When you think of rats (Rattus rattus), you might think of household pests or domestic pets. Rats are not creatures easily associated with coral reefs, but that is about to change.

Ratty Islands

Non-native rats that invade islands can cause a myriad of problems to the ecosystems they invade. Rats spread incredibly easily to islands and new habitats, as they are accidentally transported across oceans by ships and vessels. This has resulted in the introduction of rats to many islands with sensitive ecosystems and unique biodiversity.

Due to their omnivorous diets, rats are able to eat everything, which includes the eggs of nesting birds, the chicks, and even adult birds. It is estimated that non-native rat introductions have resulted in a 90% reduction in seabird populations on the world’s temperate and tropical island groups, and the impact of rats are now extending to surrounding coral reefs.

To determine the exact impact that rats have on the avian biodiversity and surrounding reef ecosystems of islands, scientists from UWA, Lancaster University, ARC Centre of Excellence for Coral Reef Studies and Dalhousie University studied the rats on the northern Chagos Islands – a group of remote tropical islands in the central Indian Ocean that provide habitat for frigate birds, noddies, boobies, terns and shearwaters. 

These islands were inhabited by humans in the 1800s, but since then humans have left the islands, leaving only rodents behind. The scientists compared six islands infested with rats with six neighboring islands where no rats, or humans, ever existed and found clear evidence that rats were not only bad news for the seabird populations, but also for the surrounding reef ecosystems. On the Chagos Islands, rats have decimated native seabird populations by feeding on the eggs and chicks, whereas the islands absent of rats had flourishing seabird populations.

Why Does This Matter?

Coral reefs make up less than 0.1 percent of the ocean’s surface area, but they are home to one-third of the ocean’s biodiversity. Coral reefs are now in grave danger, so anyone concerned about extinction and biodiversity should be concerned about their future. Millions of people around the world depend on coral reefs and their wealth of marine life for their livelihoods, so the loss of coral reefs is poised to become a humanitarian crisis.
Although adding rats to the growing list of threats to reefs can seem discouraging, the discovery of these negative consequences points to a particular strategy that could help reefs delay their decline.

A Series of Unfortunate Events

The impacts of rats on coral reefs come down to one substance – poo. Islands with fewer birds also have less guano – nitrogen-rich seabird excrement. This nitrogen is obtained from food that the birds consume during foraging trips over the ocean to waters that are 100 to 100 000 times more productive than waters in the immediate vicinity of the breeding islands. Fewer seabirds results in less guano, which means less nitrogen and other nutrients are being taken up in the soil. 

During rain events, soil nutrients leach out into the sea, where it provides food for marine organisms, such as algae, filter-feeders and fish. As a result, the marine biodiversity around islands with seabirds are richer than around islands without seabird populations, due to an abundance of nutrients in the water.

Fish play an important part in maintaining coral reef health, as well as assisting reefs to recover from disturbances. Herbivorous (algae-eating) fish are particularly important for coral reef health, as their grazing prevents algae from overcrowding and killing corals.

One of the greatest effects of the rats was a 760-fold decline in the number of nesting seabirds per hectare on the rat-infested islands compared to the islands absent of rats, and therefore fewer nutrients and nitrogen in the islands’ soil. The scientists found that reefs near the rat-infested islands also have fewer nutrients, fewer fish, and therefore reduced numbers of fish grazing on the seaweed and algae. 

The total biomass of the fish population was 48% higher around the rat-free islands than the islands infested with rats. They also found that the abundance of algae-eating fish was negatively affected by the presence of rats on the islands. Grazing of algae by fish – an important function where fish consume algae and dead coral, providing a base for new coral growth – was 3.2 times higher in the waters surrounding rat-free islands. This is a direct result of the abundance of seabirds on the rat-free islands.

Plans For The Future

The dramatic effects of the study of the rats of Chagos Archipelago provide a comprehensive picture of how coral reefs are interconnected with both land- and seascapes. The movement of animals and other organisms around these habitats generates genetic connections between regions. In marine conservation, a network of marine protected areas should take into account this genetic connectivity between coral reefs and the surrounding environment.

Coral reefs cover less than 0.1% of the ocean’s area, but house one third of ocean biodiversity. Coral reefs have historically been threatened by overfishing, however one of the most recent major threats to reefs is climate change, which results in mass coral bleaching and the eventual death of corals.

The findings of the study provide convincing evidence that rats are detrimental to not only the islands’ bird populations, but also for the fish and coral reefs surrounding the islands. Adding rats to the list of threats to corals may seem disheartening, however it does provide a strategic plan to slow the pace of reef degradations. Rats have successfully been eradicated from hundreds of islands, which has resulted in beneficial effects on the biodiversity of those islands.

There is thus an urgent, yet controversial, need to eradicate rats from islands – not only the Chagos islands, but all islands harboring seabird colonies. Unfortunately, sometimes conservation can be a bloody business, and doing the right thing for a sensitive ecosystem means the need to kill invasive species. Lessons learned from those islands where rats have successfully been eradicated can be extended to islands in the Chagos Archipelago. 

Rats are eradicated from islands by dropping poison-laced pellets from helicopters and by hand. The poison is designed to not bioaccumulate, thereby only ridding the islands of rats and not harming other organisms. Therefore, the eradication of rats will not harm the surrounding island biodiversity, and will lead to increased numbers of seabirds, thereby bolstering the coral reefs.