By Matt Simon
Few things are as good for an island as bird droppings. Guano is full of essential nitrogenâ€”the stuff in fertilizer that helps plants growâ€”and flying flocks provide a robust supply.
And few things are as bad for an island as invasive rats, which show up and devour everything in their pathâ€”including the eggs and chicks of native bird species, which havenâ€™t learned to contend with mammalian predators. But the invasionâ€™s consequences ripple even farther across the ecosystem, into places youâ€™d never expectâ€”including all the way into the islandsâ€™ surrounding coral reefs.
That nitrogen-rich guano also washes into the sea, where it nourishes reefs. So when the rats arrive and the birds disappear, so too does their life-sustaining poop. Just how bad the situation can get, scientists report today in the journal Nature. By comparing six rat-free islands and six rat-infested islands in the Indian Oceanâ€™s Chagos Archipelago, they quantified the rodentsâ€™ startling ecological damage.
To track the impact, Lancaster University coral reef ecologist Nick Graham and his colleagues looked for a specific type of nitrogen on the island. The seabirds that populate these islands forage on the open ocean for small fish like sardines, which loads their guano with a heavy isotope of the element. â€œBirds on land that have fed on grains, their diets would have a much lighter isotope signature,â€� says Graham. But his team found the heavier nitrogen from the seabirds everywhere on the islandsâ€”in soil, leaves, and even the coral reefs. So this nitrogen came from the sea, not the island itself.
By tracking that isotope, Graham could see how rat-ravaged seabird populations were changing the islandsâ€™ nitrogen stores. Soil samples showed that on islands where rats havenâ€™t invaded, nitrogen input from bird droppings was a staggering 250 times greater than on rat-infested islands. The team also found higher levels of nitrogen in reef algae and fish near rat-free islands. â€œThe rats are completely interrupting that system,â€� says Graham. â€œThe seabirds then avoid those islands, and as a result nutrients aren’t being deposited.â€�
The team paired this data with surveys of wildlife on the rat-filled and rat-free islands. Fish biomass in the reefs surrounding rat-free islands was 50 percent greater than reefs near the invaders. And the birds? â€œWe found that where there were not rats present, there were huge seabird populations, over 750 times more seabirds than on the islands with rats,â€� says Graham.
The dearth of seabirdsâ€”and therefore their nitrogen-infused poopâ€”dramatically affects coral populations. â€œThe balanced input of nitrogen and phosphorous has been shown to be very beneficial for coral physiology,â€� says Graham. â€œSo corals grow faster when they have a balanced input, and they’re more thermally tolerant so they can cope with heat stress more than corals that don’t have that input.â€�
But there are even windier ways the lack of nitrogen can impact the reefs. In healthy island ecosystems, species like the parrotfish feed on algae that grow on the coral reefs. Parrotfish grazingâ€”which comes with a healthy algae population, which comes from healthy nitrogen levels, which come from healthy seabird populationsâ€”helps coral reproduction. â€œBaby corals don’t like to settle on seaweeds,â€� says Smithsonian Institution coral reef ecologist Nancy Knowlton, who penned a commentary on the new study. â€œThey like to settle on nicely grazed clean surfaces, dead coral skeletons. So reefs are much more able to bounce back when they suffer from one of these large catastrophes if they’re well grazed.â€�
Grahamâ€™s group found that species like the parrotfish would completely clean off the surface of reefs around non-invaded islands nine times a year. But when the rats showed up, that dropped to three times a year.
Get rid of the rats, then, and you can bolster coral reefs. Which isnâ€™t as daunting as it sounds. Conservationists have attempted rat eradications on nearly 600 islands at this point, with a success rate of 85 percent. Their weapon of choice: rodenticide. This of course comes with the risk of poisoning other inhabitants of the islands, so oftentimes scientists will remove species like raptors, which might feed on poisoned rat carcasses. But itâ€™s certainly doable, and indeed essential in this age of worldwide commerce (rats are incorrigible stowaways).
Because the hidden impacts of invasive rats donâ€™t end at nutrients in coral reefs. On Palmyra Atoll near Hawaii, for instance, rodents were even mucking with terrestrial vegetation. â€œThey were predating the young saplings that were trying to sprout, and eating the seeds,â€� says Nick Holmes, director of science for the group Island Conservation. â€œSo the rats are sort of unwanted engineers of the forest, and of course the forest is what provides this baseline system for everything else, whether it’s invertebrates or seabirds.â€� In 2013, just a year after scientists had initiated the eradication of the invaders, native seedlings had increased 130 percent, and populations of arthropods, like insects and crabs, skyrocketed over 350 percent.
In the end, the biggest enemy of island and coral ecosystems isnâ€™t rats, but humansâ€”weâ€™re the ones that transplant the things, and pollute the oceans, and warm the atmosphere. But weâ€™re also the ones who can do something about it.