Scientists have discovered for the first time that invasive rats on tropical islands are affecting the territorial behavior of fish on surrounding coral reefs.
The new study, led by scientists from Lancaster University in the UK and involving researchers from Lakehead University in Canada, shows that the presence of invasive black rats on tropical islands is causing changes in damsel territorial behavior joia, a herbivorous species of tropical islands. reef fish that “grow” algae on the branches of corals.
The study, published in Nature Ecology and Evolution, compared five rat-infested islands and five rat pounds in a remote island archipelago in the Indian Ocean. Rats, which in many cases arrived on the islands as stowaways on ships in the 1700s, change damsel behavior by disrupting an important nutrient cycle. Seabirds travel to the open ocean to feed and return to nest on islands. Seabirds then deposit nutrients, through their excrement, on the islands, and many of these nutrients are subsequently washed out to sea, fertilizing the surrounding coral reef ecosystems.
On islands with invasive rats, the rodents attack and eat small resident seabirds and their eggs, decimating their populations to the point where seabird densities are up to 720 times lower on rat-infested islands.
This causes nutrient depletion in the seas surrounding the rat-infested islands, with 251 times less nitrogen flowing into the coral reefs around these islands, reducing the nutrient content of the algae for herbivorous fish.
Around islands with intact seabird populations, damselflies aggressively defend their small patch, usually less than half a square meter, of reef to protect their food source: lawn algae .
But the scientists observed that damselflies on reefs adjacent to rat-infested islands were much more likely to have larger territories and five times more likely to behave less aggressively than those living on reefs adjacent to islands no rats
Damsels around rat-infested islands must have larger territories (an average of 0.62 m² compared to the average around rat-free islands of 0.48 m²) because the algal lawns in around the rat-infested islands was less nutrient-rich due to the lack of nutrients derived from seabirds. .
Dr Rachel Gunn, who carried out the research as part of her PhD studies at Lancaster University and is now at the University of Tuebingen, Germany, said: “Jewel damsels around the free islands of rats aggressively defend their turf because the richer nutrient content means they get “more bang for their buck,” making it worth the energy cost to defend. By contrast, fish around the islands infested with rats behave less aggressively. We think the presence of rats is reducing the nutritional benefit of the turf to the point where it’s almost not worth fighting for, which is what we’re seeing with these behavioral changes.”
Nutrient reduction due to the presence of rats, and these associated changes in fish behaviour, could have wider implications for the spread of different coral species, the distribution of other reef fish and, along the generations, the damsel’s resilience due to changes in hereditary traits.
Dr Gunn: “Damsel algae farming 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. Not yet we know what the consequence of this change in behavior will be, but ecosystems develop a delicate balance over long time scales, so any disruption could have knock-on consequences for the wider ecosystem.”
Dr Sally Keith, senior lecturer in marine biology at Lancaster University and lead researcher on the study, said: “Changes in behavior are often the first response of animals to environmental change and can increase to affect whether, as and when species can live. side by side. Our research is the first to show that these broader impacts can even be felt across biomes, from land invaders to marine farmers. It also shows the power to harness real-world environmental variation across multiple locations as an approach to understanding animal behavior.”
The study adds further to the evidence base behind the need to eradicate invasive rat populations from tropical islands.
Dr Gunn said: “We have provided further evidence that invasive rats have a major impact on both terrestrial and marine ecosystems. Rat eradication has the potential to have multiple benefits across ecosystems. Removing invasive rats could restore the territorial behavior of damsel farming, which could increase to benefit coral reef community composition and resilience.
The results of the study, which was supported by the Bertarelli Foundation and the Natural Environment Research Council (NERC), are published in the paper “Terrestrial invasive species alter the behavior of marine vertebrates”.
The authors of the study are: Rachel Gunn, formerly of Lancaster University and now of the University of Tuebingen; Cassandra Benkwitt, Professor Nicholas Graham, Ian Hartley and Sally Keith from Lancaster University and Adam Algar from Lakehead University.
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