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It’s hardly news that the world’s corals are in trouble. Distressed by warmer and more acidic water and beset by pollution and development, many reefs have shifted from bountiful, diverse technicolor displays into drab expanses of uniform algae. On the Ulithi atoll, in the Federated States of Micronesia, however, some reefs are struggling with a different kind of ecological shift: getting smothered by a single type of weedy coral.

A healthy coral reef is full of life. Reef-building corals—the engineers of myriad underwater structures—create maritime megalopolises dense with crevices and hidey-holes for fish and other sea creatures. But when one type of coral replaces the usual mix of coral species, the abundance and diversity of life can disappear. That’s what happened to some of the coral reefs around the Ulithi atoll, which has a population of about 1,000 people, and where a fast-growing species of Montipora coral, known as rice coral, has been smothering other species. The result is an eerie terrain that, while still flush with thriving coral, is otherwise largely devoid of life.

“People started to say that they can’t fish anymore, that the triggerfish disappeared because this coral seems to be covering up all the holes,” says Magul Rulmal, an Ulithi local working with One People One Reef, a Micronesian organization dedicated to protecting coral reefs and the people who rely on them.

The octopus—a delicacy among locals—is also notably absent from the affected reefs, and the fish that still swim among the cabbagelike sheets of rice coral are much smaller than their counterparts elsewhere around the atoll.

Locals’ lives and the health of the reefs are intertwined. For thousands of years, Ulithi residents have tended nearby reefs with care and relied on them as a crucial source of food. Over the past few decades, however, as some of the reefs transformed into rice coral monocultures, a decline in catches prompted Ulithi residents to ask scientists for help. In particular, they wanted a better understanding of how and why this coral has been outcompeting all the other species on these reefs.

By collecting coral samples from different reefs around the atoll and analyzing their genetics, the researchers investigated the expansion of rice coral. Contrary to their expectations, the scientists found no obvious signs of people driving the coral’s spread—such as by physically breaking pieces off parts of the reef and transporting them around.

The outbreak is probably due to a combination of environmental and human factors, says Michelle Paddack, a conservation scientist who coauthored the study, and a board member for One People One Reef. “It might be that the new environment that we’ve created in our oceans favors this coral.”

Despite how placid corals appear, in reality they’re constantly competing with each other, explains Giacomo Bernardi, a molecular ecologist at the University of California, Santa Cruz, and the lead author of the new study. Once one species has an advantage—for instance, being more resilient against warming water, acidification, or different fishing practices—it will outcompete other species, Bernardi says. “It’s going to overgrow the other ones.”

The unfortunate example of Ulithi’s overgrown reefs serves as a warning. Many coral restoration efforts around the world focus on highly prolific and resilient “supercorals” better adapted to hot water. That approach makes intuitive sense in a warming world where many reefs are under threat. However, the researchers are careful to highlight the risks posed by a coral that is too successful.

“There’s a big push to do coral restoration today because we’ve messed up reefs so much,” says Bernardi. “What we found in this study is that, well, this is great if you have the right coral.” But when the reef turns into a rice coral monoculture, “it’s terrible for the reef.”

Gergely Torda, an adjunct reef ecologist at James Cook University in Australia who was not involved in the research, echoes Bernardi’s worries. “It is a scary concept,” he says. “If you give one species an ecological advantage, it will overgrow the others—this is what weeds are.”