Introduction
When all the crop losses and control and containment costs are added up, non-native invasive species (including weeds and insects) cost the United States alone an estimated $137 billion annually. The more intangible effects of invasive species on natural ecosystems are also serious. Invasive species are sometimes termed "biological pollutants," because predation and competition by invasive species can reduce populations of native species and cause extinctions. Indeed, half of the known cases of bird extinctions on islands are linked to introduced mammalian predators, such as cats.
The top invasive mammal pests worldwide are rats, mice, cats, dogs, cattle, burros, horses, goats, hedgehogs, foxes, gray squirrels, coypus, pigs, possums, rabbits, deer, weasels, mink, and the mongoose. Globally, rodents like rats and mice consume an estimated 5–15% of grains like rice, wheat, and corn in the fields before harvest. East African countries have lost as much as 80% of the crop during severe twentieth century rodent outbreaks. After the harvest, the combined actions of insects, rodents, fungi, and other organisms may destroy another 5–15% of stored grains, though some areas experience 20% losses. Stored grain losses are estimated at $5 billion per year in India alone, and may run as high as $1 billion per year in the United States. According to one estimate, rodents destroy enough food to feed 200 million people. These losses may be just the tip of the iceberg, as there is little economic documentation on invasive mammals.
Although they are considered invasive pests when feral in the wild, many of the top invasive mammals in the world lead a double life, as they are also desirable as pets and valuable as agricultural livestock. For example, cats, dogs, and rabbits are favored domestic pets and companions, but can be both a nuisance and a menace to ecosystems when turned loose in the wild. Similarly, horses and burros are used as pets and livestock, but in the wild they can damage ecosystems and deprive other species of food and water.
An understanding of what constitutes a pest helps clear up this seemingly contradictory duality of an animal being both beneficial and a pest. The term "pest" is not an absolute term, rather it is subjective. When an organism is in the wrong place at the wrong time and is unwanted it is deemed a pest. When humans want the same organism around, it is no longer labeled a pest. With the definition of a pest so much in the eye of the beholder, reasonable people can, and often do, disagree about whether a particular organism is or is not an invasive pest.
For example, wild horses and wild burros, which were introduced on the North American continent as a consequence of the European colonization, are viewed positively, often nostalgically, by many people as a historical living legacy of America's frontier days when the wild West had miners with burros and cowboys on horseback. But to ranchers grazing public lands, wild horses and wild burros are often viewed as little more than hoofed locusts, stealing valuable forage from livestock.
Indeed, at one time wild horses were herded into dead-end canyons and shot, though now they are captured and adopted. Even environmentalists can alternately wax positive or negative, depending upon whether the wild horses or wild burros are befouling a sensitive area and threatening the food sources and drinking water of native species such as mountain sheep.
Thus, viewing an invading species in a negative light and designating it as pestiferous or alternatively viewing an invader in a positive light is the product of underlying assumptions, ideologies, and value judgments. Sometimes these underlying ideologies, assumptions, and value judgments are implicit, below the surface, and hard to discern. Other times, as with the case of the American mink (Mustela vison) in the United Kingdom, the rhetoric is as heated and open as the most contested political and ideological issues debated in the British Parliament.
In the United Kingdom, a European island nation, the American mink was imported for small-scale fur farming in 1929, and sometime thereafter began escaping into the wilds of England and Scotland. Being a polyphagous predator, American mink have gobbled up ground-nesting seabirds in the firths and lochs of northwest Scotland, as well as an endangered native mammal whose riparian habitat is threatened, the northern water vole, Arvicola terrestris. In some countries water voles could easily be depicted as just another water rat, and the case for their preservation dismissed as another example of radical environmental lunacy. But in the UK, water voles are a beloved icon of English literature, populating novels like Evelyn Waugh's Scoop and starring as Ratty, Toad's companion in The Wind in the Willows.
While the water vole has deep roots in English culture and supporters in high places direct Heritage Lottery Funds its way, getting rid of the invading American mink to aid the endangered vole is a cause that runs into the ideological concerns of animal rights activists who want to free the minks. When Great Britain passed the Mink (Keeping) Regulations of 1975, which mandated measures to stop the further escape of American minks from fur farms into the wild, little thought was given to activists in the animal rights movement invading fur farms and setting minks free. Not only have law-breaking animal rights activists invaded fur farms and illegally set loose more American mink into the British wilds, where they threaten endangered water voles, but many also question the premise that the American minks are an ecological threat.
Not all invasive mammal stories are quite as dramatic or filled with such emotional passion, even in the UK, where during the past century introduced North American gray squirrels, Sciurus carolinensis, have quietly displaced the UK's only native squirrel, the red squirrel, S. vulgaris. The red squirrel, whose British lineage dates back to the last Ice Age, is now rare in southern England, with only remnant populations remaining in places like the Isle of Wight and Poole Harbor. The picture is equally bleak in central England, with remnant red squirrel populations in East Anglia, Staffordshire, Derbyshire, and Merseyside. Even in their current strongholds of northern England, Wales, and Scotland, red squirrels are losing territory to gray squirrels.
Local red squirrel extinctions have always been relatively common. But before the intentional introduction of gray squirrels began in the 1870s (an era when species were still freely moved from continent to continent with little concern for ecological consequences), red squirrels almost always recolonized areas several years after local extinctions. The outcome may have been different if there was just one release of gray squirrels. However, there were repeated releases of gray squirrels from the 1870s until the practice became illegal in 1938. No doubt these well-intentioned human releases of gray squirrels to recolonize habitats put the red squirrels at a great disadvantage and contributed to their relatively rapid loss of home range.
Nevertheless, the best explanation put forth today for the continuing displacement of native red squirrels by introduced gray squirrels is ecological competition. In other words, gray squirrels are believed to be outcompeting and thereby replacing red squirrels in their former ecological niches such as parks, gardens, broadleaved woodlands, and conifer forests. Biodiversity and timber values (gray squirrels damage the bark) may ultimately be affected by this shift in squirrel species, which is still in progress.
The importation of the coypu, or nutria (Myocastor coypus), from South America to the North American continent is, like the gray squirrel and American mink, another case of seemingly good human intentions gone awry. Business people originally imported the herbivorous coypus from southern Argentina to the United States for fur farming in 1899. Coypu farms rapidly spread from California to Oregon, Washington, Michigan, Ohio, Louisiana, and other states. But the coypu fur craze and the demand for coypu meat eventually collapsed. Depending on the locale, and local stories vary, the coypus either escaped on their own or were intentionally let loose by failed fur farmers in the late 1930s.
Coypus are now well-established in the United States, including key coastal states like Louisiana, Texas, and Maryland. Without the predators, diseases, and other natural factors controlling their populations in South America, coypus are running amok in North America. Along Maryland's Chesapeake Bay, coypus gobble up patches of marsh plants and accelerate the conversion of rich wetland habitats into eroded ponds and bays. When not chewing up wetland vegetation in Louisiana and Texas, coypus are attacking rice and sugarcane fields.
Attempts to turn coypus into gourmet fare have yet to rekindle an interest in harvesting them for either their fur or meat. In any event, trapping is not a feasible solution for the North American continent. Hopefully, humans will devise a solution that mitigates North American coypu wetland damage, which, like most mammalian invasive species problems, was created as a consequence of human activities.
Humans, the most successful invasive species
The magnitude of the invasive species problem in agricultural and natural ecosystems prompted U. S. President Bill Clinton to organize the heads of eight federal agencies into the National Invasive Species Council in 1999. Actually, humans rank among the most successful invasive mammal species. Humans are believed to have spread from Africa to Europe and Asia over 100,000 years ago, and reached the island continent of Australia between 40,000 and 60,000 years ago. But humans are apparently relative newcomers to the Americas, having arrived between 15,000 and 20,000 years ago. The human invasion did not reach many Pacific Ocean islands until 1,000 to 2,000 years ago. A small human presence on the continent of Antarctica is a twentieth-century phenomenon.
Between 20 and 40 bird species have become extinct in North America over the past 11,000 years, a period when the presence of humans is well documented and not controversial. Many of these bird species likely disappeared because they had narrow ecological niches dependent upon now extinct large mammals like mammoths, mastodons, horses, tapirs, camels, and ground sloths. Human hunting likely played a role in the extinction of large mammals like the mammoth. But the magnitude of the prehistoric human role in extinctions involves conjecture and is still being debated.
Several species of long-legged, flightless moas and an eagle, Harpagornis moorei, are among the birds possibly hunted to extinction by New Zealand's first human inhabitants, the Maori. The loss of 62 endemic bird species in the Hawaiian Islands is associated with the arrival of the first human inhabitants from Polynesia. In North America, more recent European immigrants hunted the passenger pigeon to death at the end of the nineteenth century.
Rhinoceros species were hunted to the brink of extinction for their horns in Africa by the end of the twentieth century. Thanks to a new ecological consciousness sweeping the planet, small rhino populations still exist in protected reserves at the start of the twenty-first century. Humans have also extinguished species via habitat loss. This is among the problems addressed in the United States by legislation like the Endangered Species Act of 1973.
Invasive mammal species seem to be most serious on isolated islands where the native organisms have not evolved defenses against the mammals common to the major continents. In Great Britain, an island close to mainland Europe, only 22% of the mammal species are considered exotic. But on New Zealand's islands, which were only settled by humans within the last 2,000 years, 92% of the mammal species are recent introductions. Indeed, New Zealand and some other islands were free from mammalian predator pressure for so long that flightless bird species evolved.
In North America and the islands of the Pacific, other mammal species accompanied the human invasions. The first Asians entering the Americas brought along dogs. When the Polynesians set sail for new Pacific islands, they brought along their pigs, plants, and stowaways like lizards and rats. European colonialism from the fifteenth to twentieth centuries was a major driving force behind biological invasions of North America, Australia, New Zealand, and other areas. Like the ancient Polynesian voyagers, European colonists brought along their plants and animals to help settle these "new worlds." Besides livestock like sheep, goats, cattle, pigs, and horses, there were stowaway species like rats. Later, predators like the mongoose were deliberately introduced to help control the rats.
Feral cats in Australia
A good example of the pest/not-pest duality is the domestic cat on the island continent of Australia. Between 4 million and 18 million feral cats (Felis catus) live wild in Australia. Until recently most of these cats were believed to be descendants of European cats brought to the continent in the late eighteenth century, with a few earlier arrivals via trading ships and shipwrecks. However, Australia's aboriginal people regard cats as native. Genetic analysis indicates that Australian feral cats may have more in common with Asian than European cats, supporting the aboriginal view for an earlier arrival of cats on the continent.
But the debate of more practical consequence is whether feral cats threaten native species such as tammar wallabies (Macropus eugenii). If viewed as an invasive pest, then feral cats need to be hunted down, poisoned, given birth control, or otherwise controlled. If viewed as beneficial predators helping control other pests such as rabbits, rats, and mice, then feral cats should at least be tolerated.
In the late nineteenth century feral cats were viewed as beneficial. Cats were deliberately acclimatized and released into the Australian wild to hunt pestiferous (nuisance) European rabbits (Oryctolagus cuniculus). Indeed, Australia's Rabbit Nuisance Bill of 1883 supported releasing feral cats to help control rabbits that were damaging agricultural grazing lands.
But later in the twentieth century, feral cats were no longer welcomed as rabbit killers. Feral cats began to be viewed as invasive pests, threatening to native birds and mammals. Anticat forces pointed to the case of Marion Island, where five domestic cats released in 1949 had become a colony of over 2,000 by 1975. The Marion Island feral cat colony was destroying nearly half a million burrowing petrels per year.
On the Australian continent and on Australian offshore islands, feral cats were blamed for the regional extinction of several native bird and mammal species. The vanishing species were ground dwellers living in open habitats (favorable to cat hunting) and were the right size to be cat prey. The anti-cat forces also suspected that toxoplasmosis, a disease vectored by cats, may have played a role in mammal and carnivorous marsupial population declines many years earlier.
Feral cats were suspects in Western Australia, where a red fox (Vulpes vulpes) removal program did not stop the population decline of native fauna. Feral cats were suspected of filling the niche vacated by the red fox, and adding native species to their predominately rabbit and rodent diet. To determine whether feral cat control is a necessary policy, researchers like Robyn Molsher set up studies in New South Wales to evaluate the ecological relationships among feral cats, red foxes, and other fauna.
Since feral cats are difficult to follow in the wild, their scats (fecal droppings) were analyzed for dietary clues. Rabbits were the primary feral cat prey in New South Wales; rabbit remains were present in 82% of the scats and constituted over 68% of scat volume. The majority of the other prey was carrion, primarily sheep and eastern gray kangaroo (Macropus giganteus). Even after rabbit populations plummeted following application of a biological control agent known as Rabbit Calicivirus Disease, rabbits remained the dominant prey of feral cats. Feral cats consumed more of the house mouse (Mus musculus) following the rabbit population decline.
Foxes and feral cats tracked via radio collars shared similar habitats and prey. Foxes displayed aggression and killed some of the feral cats competing for the same food resources. When foxes were present, feral cats ate mostly rabbits and left the carrion for the foxes. In fox removal experiments, feral cats ate more carrion and hunted more at night in the same prey-rich grassland habitats favored by foxes.
Molsher concluded that integrated rabbit control programs needed to also consider fox and cat control to prevent native fauna from becoming prey in the absence of rabbits. However, rabbits are so well established across such a vast area that the goal of rabbit eradication in Australia has been abandoned in favor of long-term population suppression. This seems to vindicate a continuing beneficial role for feral cats as rabbit and rodent predators in Australia.
Integrated rabbit control
Prolific reproductive potential has helped the European rabbit become a very successful invasive species. Originally from North Africa, the European rabbit spread north through Italy to the British Isles and then around the world, causing ecological havoc in some countries. On the Hawaiian island of Laysan, the rabbit is credited with wiping out 22 of 26 native plant species at the beginning of the twentieth century.
Since its mid-nineteenth century introduction into Australia, the European rabbit has been a major plague. Vegetation is grazed from vast stretches of land that become more desert-like and less suitable for livestock grazing despite the killing of millions of rabbits every year. Over a century after starting rabbit mitigation programs, Australia still spends an estimated $373 million per year on rabbit control.
Eradication is deemed feasible only on small islands or in small localized areas where rabbit populations are newly established. The few small islands off the coast of Western Australia where rabbits have been eradicated are the exception, not the rule. More typical is 46 mi2 (120 km2) Macquarie Island and the main Australian continent, where rabbits are so well-established that eradication has been replaced with the more realistic goal of population suppression.
Population suppression is accomplished using a suite of varied biological, mechanical, and chemical control techniques. This integrated pest management approach includes predators, microbial control agents, warren ripping, and electrified and wire-net fences. Wild rabbits are also hunted and "harvested" as a commercial product. Barrel or soft catch traps are still used against small isolated rabbit populations. (Humane considerations have largely precluded continued use of the traditional steel-jawed leg-hold trap.) But rabbit populations are so high and the species is so prolific that shooting and trapping have no significant impact on populations.
A variety of poisons and fumigants are still used against feral rabbits, though safety, environmental, and humane concerns have been raised. The most widely used vertebrate control pesticide is 1080 (sodium monofluoroacetate), which is formulated into paste, pellet, food cube, grain, and carcass baits to poison animals such as rabbits, feral pigs, wallabies, wombats, dingos (wild dogs), possums, rats, mice, and foxes. Food chain risks are inherent in poison baiting, particularly when individuals lack baiting expertise. Another drawback is that sheep, cattle, horses, goats, cats, dogs, some native wildlife, and humans are also very susceptible to 1080, and there is no known antidote to the poison.
Destroying warrens by ripping or plowing is a less controversial alternative to poisons, though the two techniques are sometimes combined. Sometimes rabbit kill is maximized by using dogs to drive rabbits into their warrens before burrow destruction commences. But rocky areas, riversides, and steep sandbanks with rabbit warrens are impossible to rip up and destroy, short of explosives.
In some locales rabbits prefer surface refugia rather than warrens. This means habitat management may be needed. However, the same shrub, blackberry, and log debris habitats favored by rabbits are also home to desirable species of birds, reptiles, amphibians, and other small mammals. So, it is not always desirable to modify the habitat to fight rabbits.
Biological control using predators, parasites, or microbes can be part of integrated rabbit control programs and help overcome the limitations of poisoning and habitat modification. One of the more famous instances of biological control was the introduction of the myxoma virus to fight rabbits.
Resources
Books:Crosby, Alfred. Ecological Imperialism: The Biological Expansion of Europe, 900-1900. Cambridge: Cambridge University Press, 1986.
Kirch, Patrick, and Terry Hunt. Historical Ecology in the Pacific Islands: Prehistoric Environmental and Landscape Change. New Haven, CT: Yale University Press, 1997.
Perrings, Charles, Mark Williamson, and Silvana Dalmazzone. The Economics of Biological Invasions. Northampton, MA: Edward Elgar Publishing, 2000.
Staples, George, and Robert Cowie. Hawaii's Invasive Species: A Guide to the Invasive Alien Animals and Plants of the Hawaiian Islands. Honolulu: Mutual Publishing and Bishop Museum Press, 2001.
U.S. Congress, Office of Technology Assessment. Harmful Non-Indigenous Species in the United States, OTA-F-565. Washington, DC: U.S. Government Printing Office, 1993.
Vitousek, Peter, et al. Biological Diversity and Ecosystem Function on Islands. Heidelberg, Germany: Springer-Verlag, 1995.
Periodicals:Flux, J. E. C. "Relative Effect of Cats, Myxomatosis, Traditional Control, or Competitors in Removing Rabbits From Islands." New Zealand Journal of Zoology 20 (1993): 13–18.
Molsher, Robyn. "Trappability of feral cats (Felis catus) in central New South Wales." Wildlife Research 28 (2001): 631–636.
Pimentel, David, et al. "Environmental and Economic Costs Associated With Nonindigenous Species in the United States." Bioscience 50 (2000): 53–65; 309–319.
Roemer, Gary W., et al. "Feral Pigs Facilitate Hyperpredation by Golden Eagles and Indirectly Cause the Decline of the Island Fox." Animal Conservation 4 (2000): 307–318.
[Article by: Joel H. Grossman]
