Rats on tropical islands change reef fish behavior

Rats have been introduced to many offshore islands in the past, mostly arriving by accident aboard visiting ships. They can cause devastation to the indigenous wildlife of these islands, particularly if the native species include ground-nesting birds. This has led to a call for rat eradication programs on many islands, in order to save wildlife from the deleterious impacts of these introduced rodents. 

A study led by researchers at Lancaster University has now identified an unexpected way in which invasive rats on tropical islands have affected the lives of fish inhabiting the reefs around the islands. This is an unusual instance where the presence of terrestrial rats leads to a change in the behavior of species in an entirely different biome. It reveals how species in different biomes are linked, despite the fact that they will never meet one another.

The study, which also involved researchers from Lakehead University in Canada, compared the territorial behavior of jewel damselfish (Plectroglyphidodon lacrymatus) on reefs around five rat-infested and five rat-free islands in a remote archipelago in the Indian Ocean. These small reef fish are highly territorial and readily chase away other fish that enter their territory. They “cultivate” patches of algae between the branches of corals within a territory, and mostly direct their aggression towards intruders that are also herbivorous and in search of algal turf for food. 

Previous research has shown that the damselfish around rat-free islands benefit from the nutrients that leach from the guano of seabirds breeding and roosting on the islands. These seabirds, including mostly terns and noddies, forage at sea and return to their nests on the islands. They produce nutrient-rich guano that enters the waters surrounding the islands and provides a sort of fertilizer. Around the rat-free islands, algal productivity and damselfish growth and body sizes are higher than they are in the waters around rat-infested islands. 

The black rats (Rattus rattus) are thought to have been introduced to the archipelago in the late 18th and early 19th centuries, but are not present on all the islands. This provided the researchers with a natural experiment in which they could assess the impacts of the rats by comparing the behavior of damselfish around rat-infested and rat-free islands. 

The rats feed on the eggs and chicks of the resident seabirds, causing significant decreases in bird population numbers. The researchers recorded an average of 1,243 birds per ha on rat-free islands, and 1.6 birds per ha on rat-infested islands, which equates to a seabird density that is 720 times smaller where rats are present. As a result, there is less guano on rat-infested islands, and less nutrient-enrichment in the surrounding waters.  

The findings of the current study, published in Nature Ecology and Evolution, show that damselfish on reefs around rat-infested islands defend larger territories (average 0.62m² compared with 0.48m² around rat-free islands), presumably because the algae receives less nutrient-enrichment and is less productive where the seabird populations have been decimated; the damselfish therefore need a larger “farm” area to fulfil their needs.

In addition, while damselfish aggressively defended their small territories and algal turf patches around rat-free islands, the fish on reefs around rat-infested islands were far less likely to behave aggressively towards other fish that entered their territory. The researchers hypothesize that the algal food in waters around rat-infested islands was simply not nutritious or productive enough to make it worth defending. 

Dr. Rachel Gunn conducted the research as part of her PhD studies at Lancaster University and is now at Tuebingen University, Germany. 

“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,” said Dr. Gunn. “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.” 

Seabirds are drivers of nutrient cycling in numerous locations around the world, as they forage out in the oceans and return to land to roost and breed. The results of this study reveal that the reduction in nutrient availability, due to the effects of invasive rats on the seabird populations, can also affect the behavior of fish in the shallow waters around tropical islands. It is possible that the reduced nutrient levels could also affect other reef species, and influence the spread of corals, the distribution of reef fish and the genetic traits of damselfish. 

“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,” said Dr. Gunn.

“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,” explained Dr. Sally Keith, the principal investigator of the study. “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 understand animal behavior.”

The findings also add evidence in support of rat eradication programs on tropical islands. Rats have been removed successfully from more than 580 islands worldwide, and in many instances resident seabird populations have benefitted. It is clear from this study that the return of seabirds and their guano would be advantageous, not only for the islands’ ecosystems, but also for the nearshore marine ecosystems that receive nutrient enrichment as a consequence of seabird presence. Rat eradication should be conservation priority for these tropical islands. 

“We have provided more evidence that invasive rats have a large impact on both terrestrial and marine ecosystems,” said Dr. Gunn. “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.”

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By Alison Bosman, Earth.com Staff Writer

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