Jobs, ecotourism and diving industries can benefit from having a diverse supply of weed-eating fish on the world’s coral reefs, scientists say.

Despite their small size, relative to the sharks, whales, and turtles that often get more attention, herbivorous fish play a vital role in maintaining the health of coral reefs, which support the livelihoods of 500 million people worldwide, according to a study published this month in the journal Ecology.

“Herbivorous fishes protect coral reefs by limiting the growth of algae, or seaweed,” says Loïc Thibaut, lead researcher of the study. “Seaweeds grow rapidly and compete with corals for space. If left unchecked, they can smother the corals and take over the reefs. This shift, once it happens, is extremely difficult to reverse.”

The study shows that having high biodiversity of herbivorous fishes provides strong “insurance” for coral reefs. A diverse set of herbivores ensures that seaweeds are kept under control, because when some fish species decline, others take their place. This makes seaweed control more stable over time, something researchers call the “portfolio effect”.

“It’s like having a diverse stock portfolio – you wouldn’t put all your money into one particular stock, because if that company goes down, so will your life savings,” says Professor Sean Connolly, a Chief Investigator at the Centre. “A very similar principle works in ecosystems.”

An example of the disastrous effects of having only one herbivore as ‘gatekeeper’ is the extensive coral loss in the Caribbean in the 1980s.

“In the 80s, overfishing left a species of sea urchin as the only animal controlling seaweed growth on Caribbean reefs. When a disease broke out, the sea urchin population collapsed – and there was nothing to keep the weeds in check. This was followed by an explosion of seaweeds, which smothered the coral and hit tourism pretty hard,” says Prof. Connolly.

“There are three main groups of herbivorous fishes: territorial grazers that bite at the algae and are site-attached and actively defend a small patch of reef; roving grazers that feed in the same way but move around the reefs; and scrapers who range widely and feed by biting the algae back to the limestone surface of the reef, making clear patches where corals can establish. All are critically important,” says study co-author Dr Hugh Sweatman, a Senior Research Scientist at the Australian Institute of Marine Science.

“The more diversity you have, the lower the risk that all the fish that play a particular role on the reef – like controlling seaweeds – will crash at the same time. This is greatly beneficial for the health of any type of ecosystem. And it is also beneficial to the people whose jobs and livelihoods depend on that system,” says Loïc Thibaut.

Further Reading:
Thibaut, Loïc M., Sean R. Connolly, and Hugh P. A. Sweatman. 2012. Diversity and stability of herbivorous fishes on coral reefs. Ecology 93:891–901. http://dx.doi.org/10.1890/11-1753.1