Island-dwelling animals across the world have been devastated by predators introduced by man. In the Aleutian islands, this age-old problem has gone one step further. There, the introduction of Arctic foxes has changed the very nature of the land itself.
Nizki island has changed. If you had visited Nizki in the 19th century, you would have been greeted by the chorus of massive colonies of seabirds, and set foot on verdant grassland. But travel to the island now and you would find a land transformed. The tall grasses and most of the seabirds have gone. The landscape is now tundra, dominated by low-lying shrubs and suffering from poor soil quality.
And if you looked carefully, you could probably spot the perpetrator behind the altered terrain – the Arctic fox. According to literature and folklore, the fox had exceptional powers of cunning and trickery. But science now reveals that they have another trick up their sleeve – the power to change entire landscapes.
The foxes arrive
Nizki Island is part of the Aleutian archipelago, a band of sub-Arctic islands that spans the gulf between Russia and Alaska.
In the late 19th century, the island chain was visited by fur traders. Seeking to forestall losses from declining sea otter numbers, the traders introduced Arctic foxes to the islands to act as a readily available future source of fur.
A century later, Donald Croll and James Estes from the University of California, Santa Cruz, were carrying out conservation work in the Aleutians. They noticed that islands infested by foxes had changed from grassland to low-lying tundra and wanted to work out how the furry predators had affected the Aleutian archipelago.
Thankfully, the fur traders had failed to introduce foxes to some of the islands, and many remain fox-free to this day. They had unwittingly set up a massive natural experiment, which Croll and Estes took advantage of. Backed by a team of researchers, they surveyed 18 islands, comparing those that were ridden with foxes and those that lacked them.
Goodbye seabirds, farewell gauno
They found that when foxes first invaded the islands, they began doing what natural selection had designed them to do – killing. Their prey were the local seabirds, and only species that nested on unreachable cliff faces escaped them.
Burrow-nesters like puffins and surface-nesters like gulls were easily taken and their populations were decimated. Today, seabirds are a hundred times more common on fox-free islands than on their fox-infested neighbours.
Bird droppings, or ‘guano’ were the main source of fertiliser for the Aleutian vegetation. By feeding in the productive ocean waters and defecating inland, the birds transferred nutrients from the rich sea to the poor land.
As the seabirds died, guano levels fell by over 60 times, and the soil was quickly rendered infertile. With foxes around, the levels of phosphorus – a key nutrient in most ecosystems – on an island plummeted by three times.
This newly depleted land could not longer support lush grassland, and shrubs became the dominant plant as the grasses died out.
As a final test of their theories, the scientists artificially added fertiliser to parts of fox-infested islands over three years, to mimic the effect of guano. On the fertilised terrain, grass rapidly re-established itself as the dominant plant group, increasing in numbers by 24 times.
The problem of introduced predators is, sadly, not uncommon. All around the world, people have transferred predators to places they don’t belong with devastating consequences. The problem is especially
serious on islands, where the local wildlife is naïve about the threat of predators, or has lost defensive adaptations such as flight.
In Australia and New Zealand, foxes, cats and stoats have hunted their way through local bird populations, driving many to the brink of extinction. And as the Aleutian problem demonstrates, the effects of introduced killers can ripple out to affect more than just their prey. In this example, entire ecosystems can be changed over a very large area.
Deporting the problem
In the Aleutians at least, the problem seems solvable. The US Fish and Wildlife Service has been removing foxes from Aleutian islands for over 35 years. As a result, the seabirds are staging a comeback and the lush vegetation is returning. Even so, it may take several more decades for the islands to return to their former glory.
Until then, the Aleutians serve as a stark reminder of the disastrous effects of placing top predators where they don’t belong. Conserving these animals in their original homes is just as important – other studies have shown that removing top predators can wreak equally dramatic changes in an ecosystem.
Many of the world’s key predators – sharks, big cats, polar bears and many more – are facing extinction across a wide range of habitats. The need to conserve these decisive and often charismatic animals has never felt stronger.
Related posts on introduced predators:
Attack of the killer mice – introduced rodents eat seabird chicks
Shark-hunting harms animals at bottom of the food chain
Farmed salmon decimate wild populations by exposing them to parasites
Images: (Photos from Anthony DeGange and Donald Croll)