Rats and Reefs: What Happens On Land Doesn’t Stay On Land

Rats and reefs on tropical islands are a bad combination.
Underwater habitats – that is, coral reefs – fare much better in the absence of rat populations on land.

A NEWLY PUBLISHED STUDY ON THE LINK BETWEEN RATS AND REEFS has found a substantial link to the health of the coral reefs in the waters around the islands. 

The reason: Seabird poop on land is good for a broad range of reef denizens underwater. And the rats kill off seabirds, whose guano provides nutrients that enhance the reef’s health.

An international team of scientists studied the ecosystems of rat-infested and rat-free islands in the Chagos Archipelago in the Indian Ocean. They reported their findings in the journal Nature.


“Herbivorous damselfish on reefs adjacent to the rat-free islands grew faster, and fish communities had higher biomass across tropic feeding groups, with 48% greater overall biomass,” the study says.

“Rates of two critical ecosystem functions, grazing and bioerosion,” they added, “were 3.2 and 3.8 times higher, respectively, adjacent to rat-free islands.”

To foster the health of reefs surrounding tropical islands, it’s important to exterminate the rats’ invasive populations, the researchers say.

The hope is that more resilient reefs would deal with climate change better. They’re urging governments to consider pest control as a strategy to protect corals.

Rats and reefs on tropical islands are a bad combination.
This four-minute video describes the study with both people and graphics.


The Chagos Archipelago is a group of some 60 islands about 300 miles south of the Maldives. Some of the islands are infested with wild rats thought to have arrived there on ships hundreds of years ago. Others have evaded rat invasions and are rat-free.

The researchers were able to study the rats and reefs connection by comparing the ecological health of six rat-free islands with six rat-infested islands.

Invasive rats, which feed on eggs, chicks and adult birds, are believed to have decimated seabird populations on as many as 90 percent of temperate and tropical islands worldwide.

Rats and reefs on tropical islands are a bad combination.
Looks cute, but wild rats like this guy can wreak havoc on seabird populations on tropical islands, with consequences for surrounding reefs.


A separate study in the online journal PLOS ONE focused on rats’ impact on native trees at Palmyra Atoll, south of Hawaii. That study found that counts of seedlings for native tree species jumped from under 150 to more than 7,700 within five years of rat elimination.

It’s not just about the trees. The islands’ rainforests, of course, constitute important habitat for nesting seabirds, crabs, insects and other species.

PRINCIPAL SOURCES:  “Seabirds enhance coral reef productivity and functioning in the absence of invasive rat,” Nature; “Of Rats and Reefs: How rodents are harming tropical coral,” Nature Video; “Eradicate rats to bolster coral reefs,” Science Daily; “5,000 percent increase in native trees on rat-free Palmyra Atoll,” Science Daily.