Coastal Management Invasive Species

Rat infestations: how rats affect the ocean

Gunn, R. L., Benkwitt, C. E., Graham, N. A. J., Hartley, I. R., Algar, A. C., & Keith, S. A. (2023). Terrestrial invasive species alter marine vertebrate behaviour. Nature Ecology & Evolution, 7(1), Article 1.

Rat’s Island Infestation

The black rat (Rattus rattus) is also known as the ship rat. It originated in the Indian subcontinent, but can now be found worldwide.

Rats have been sailing the seas as long as people, moving from island to island with Polynesian explorers or across the ocean with European settlers. Although rats have become just nuisance for some, they can destroy a natural area through efficient foraging and breeding. Rats also tend to aggressively defend their resources and territory, scaring off other animals that once inhabited the area. But what about the ocean? There is no way a small land-dwelling rodent could affect fish, right?

Seabirds vs Rats

Seabirds like the sooty tern feed at sea, but return to islands to rest, nest, and most importantly: poop.

Even though rats tend to stay grounded, they can decrease seabird populations through predation. Seabirds get most of their prey from the open ocean, but they return to islands to nest or to take a break. There, they do what all creatures do: they poop. Bird poop, also known as guano, is extremely nutrient rich and important for the health of the island as a whole. (Guano is so packed in nutrients it was widely harvested as agricultural fertilizer before fertilizer could be easily manufactured.) Without seabirds and other species disturbed by rats on an island, the surrounding ocean could feel the effects. A team of scientists set to figure that out.

Fish, Seabirds, and Rats

The Chagos Archipelago (named after the native Chagossian people who were expelled from the island in the early 1970s) is a group of over 60 islands in the Indian Ocean. (taken from

The scientists looked at islands in the Chagos Archipelago. They studied five islands that were rat free and five islands that were rat-infested. Within each of those islands, seabirds were present at some an absent from others. To understand how the presence of rats affected fish the team of scientists looked at damselfish territories. These fish are herbivorous farmers, meaning they aggressively defend their territory and tend to the algae within it. Tracking their behavior is a good indicator of their nutritional resources. As it turns out, islands with rats and without the seabirds, damselfish need to alter their behavior. Without the nutrients brought to the island through bird poop the reef was in worse condition and damselfish territories became larger as they had to go further to forage. With larger territories, the fish also became less aggressive, seemingly not seeing the worth in defending such poor territories.

So What?

While these damselfish are just one species, they are a canary in the coalmine for a much larger issue. Faster-growing and more aggressive fish near rat-free islands are likely contributing to a much healthier ecosystem along with nutrients brought to the island by seabirds. The territorial nature of damselfish and other similar species influence the social organization of reef fish communities and therefore the health of the surrounding reef as a whole. The ecosystem-wide disruption caused by just rats truly cannot be understated. Efforts to remove rats from islands are taking place globally, but it will not be an easy feat. Rats are just one of a long list of invasive species (non-native species that wreak havoc economically or environmentally). While you may not be able to irradicate rats off a distant island, it is important to consider how you could prevent invasive species in your area.



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