Invasive rats are changing how reef fish behave. It's because there's not enough bird poop.

It's no secret that invasive rats can cause widespread and long-lasting damage to tropical island ecosystems, but now, a new finding reveals that the invasive rodents' impacts may extend even further than the land on which they scamper; these ravenous critters can also disrupt the surrounding marine ecosystem. 

In the Indian Ocean, invasive island rats have altered the behavior of tiny farming fish known as jewel damselfish (Microspathodon chrysurus) that live on coral reefs located miles offshore, a new study shows. Affected damselfish have become much more docile than normal due to a weird link between the fish and the rats: bird poop. 

Jewel damselfish are normally highly territorial fish that aggressively defend and cultivate small patches of algae that grow on corals surrounding tropical islands. The plant patches are greatly desired by the damselfish because the algae are highly nutritious, thanks largely to the nutrients that run off from the islands. One main source of island-derived nutrients is the poop, or guano, excreted by seabirds that nest on the islands, which then gets washed into the ocean. 

However, invasive rats, which were historically introduced to islands after stowing away on ships, are known to decimate bird populations on their new homes by sneaking into nests and feeding on unhatched eggs. This can cause seabird densities to decrease by up to 720 times on rat-infested islands, which can decrease the nutrients in surrounding waters by up to 251 times, the study authors explained in a statement. As a result, the damselfish's algal patches have become less nutritious, which means the feisty farmers are less inclined to defend them.

Related: Glowing, red-eyed rat fetus is global photo contest's gorgeously creepy winner

In the new study, published Jan. 5 in the journal Nature Ecology & Evolution, researchers compared the damselfish behavior on coral reefs surrounding five rat-free islands and five rat-infested islands in a remote archipelago in the Indian Ocean. 

The team found that damselfish surrounding rat-infested islands had larger territories, measuring an average of 6.7 square feet (0.62 square meter), compared with an average of 5.2 square feet (0.48 square meter) on rat-free islands. The larger territories are a sign that the fish have become less territorial over specific patches, the researchers said.

The team also observed that the fish were up to five times less likely to act aggressively toward one another.

Jewel damselfish around rat-free islands fiercely defend their patch of algal turf because it is packed with  nutrition, which means the fish get "more for their money," by using energy to defend it, said study co-author Rachel Gunn, a behavioral ecologist at Tübingen University in Germany and a former doctoral student at Lancaster University in the U.K. at the time of the study. "We believe that the presence of rats is lowering the nutritional benefit of the turf to the extent that it is almost not worth fighting for, which is what we are observing with these behavior changes." 

The researchers think the change in the damselfish's behavior is likely to have further knock-on effects on their coral reef ecosystems, which have not yet been properly studied.

"The algal farming of damselfish affects the balance of corals and algae on the reef," Gunn said. "Their aggression towards other fish can [also] influence the way those fish move around and use the reef."

The decrease in the available nutrients in the water could also have implications for other algae eaters, as well as for filter-feeding organisms such as sponges, which could, in turn, affect food chains across the reefs, the researchers warned. "Ecosystems evolve a delicate balance over long time scales, so any disruption could have knock-on consequences for the wider ecosystem," Gunn said.

The team believes the new findings show how important it is to control or completely kill off invasive rat populations where possible.

"Rat eradication has the potential to have multiple, cross-ecosystem benefits," Gunn said. The removal of rats from tropical islands would likely restore the territorial behavior of jewel damselfish and provide other benefits to the rest of the coral reef ecosystem, she added.