© Shawn O'Brien

On a bleak December afternoon in 1989, with the color drained from the skies of the Cambridgeshire Fens, the campaign to eradicate a large semiaquatic rodent called Myocastor coypus from the British Isles finally ended. An aged, fight-scarred male, which the previous night had made the fateful decision to climb onto a raft set for it by the Ministry of Agriculture, Fisheries, and Food, spent most of the day encaged in a steel live-trap before being humanely dispatched. A single close-range shot thus consigned the coypu, or nutria, to the memories of the few remaining "marshmen" – inhabitants of the windswept fens and an endangered breed themselves. But this event, played out against a backdrop of ancient reed beds and crumbling windmills, still stands out as a landmark achievement in conservation biology.

Invasive species cause ethical, economic, and ecological problems the world over. Fire ants, zebra mussels,...


Theory has begun to complement practical applications. For example, the "10s rule" gives a loose approximation of the expected scale of the problem facing conservationists: 10% of species that are introduced or occupy new ranges become established, and 10% of established species reach invasive levels. "The 10s rule should be taken seriously but not too seriously," says Mark Williamson, emeritus professor at York University, UK, the rule's originator. "It's really only a rough guide and doesn't always hold true." Another dictum – "the larger the animal, the smaller the area, the easier to eradicate" – provides a more reliable rule of thumb, he says.

This underlines one of the lessons learned through costly trial and error: that islands are much easier to defend than continental landmasses. In the late 1990s, for example, the Royal Society for the Protection of Birds succeeded in completely eradicating brown rats from Ramsay Island off the Pembrokeshire coast. Such an accomplishment would, of course, be unthinkable in London or New York, where one is famously never more than a few feet away from a rat.


Courtesy Ellen M. Vangelder

ANIMAL: Little fire ant, Wasmannia auropunctata

ALIEN TO: Globally restricted to warm climates. Particularly troublesome in ecologically important island systems, such as the Galapagos archipelago.

INTRODUCED: Transport of plant material, especially trees. Easily imported with food items, particularly raw products, such as fruit and cereals.

IMPACT: This tiny, highly social insect, overwhelms local insect fauna. Although extremely damaging, the ant is actually encouraged in the Cameron Islands, as it helps keep local insect pest species in check. Estimated costs to agriculture run into hundreds of millions of dollars each year in the US alone.

CONTROL: poison, fire and clearance of vegetation. Integrated Management techniques, which take a more holistic approach, are being developed. An eradication program in the Galapogos, costing $8 million over five years, has successfully cleared several islands.

But control measures, even biologically directed ones, are rarely a panacea. The spiraling rabbit population in the UK was decimated by the introduction of the Myxoma virus in 1953, which is carried by fleas, but in Australia, the virus is mosquito-borne and thus can pass effectively only during certain seasons. Early trials in the 1920s were conducted outside of mosquito season, and as a result the method almost failed.

Regardless, in both hemispheres rabbits and the Myxoma virus rapidly reached a standoff, because rabbits became increasingly resistant to an increasingly less-virulent pathogen. Laura Meyerson, an invasive species biologist at the University of Rhode Island, says that investigators need to be looking for more than a straightforward natural history. "Research should include not only native habitats," she says, "but should also investigate any adaptations or changes that have occurred in the introduced habitat." Coupled with their pioneering characteristics, many introduced species find themselves released from natural predators and faced with new niches to exploit. Many easy targets are those where humans have already offset natural balances such as agricultural regions. "Although the data are sketchy, the evidence suggests that invasive species tend to exploit degraded ecosystems more effectively," says Slimak.

Such factors can actually help make the economic argument to eradicate a pest. Balancing costs is another issue. The UK coypu program cost around £3 million. Trappers had to be dealt with smartly, as a pertail bounty risks engendering a so-called game-keeping culture. Any rational trapper would forego the prospect of a payment for a single animal now in favor of multiple payments (for its offspring) later. So time limits were set with bonuses for early completion.

Thus in 1981 a team of 24 trappers was placed under the direction of a research laboratory dedicated to understanding coypu behavior and population dynamics. Morris Gosling, who now holds a chair in animal behavior at the University of Newcastle-Upon-Tyne, UK, directed the laboratory.* Understanding the biology of coypus was instrumental in making the economic argument. "Such a biological underpinning makes it possible to define how long an eradication program will take and how much it will cost," he says, referring to the then-revolutionary use of computer models to predict how the coypu population would respond to trapping pressure. The population models were developed using data from both the large-scale field experiments and carefully maintained trapping records. "Nobody will commit funds unless they have that information," Gosling says.



Rondi & Tani Church/Photo Researchers, Inc.

ANIMAL: Sea Lamprey, Petromyzon marinus

ALIEN TO: Great Lakes and surrounding river systems.

INTRODUCED: Welland Canal built in the 1920s created a 'bridge.'

IMPACT: This predator of large fish kills an estimated 40 lbs during the final parasitic stage of its life cycle, decimating native sport fish populations within the Great Lakes area. An eight-year study carried out by the University of Vermont found that lamprey control at Lake Champlain generated benefits of around $29.4 million with associated control costs of about $8.4 million.

CONTROL: Chemical treatment of spawning rivers to kills larvae. Newly developed pheromone bio-control shows promise. Release of sterile males to lower reproduction rates. Physical barriers along streams and rivers to prevent migration of spawning adults. Annual control costs for the Great Lakes basin are estimated to be around $15 million.

Eradicating an invasive species from a continental area is a major challenge. "Nonisland systems make eradication difficult, practically impossible," says Slimak. "You're looking at control." One exception, called the island effect, occurs where a target area can in some way be isolated from its surroundings, as are lakes or mountains. Attempts to eradicate the sea lamprey from the Great Lakes illustrate how this island effect can be used to advantage. Sea lampreys, primitive eel-like fish that prey on other, larger species, have infested the Great Lakes for several decades. Until recently, environmentally damaging chemical treatments were the only effective way to keep them under control, but studies of the fish's mating system have enabled scientists to develop a finely tuned weapon. By using the lamprey's own sex pheromones as bait, large numbers have been rounded up and in many places native fish stocks have started to recover.

Nature reserves can sometimes fit the "island" bill too, as in Maryland's program to eradicate coypu. At the Blackwater National Wildlife Refuge, coypu (commonly known as nutria in the United States) damaged thousands of acres of marshland, much of which is unlikely to ever recover naturally. By feeding on the vegetative root mat that binds the structure of the marsh together, they exposed the underlying mud, leading to erosion and subsequent flooding. This takes the system past an ecological "tipping point," causing a flip to another relatively stable state, in this case pools of open, stagnant water replacing dense reed bed.

Realizing that control was proving ineffective, the US Fish and Wildlife Service, which is responsible for the management of the National Wildlife Refuge network, undertook an eradication program modeled closely on the successful UK campaign. Research included the establishment of exclosures – fenced areas in which coypus were prevented from grazing – to determine the specific impact of coypu grazing on the marsh, and thus helping to make the economic case for their eradication. A team of trappers was employed to catch and shoot coypus and in November 2004, just two years into the campaign, a statement was issued declaring the Blackwater Refuge free of nutria. This objective, in which tens of thousands of animals were killed, had been met at a cost of $2 million.

This is perhaps small change when the economic impact of coypus is evaluated fully. Jacoby Carter, one of the scientists at the National Wetlands Research Center in Lafayette, La., who was closely involved with the nutria eradication program in Maryland, points out the loss of marshland through coastal saltwater intrusion, erosion, and other abiotic factors. Coypus have compounded the problem, exposed to devastating effect by Hurricane Katrina. "Two hundred years ago there were miles of marsh and barrier islands to protect New Orleans from storm surge and flooding. Now they are gone," says Carter. Perhaps that $2 million in Maryland was wisely spent.

Once eradication is achieved, keeping continental areas clear is no easy task. The larger the cleared "island," the greater the perimeter bordering it. Carter says the Maryland example shows that an invasive species can be removed permanently with the right approach. Others, such as Slimak, are more skeptical. "I would bet you a pint of Guinness, in Dublin, that I could go to the Blackwater Refuge and find nutria." He sounds confident.

*The author worked at this laboratory until 1990.


© Getty Images

ANIMAL: Cane toad, Bufo marinus

ALIEN TO: Global, mainly in sugarcane and banana growing areas, many fragile island ecosystems.

INTRODUCED: Deliberate, as a means of biological control of rats, mice and insects.

IMPACT: A voracious predator and lethally toxic to other predators, cane toads threaten many endangered species. For example, some 49 species of Australian snake are at risk: Their preferred amphibian diet makes them extremely vulnerable to cane toad poisoning.

CONTROL: Local collection initiatives, fenced enclosures. Research underway to develop viral and sex pheromone biological control methods.

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