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Few things are as good for an island as bird droppings. Guano is full of essential nitrogen – a fertilizer that helps plants grow – and flying flocks provide a robust supply.
And little is as bad for an island as invasive rats, which manifest themselves. devour everything in their path – including eggs and native bird species chicks, who have not learned to fight mammalian predators. But the consequences of the invasion spread even further into the ecosystem, in unexpected places, including in the surrounding coral reefs.
This nitrogen-rich guano also flows into the sea, where it feeds the reefs. Thus, when the rats arrive and the birds disappear, their vital shit also continues to disappear. How bad the situation may be, report the scientists in the journal Nature . By comparing six rat – free islands and six rat infested islands in the Chagos archipelago of the Indian Ocean, they quantified the surprising ecological damage of rodents.
To measure the impact, the coral reef ecologist Nick Graham and his fellow type of nitrogen on the island. The seabirds that populate these islands feed on the high seas small fish like sardines, which load their guano with a heavy isotope of the element. "The birds on the land that fed on grain, their diet would have a much lighter isotope signature," says Graham. But his team found the heavier nitrogen of seabirds all over the islands – in the soil, leaves and even coral reefs. So this nitrogen came from the sea, not from the island itself.
Following this isotope, Graham could see how rat-ravaged seabird populations were changing the islands' nitrogen reserves. Soil samples showed that on the islands where rats did not invade, the nitrogen intake from bird droppings was 250 times greater than on the infested islands of rats. The team also found higher levels of nitrogen in reef and fish algae near the rat-free islands. "The rats completely interrupt this system," says Graham. "The seabirds then avoid these islands and, therefore, the nutrients are not deposited."
The team combined this data with wildlife surveys on rat-filled and rat-free islands. The fish biomass in the reefs surrounding the rat-free islands was 50% larger than that of the reefs near the invaders. And the birds? "We found that where there were no rats, there were huge populations of seabirds, more than 750 times more than on the islands with rats," explains Graham
. affects the coral populations. "The balanced intake of nitrogen and phosphorus has proven to be very beneficial to coral physiology," says Graham. "So corals grow faster when they have a balanced intake, and they are more thermally tolerant, so they can withstand heat stress more than corals that do not have that intake."
But there are even more windy ways the nitrogen can impact the reefs. In healthy island ecosystems, species like parrotfish feed on algae that grow on coral reefs. The parrot fish – which comes with a healthy algae population, which comes from healthy nitrogen levels, which come from healthy seabird populations – contributes to the reproduction of the corals. "Coral babies do not like to attach to algae," says coral reef ecologist Nancy Knowlton of the Smithsonian Institution, who wrote a commentary on the new study. "They like to settle on clean, well-groomed, dead coral skeletons.The reefs are much more able to bounce back when they are suffering from one of those big disasters they're good at."
Graham's group discovered that species like parrots completely cleaned the surface of reefs around uninvaded islands nine times a year. But when the rats showed up, it dropped to three times a year.
Get rid of rats, then, and you can strengthen the coral reefs. Which is not as discouraging as it may seem. Conservatives have attempted to eradicate rats on nearly 600 islands at this stage, with a success rate of 85%. Their weapon of choice: rodenticide. Of course, this may poison other islanders, so scientists will often kill species such as raptors, which could feed on poisoned rat carcasses. But it is certainly feasible, and indeed essential in this age of world trade (rats are incorrigible stowaways).
Because the hidden impacts of invasive rats do not stop at nutrients in coral reefs. On the Palmyra Atoll near Hawaii, for example, rodents were even clearing the land vegetation. "They were ahead of the young trees that were trying to sprout and eat the seeds," says Nick Holmes, science director for Island Conservation. "Rats are therefore undesired forest engineers, and of course the forest provides this basic system for everything else, whether it's invertebrates or seabirds. " In 2013, just one year after the invaders were eradicated Native plant populations grew by 130 percent and arthropod populations, such as insects and crabs, exploded by more than 350 percent.
In the end, the biggest enemy of island and coral ecosystems is not the rats. those who transplant things, and pollute the oceans, and warm the atmosphere. But we are also the ones who can do something about it.
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