Let rats loose on an island and they don’t just scamper around eating birds’ eggs, a study reports in a recent Ecology Letters. They cut nutrient levels, make the soil nearly 100 times less acidic, and topple populations of six out of eight kinds of – well, let’s call them creepy crawlies (springtails, rotifers, nematodes, snails, and so on).
It’s called a trophic cascade – the domino effect of the ecological world. And in this case, rats seem to have pushed against the sort of domino that sets several new rows falling.
The rats weren’t scarfing down tiny soil organisms themselves, or leaching nutrients through some ingenious filtration scheme. All they had to do was drive away the seabirds – graceful, wave-trotting shearwaters and the like – that nest here precisely because there are no predators around.
The study team, led by Tadashi Fukami and composed of scientists from New Zealand, Hawai’i, Alaska and Sweden, chose 18 very similar hunks of rock off northern New Zealand. Rats had jumped ship onto nine of these islands between 50 and 100 years ago. The other nine islands had no rats – and 24 times the number of seabird burrows.
Nitrogen isotopes measured in soil and plant leaves revealed the signature of marine nutrients on the rat-free, guano-splattered islands. Guano splattering is a good thing: carbon, nitrogen and phosphorus levels were at least 45% higher on the rat-free islands. When researchers grew plants in the same greenhouse using soil taken from each of the islands, plants growing in rat-free soil grew to 40% heavier.
Rats did seem to encourage plant growth, with bigger trees, more seedlings and more leaf litter found on invaded islands. The authors suggest that on the other islands, the seedlings may have been kept down by “seabird trampling,” a wonderful thing to imagine.
We live in a world where everything we see runs downstream. So it’s easy to understand problems caused by runoff (dead zones, for example). It’s less straightforward, but more fascinating, to learn of the various ecological rivulets that carry nutrients back up onto land.
In recent years, researchers have found the marine signatures of salmon flesh in Alaskan forests and pollutants concentrating at the feet of nest-cliffs in the otherwise pristine Arctic. Add this study to the list – a neat demonstration of both the seabirds’ ocean-harvesting ability and the shockingly simple way the system can be dismantled.