Coral reefs are fraught with danger for herbivores such as damselfish and tangs. Venturing out from the safety of the reef’s colorful cracks and crevices to feed means risking being devoured by predators that patrol the warm waters. As a result, the small fish tend to stay close to the reef when grazing.
Marine biologists have long recognized this cautious behavior, commonly documenting “halos” of bare sand stripped clean of seaweed around coral reefs. Now, using publicly available satellite images from Google Earth, researchers have for the first time observed these halos from space, providing insights into the size, frequency, and distribution of these underwater sand spots. They report their findings in the premiere issue of Nature Publishing Group’s new open access journal, Scientific Reports.
“This is a very imaginative use of remote imagery,” said Robert Warner, evolutionary ecologist at University of California, Santa Barbara, who was not involved with the study. “The halos have been studied before, but never before have we been able to ascertain just how common they are." This information can yield valuable insight into how predators shape the feeding patterns of reef herbivores, he added. "Imagine seeing the effects of marine predators from space!”
It was bad luck that first led postdoctoral researcher Elizabeth Madin and her husband—authors of the paper—to this project. Stormy weather had left the couple landlocked while on a field trip to Australia’s Heron Island for an unrelated project. With nothing to do, they decided to look for grazing halos in the island’s shallow lagoon using Google Earth. Once they had identified spots that had the telltale signs of a halo, they obtained their GPS coordinates and set out on foot to investigate.
Sure enough, the light blue rims surrounding the reefs that they had observed in the satellite images were indeed bare patches of sand devoid of sea grass. “Seaweeds increased dramatically the farther away from the reef we got,” Madin explained.
But to corroborate that the halos were caused by herbivores under fear of predation, the researchers transferred clumps of seaweeds from farther out in the lagoon into the bare areas. “When we came back three days later, they had been eaten down in the same exact pattern.”
Although the findings are still preliminary, Madin said, “it is the first step in the quest to understand the large-scale significance of these halos.” Madin is currently working on a World Wildlife Fund-funded project that aims to investigate how freely available satellite images of grazing halos can help marine biologists and conservationists monitor the health of coral reef ecosystems around the world.
E.M.P. Madin, et. al., "Landscape of fear visible from space," Scientific Reports, doi:10.1038/srep00014, 2011.