seaweeds, and the corals in those areas increased in cover by more than 20 percent during ten months,” says Hay. “That is a dramatic rate of increase for a Caribbean reef.”

Areas with only one type of fish or no fish at all lost as much as 30 percent of their live coral coverage during the research, while areas with two species of fish increased their live coral coverage from four to five percent to six to seven percent.

“Species diversity is critically important, but we are losing critical components of the Earth’s ecosystem at an alarming rate,” says Hay. “There has been little work on the role of diversity among consumers and the effect that has on communities. This study will help add to our knowledge in this critical area.”

After the initial 10-month experiment, Hay and Burkepile launched a second study where the Ocean surgeonfish (Acanthurus bahianus) was substituted with Princess parrotfish (Scarus taeniopterus). Unfortunately, the cages only stayed on the reef for seven months before being wiped away by Hurricane Dennis in July 2005.

The research was conducted at the National Undersea Research Center in Key Largo, Florida and supported by the National Oceanic and Atmospheric Administration, the National Science Foundation and the Teasley Endowment at Georgia Tech.

You can read more about Hay’s and Burkepile’s work at

http://www.biology.gatech.edu/faculty/mark-hay/ http://www.biology.gatech.edu/faculty/mark-hay/lab.php
http://www.fiu.edu/~dburkepi/front.htm
http://www.fiu.edu/~dburkepi/research.htm