Birds provide a potent natural fertiliser in the form of their droppings, which benefit not only the plants growing on land but also underwater coral gardens.
Unfortunately, birds are also the preferred prey of the rats that have invaded many of the world’s coral-fringed islands.
In a new study, scientists used a set of tropical islands as a massive natural experiment to investigate the knock-on effects of rat invasion on seabird populations and coral reef decline.
Ever since humans first began exploring the world’s oceans, rats have stowed away in the bowels of their ships and colonised every island they stopped at.
As rats will happily feast on eggs and even fully grown birds, this trend has led to seabird populations on 90 per cent of the world’s tropical island groups being exterminated.
However, some islands have miraculously escaped this rodent menace, and their populations of boobies, frigatebirds and terns have thrived.
Dr Nicholas Graham from Lancaster University saw this contrast play out dramatically when he visited the Indian Ocean’s Chagos Archipelago. Of the 55 islands there, 18 have remained rat-free.
“You step onto one of these islands, the sky is full of birds, it smells because the guano [bird droppings] is really pungent – it’s quite a dramatic thing,” said Dr Graham.
“Another island which might be literally 1km further down the atoll rim, similar size and elevation everything is the same – and it’s stunning, but now all you can hear are the waves lapping on the shoreline.”
Dr Graham and his colleagues estimated that seabird densities were nearly 800 times greater without rats.
To investigate the far-reaching consequences of this major shift in the islands’ ecosystems, he set about analysing island soil samples, and measuring algae and fish numbers in the surrounding reefs.
In a study published in the journal Nature, the team studied six rat-free and six rat-infested islands to assess what difference, if any, a massive injection of bird fertiliser had on neighbouring reefs.
Their results were dramatic. Not only was the total mass of fish around 50 per cent higher around the bird-rich islands, the scientists traced chemical signatures of nitrogen from bird guano to seaweed, filter-feeding sponges and algae growing in the reefs.
Coral communities with seabirds nearby were healthier due to this stream of nutrients, and the abundance of fish kept them in good condition by nibbling away at dead coral and algae growing on the reefs.
“Adding rats to the list of dangers to reefs might seem discouraging,” wrote Dr Nancy Knowlton, a coral reef biologist who did not take part in the research, in an accompanying commentary.
“Yet the discovery of the negative impacts of rats on reefs does point directly to a specific strategy that could slow the pace of reef degradation.”
Over 500 islands around the world have already been de-ratted, and rat extermination has become a slick process – with poisonous rat pellets dropped en-masse from aircraft.
Dr Graham estimated that it would cost between $2-3m (£1.5-2.26m) to de-rat the rest of the Chagos Archipelago, but it could be worth it if it means preserving its precious reefs.
“This process is the most clear-cut example we have of a way that you could effectively enhance the functioning of a coral reef,” said Dr Aaron MacNeil, a fisheries ecologist at Dalhousie University who contributed to the research.
“There is no example that is as clear cut and effective in enhancing the functioning of coral reefs in the face of climate change.”