Nature has the answer, as supplied by a team of British and Australian researchers. Even though rats can’t live underwater, they can do fine on an island. And, thanks to their appetite for eggs, they can kill a lot of birds. The birds, they don’t live underwater either… but they make a difference because they’re really full of it:
When abundant, seabirds feeding in the open ocean transport large quantities of nutrients onto islands, enhancing the productivity of island fauna and flora. Whether leaching of these nutrients back into the sea influences the productivity, structure and functioning of adjacent coral reef ecosystems is not known. Here we address this question using a rare natural experiment in the Chagos Archipelago, in which some islands are rat-infested and others are rat-free. We found that seabird densities and nitrogen deposition rates are 760 and 251 times higher, respectively, on islands where humans have not introduced rats. Consequently, rat-free islands had substantially higher nitrogen stable isotope (δ15N) values in soils and shrubs, reflecting pelagic nutrient sources. These higher values of δ15N were also apparent in macroalgae, filter-feeding sponges, turf algae and fish on adjacent coral reefs. Herbivorous damselfish on reefs adjacent to the rat-free islands grew faster, and fish communities had higher biomass across trophic feeding groups, with 48% greater overall biomass.
In other words, the birds “deposit nitrogen.” They poop. And on rat-free islands, the nutrients from that poop fertilize an awful lot of fish food, meaning the fish were just about 50 percent bigger than elsewhere. Healthier fish make for a healthier reef.