Attack of the killer mice – introduced rodents eat seabird chicks alive

On Gough island, introduced mice have developed a grisly appetite for seabird chicks and could well drive local species to extinction.

As Charles Darwin learned several centuries ago, islands are havens for evolution. Newcomers to these isolated worlds find themselves unshackled from the predators that dogged them on the mainland. They celebrate their freedom by diversifying into a great variety of species.

A Gough Island mice sites amid a grisly pile of chick bodies.But predators still have ways of tracking them down, and following the footsteps of sailors is one of them. By killing adults and eating eggs, introduced predators such as rats, cats and stoats are responsible for nine in ten of the bird extinctions since 1600.

Now, conservation agencies are getting serious about introduced predators. As an example, they have spent increasingly large budgets in recent years on the eradication of rats from troubled islands.

Smaller stowaways like mice typically escape the conservationists’ wrath, and between 2001 and 2005, twenty-five times less money was spent on dealing with them. After all, mice are smaller and less opportunistic than rats and pose very little threat to seabirds.

Or at least that was what scientists used to think. In 2005, Ross Wanless, Peter Ryan and colleagues from the University of Cape Town found that on Gough Island in the south Atlantic, mice had developed sinister appetites. They were eating the chicks of local seabirds alive (see image below).

Infrared footage shows mice feeding on a chick.

House mice were introduced to Gough Island, now a World Heritage Site, in 1888 and are the only introduced mammals there. They share the island with large seabird colonies including many endangered species, and the last breeding populations of the Tristan albatross and the Atlantic petrel.

The mice seemed to be co-existing peacefully until the turn of the millennium, when some of the seabirds started experiencing massive breeding losses.

Wanless investigated and by watching about 300 nests, he found video evidence that the mice have developed a taste for albatross, petrel and shearwater chicks. They were attacking and eating chicks up to 300 times their weight, and often en masse.

Albatross chicks are not defenceless and will often ward off much larger avian hunters like the sub-Antarctic skua or the southern giant petrel. But they have also been spoilt through an evolutionary history free of mammalian dangers. The chicks have no idea how to react to mice, let alone defend themselves, hapless and helpless.

To save endangered birds like the albatross, ALL introduced predators must be dealt with.In fact, Wanless describes the mice as parasites rather than predators. They often don’t kill the chicks outright but slowly feed from open wounds over the course of days.

Typically, 60-75% of Tristan albatross young survive their first year. But thanks to the voracious mice, only 27% now do so on Gough Island. As a result, the population of this already vulnerable bird has crashed by over a quarter since the 1960s.

The birds of Gough Island will clearly need some assistance. Meanwhile, Wanless is sounding the alarm for other islands where introduced mice roam free.

On islands where they are not alone, larger introduced predators like cats or rats may be helping to keep their numbers down. The danger then, is that when conservationists get rid of these larger problems, they may unwittingly unleash a smaller foe on the native animals.

This very situation seems to be playing out on Marion Island in the South Indian ocean. In the 1990s introduced cats were finally eradicated from the island, leaving mice as the only mammal aliens around.

And sure enough, Ryan has found that several wandering albatross chicks have died of wounds consistent with mouse attacks. These attacks could be much more widespread than we had realised.

The message is clear – eradication programmes for introduced animals should target all potential predators, whether big or small. As in the case of sharks and scallops dealt with elsewhere in this blog, the removal of top predators can often doom animals at the bottom of the food chain by releasing hunters in the middle tier.

Reference: Wanless, Angel, Cuthbert, Hilton & Ryan. 2007. Can predation by invasive mice drive seabird extinctions? Biology Letters doi:10.1098/rsbl.2007.0120

Shark-hunting harms animals at bottom of the food chain

Overfishing has disproportionately reduces the numbers of the ocean’s ‘apex predators’ and large sharks are disappearing particularly fast. Their absence allows their prey to flourish and the consequences of that can be disastrous for animals at the bottom of the food chain, and the humans that depend on them.

On the surface, plummeting populations of sharks do not seem like much cause for concern for humans or, for that matter, other sea life. But this simple viewpoint relies on splitting animals into two groups – predators and prey.

The sand tiger shark - one of the victims of overfishing.In practice, this distinction is far too crude. Too put it bluntly, there are predators and there are predators. Those at the top kill those in the middle, and stop them in turn, from killing those at the bottom. As the old saying goes, the enemy of my enemy is my friend.

The rise in shark fishing is mainly driven by a growing market for their fins. Sharks’ fins soup is a delicacy in China, which is utterly ludicrous given that the fins themselves are tasteless and merely add texture.

China’s strong economy has put this expensive treat in the range of the expanding middle classes and the world’s sharks are paying the price for it.

Ransom Myers from Dalhousie University, Halifax, decided to study the effects of declining shark numbers by analysing a uniquely comprehensive shark census taken over the last 30 years on the American eastern seaboard.

In these waters, several shark species have all but vanished since 1972, including 99% of the bull, dusky and smooth hammerhead populations.

And because fishing expeditions tend to catch larger individuals, the average size of the survivors has plummeted. Mighty animals like the tiger and black-tip sharks are now up to half as long as they used to be.

The hammerhead shark helps to control numbers of bottom-dwelling predators like rays and skates.Unsuprisingly, as the sharks declined, their prey benefited. Great sharks mainly hunt smaller predators, including their close relatives skates, rays, and indeed, smaller sharks, whose numbers surged in their absence.

The cownose ray, for example, is now ten times more common than it was in the mid-70s.

And here’s where the domino effects begin.

It turns out that large sharks inadvertently carry out a sort of protection racket for animals at the bottom of the food chain.By taking out the mid-level predators, they prevented these lesser hunters from decimating stocks of small fish and invertebrates.

The cownose ray feeds mainly on shellfish like scallops, clams and oysters. In the 80s, their small numbers made little dent on the local scallop population which sustained the economies and stomachs of local seaside towns.

In 1996, the ray explosion started to spell the end for the scallops. By 2004, the local North Carolina scallop fisheries which had thrived for centuries were forced to close and remain closed to this day. Little did the locals imagine that the disappearance of dangerous sharks from their waters could have such strongly felt economic consequences.

The ludicrous demand for shark’s fin soup will wreak irrevocable damage on oceanic ecosystems.This is far from an isolated incident. In Ariake Sound off Japan, shark fishing is particularly intense, and a booming population of long-headed eagle rays has decimated shellfish populations just like their cownose cousins in the Atlantic.

This is one of the first times that the removal of apex predators has been so thoroughly studied in the ocean. On land, the consequences are well-known.

Just last year, Australian scientists found that in some areas, persecution of the local top dog – the dingo – has allowed introduced predators like cats and rats to kill off up to two thirds of ground-dwelling mammal species.

As for the great sharks, the responsibility for preserving these great animals lies with the only predators they themselves face – the fishermen who kill them, and the Asian restaurant-goers who create the demand for their fins.

Surely, the price of irrevocably altering an ecosystem is too high to pay for the right to eat textured soup?

Reference: Myers, Baum, Shepherd, Powers & Peterson. 2007. Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science 315: 1846-1850.

Technorati Tags: , , , , , , , ,

Related posts on conservation and introduced animals:

The fox and the island: an Aleutian fable
Farmed salmon decimate wild populations by exposing them to parasites
Attack of the killer mice – introduced rodents eat seabird chicks alive

Farmed salmon decimate wild populations by exposing them to parasites

Salmon migrations serve to protect newly hatched youngsters from the parasites that afflict their parents. But salmon farms undermine this protection and jeopardise wild stocks by exposing young salmon to large numbers of parasitic sea lice.

The next time you buy salmon from your local supermarket, think about the hidden costs in each succulent fillet. Compared to wild fish, farmed salmon is far less likely to burden your wallet. But by buying it, you may be placing a much larger burden on the environment. Fish stocks around the world are declining due to over-fishing and ‘aquaculture’ – the farming of fish – was originally thought to help. But farming brings with it a host of ecological problems.

Wild salmon suffer fewer parasites than their farmed cousins. If the farmed fish are meat-eaters, as salmon are, they must be fed on the proteins and oils of wild fish, which does nothing to alleviate the stress on wild populations. Domesticated farmed fish are also genetically different to their free counterparts, and escapees risk spreading their genes and replacing local genetic diversity.

But the biggest and most immediate problem may be to do with the spread of parasites. Large, crowded and trapped animal populations are an easy target for parasites, and salmon farms are no exception. Farmed salmon are often infested with sea lice, parasitic relatives of prawns and shrimp, that cause direct damage, starve their host and increase vulnerability to disease. But the real problem comes when infected farmed salmon pass their parasites onto wild fish.

Martin Krkošek and colleagues from the University of Alberta believe that salmon farming may be disrupting behaviour that evolved in salmon to protect their young from parasites.

Newborn salmon are especially vulnerable to parasitesSalmon are known for their massive and demanding migrations in order to mate and lay eggs. Because of these treks, the young salmon enter the ocean several months before the adults and their parasite passengers return. In this way, the youngsters gain precious months’ respite from infections during which they can develop unhindered. It’s the same strategy that human parents use when they move to the country to raise their children in safer surroundings.

But this safe period is shattered if the ocean is crammed with lice-ridden farmed populations. Across the North Atlantic from Canada to Norway, wild juveniles are being infested with sea lice. The farms are providing the lice with new routes for infecting even more hosts, and the young salmon are not ready for them.

To adults, sea lice are irritating but tolerable, but to the much smaller young, carrying more than two lice is always lethal. Not only do they take up valuable nutrients the juveniles need to grow but they make them more vulnerable to predators and weaken their immune systems.

Krkošek analysed data on salmon and lice populations off the western coast of Canada and revealed disturbing trends. The lice were decimating many local populations of salmon, killing up to 95% of juveniles in some regions.

As aquaculture continues to spread, this study provides us with a harsh reality check and consumers around the world have the power to reverse the trend by protesting with their money. Choosing wild fish over farmed varieties sends a message to the fishing industry that the benefits of buying cheaper fish are outweighed by the costs to wild populations.

The fox and the island – an Aleutian fable


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

The Arctic fox takes another Aleutian birdNizki 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.

The landscape changes from grassland to tundra without guano

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.

Killer immigrants

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.

Reference: Croll, Maron, Estes, Danner & Byrd. 2006. Science 307: 1959-1961.


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

Related posts on dogs:
Bone-crushing super-wolf went extinct during last Ice Age
Of dogs and devils: the rise of contagious cancer


Images: (Photos from Anthony DeGange and Donald Croll)