Kill a Rat, Save an Ecosystem
Worldwide, invasive alien species are second only to habitat destruction in reducing the planet's biodiversity. Their effect is especially potent on islands. Cats, rats, weasels and stoats wreak havoc on native faunas which evolved in the absence of predatory mammals.
When, in 1775, Captain Cook discovered a large glacier-riven island in the sub-Antarctic, he named it after King George III and quickly moved on, desperately disappointed that it wasn't part of the fabled southern continent. But the impact of his discovery was to have much longer-term consequences for the island and the biologically productive seas around it.
Cook returned to civilisation with news of an extraordinary abundance of seals on South Georgia's beaches. Within years, many wooden ships had descended on the island from both the New and Old Worlds, replete with sealers and their unintended cargo - rats and mice. While the sealers reaped over a million seal pelts, the rodents began devouring what was possibly the greatest concentration of seabirds in the world.
Two centuries later, the sealers and their whale-hunting successors have long since departed, and the marine mammal populations they exploited are well on the road to recovery. But their legacy lives on. Rats and mice have spread to almost every habitable part of the island.
Though South Georgia is famous for its wildlife, few visitors today realise that it is but a shadow of what it was before man's arrival. What should be the most abundant birds - the many species of burrow-nesting prions, storm-petrels and diving petrels - are essentially restricted to small offshore islands by the presence of rodents. Similarly, the endemic South Georgia pipit, the world's most southerly songbird, is clinging to survival, unable to rear young in the presence of predatory rodents.
But it's not just the birds that have been impacted by the alien invaders; the entire ecology of the island has changed. Rats eat insects, which have a hard enough time as it is. More obviously, the vegetation changes, too. By excluding the seabirds and the energy they bring back to South Georgia in the form of food for chicks (which becomes a nitrogen-rich fertiliser once excreted by the chicks), the rats profoundly impact the standing vegetation and the depth and quality of the soil that it produces. It is hard to exaggerate the cumulative effect of generations of rats on the terrestrial ecology of the island and, through the seabirds, the local marine ecology too.
But what could be done? How could rodents be removed from every nook and cranny on this 170km-long, remote mountainous island? Enter the South Georgia Heritage Trust (SGHT), a small UK charity with no previous experience of island eradications, but a steely determination to somehow fix this problem.
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Rat via Shutterstock