How rats are making reef fish less aggressive

By Pol Allingham via SWNS

Rats are making reef fish less aggressive neighbors, according to scientists calling for the eradication of the invasive rodents.

When five remote and rat-infested tropical islands were compared with five rat-free islands, scientists discovered the rodents were disrupting the lives of jewel damselfish.

Normally, damselfish on the remote Indian Island archipelago aggressively defend small patches of land surrounding the algae they farm for food.

However, rats kill off seabirds that supply the reefs algae with nutrients.

As a result, the damselfish are less defensive because it is not worth the energy to fight over the nutrient-deficient food.

Dr. Rachel Gunn, who conducted the research as part of her Ph.D. studies at Lancaster University and who is now at Tuebingen University, Germany, said: “Jewel damselfish around rat-free islands aggressively defend their turf because the higher enriched nutrient content means they get ‘more for their money’ and this makes it worth the energy cost needed to defend.

“Conversely, the fish around rat-infested islands behave less aggressively.

“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.”

Normally, small seabirds feed on the open ocean and return to the remote islands to nest.

Back on the island, they leave nutrient-rich droppings which wash into the sea and fertilize the surrounding coral reefs.

However, the invasive rats - many of which arrived as stowaways on ships in the 1700s - often eat the small seabirds and their eggs.

They have decimated small seabird populations, which are now up to 720 times smaller on rat-infested islands.

Nutrients from bird droppings are no longer getting into the sea and 251 times less nitrogen flows into the coral reef around rat-infested islands.

In turn, seaweed has become less and less nutritious for herbivorous sea life.

On rat-free islands damselfish aggressively farm and defend small patches around their food source, turf algae, which are typically less than half a square meter.

However, scientists noticed around rat-infested islands damselfish often had larger territories and were five times more likely to be less aggressive than their peers near rat-free islands.

Territories of an average damselfish by a rat-free island span 0.48 meters squared, which expands to 0.62 meters squared on average on rat-filled islands.

Researchers reported damselfish on rat-infested islands to need larger territories because the seabird killings have made their algal turf less nutritious.

The rat-induced behavior change could have a lasting effect on how coral spreads, how reef fish are distributed, and how resilient the future generations of damselfish are.

Dr. Gunn said: “The algal farming of damselfish affects the balance of corals and algae on the reef.

“Their aggression towards other fish can influence the way those fish move around and use the reef.

“We do not yet know what the consequence of this behavioral change will be but ecosystems evolve a delicate balance over long time scales, so any disruption could have knock-on consequences for the wider ecosystem.

“We have provided more evidence that invasive rats have a large impact on both terrestrial and marine ecosystems.

“Rat eradication has the potential to have multiple, cross-ecosystem benefits.

“The removal of invasive rats could restore the territorial behavior of farming damselfish, which could scale up to benefit coral reef community composition and resilience.”

Dr. Sally Keith, Senior Lecturer in Marine Biology at Lancaster University and Principal Investigator of the study, said their research is the first to show how far the impact of environmental change can range.

Writing in the journal Nature Ecology & Evolution, she said: “Changes in behavior are often the first response of animals to environmental change, and can scale up to affect if, how and when species can live alongside one another.

“Our research is the first to show that these broader impacts can even be felt across biomes, from terrestrial invaders to marine farmers.

“It also shows the power of leveraging real-world environmental variation across multiple locations as an approach to understanding animal behavior.”