Mammals and Humans: Mammalian Invasives and Pests
Mammals and humans: Mammalian invasives and pests
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.
Viruses for rabbit control
The myxoma virus was imported into Australia in 1936, and extensively studied before being released into the environment in 1950. The virulence of the myxoma virus is rated on a scale of one to five, with one being most virulent. The original myxoma virus strain released into the environment was rated one, and provided a spectacular 99% rabbit mortality when first released into the environment in the early 1950s. Without rabbits grazing on the landscape, the amount of forage available for sheep production soared and farmers were very happy with the financial windfall.
But the spectacular success did not last. There is an ecological interaction over time involving the pathogenicity or virulence of a microbe and the genetic suseptibility or immunity of the host population. A microbe that is 100% successful in killing its host would go extinct along with its host. So, ecological theory favors the evolution of a less virulent microbe that does not kill the entire host population. Indeed, the evolution of reduced myxoma virus virulence and increased rabbit immunity became a classic epidemiological case.
Over time the virus attenuated, and the virulence of field strains of myxoma virus declined from one to three. At the same time, the rabbit population developed greater immunity to myxoma virus. Releasing the highly virulent myxoma virus strain rated one into the environment no longer produces the 99% rabbit kill seen in the 1950s. At the end of the twentieth century, myxoma virus was producing a more sustainable rabbit kill of between 40% and 90%. This reduced rabbit kill is still very important to integrated control programs. Indeed, in the absence of myxoma virus rabbit populations can still soar to very high levels.
In theory, integrated pest management can incorporate multiple techniques, each providing a percentage of pest population suppression. In practice, studies in both central and south Australia demonstrate the advantage of combining multiple techniques into an integrated pest management program for rabbits. When myxoma virus is used alone, as is typically the case, rabbit populations rebound to high levels in subsequent years. However, when rabbit warrens are ripped or plowed after myxoma virus has already reduced the rabbit population, the area can remain almost devoid of rabbits for many years.
Bolstered by the half century of continuing success with myxoma virus, it has only been natural to look for additional rabbit pathogens to introduce into the environment. In 1984 Chinese scientists identified an acute infectious rabbit disease called Rabbit Calicivirus Disease (RCD). RCD was subsequently identified in Europe, Asia, Africa, and Mexico. Australia began studying RCD on wild and laboratory rabbits and non-target species in 1991. Several months after a 1995 field trial on South Australia's Wardang Island, RCD was detected on the mainland. In 1996, RCD was officially recognized as
a biological control agent under the Commonwealth Biological Control Act.
In some parts of South Australia RCD has killed over 90% of the rabbits, and longer term rabbit populations are down 17%. RCD has been spreading at the rate of 250 mi (400 km) per month. An insect vector traveling in the wind is believed responsible for this rapid spread. However, humans and implements in contact with carrier rabbits may also spread the disease. More time is still needed to see how well the combination of myxoma virus and RCD work against rabbit populations.
Less lethal fertility control agents, a humane solution favored by animal welfare groups, are also under development for rabbits, foxes, and mice. One idea is to engineer a virus like the myxoma virus with an antigen causing animals to produce antibodies that reduce fertility. An estimated 60% to 80% of female rabbits need to be stopped from breeding in order to reduce rabbit populations. Immunocontraception will likely be tested on wild rabbits sometime before 2010.
Even if not wildly successful by themselves, new techniques like immunocontraception will likely play a role in integrated pest management systems targeting rabbits and other pest mammals. It is clear from over a century of rabbit control efforts that no single pest control technique by itself will work everywhere. Even myxoma virus, which looked so promising with its initial 99% rabbit control, is no longer viewed as a stand-alone solution. The integrated pest management paradigm of combining multiple control techniques and ecological principles is the wave of the future for combating invasive pests.
Invaders in paradise
The 6,393 mi2 (16,558 km2) Hawaiian Islands chain, a collection of 132 islands, reefs, and small shoals, has only one native land mammal species, the Hawaiian hoary bat (Lasiurus cinereus semotus). Thus, the Hawaiian Islands are a good "laboratory" for the studying the ecological effects of introduced mammals.
Even such usually ubiquitous land organisms as ants and mosquitoes were absent from the Hawaiian Islands' native fauna. For much of its geological history the world's largest ocean, the Pacific Ocean, acted like a giant moat, keeping the Hawaiian Islands relatively isolated and deterring invading species. The winds, ocean currents, and migrating birds brought species to the Hawaiian Islands, but very few became established. On average over the last 70 million years, only one invading species per 35,000 years successfully established in the Hawaiian Islands.
However, a diverse topography, a warm tropical climate and an absence of predator species during most of the past 70 million years made the Hawaiian Islands a good evolutionary locale for new species formation via adaptive radiation. A spectacular example of new bird species formation by adaptive radiation in the Hawaiian Islands is the 54 species of Hawaiian honeycreepers (Drepanididae). Compared to the Galápagos Islands and its 14 Galápagos finch species that inspired Charles Darwin's theory of evolution, the Hawaiian Islands have more habitat diversity and a longer evolutionary history.
A mammalian invasion began transforming the Hawaiian Islands approximately a.d. 400, when the first Polynesian sailing canoes arrived. The Polynesian voyagers brought animals and
plants with them, and changed the landscape with their agriculture. New species in the Hawaiian Islands increased to three to four per century after the Polynesians arrived, a huge increase from the pre-human one species per 35,000 year average.
Even before the Europeans arrived in the late eighteenth century, the Hawaiian Islands had a few hundred thousand people of Polynesian descent. The fauna introduced by humans included the Polynesian (or Pacific) rat (Rattus exulans), dogs, pigs, fowl, and reptiles. The Polynesian rat is a native of Southeast Asia that spread across the Pacific Ocean islands to the Hawaiian Islands with the Polynesians, but never reached the mainland of the United States.
Polynesian rats attract a lot of attention as pests of plantation agricultural crops like sugarcane and pineapple, though a broad range of crops are attacked. Polynesian rats are omnivorous, and studies show adverse impacts on coastal tree and lizard species in New Zealand and on seabirds on several Pacific islands. There are little data available on Polynesian rat ecological effects on now extinct Hawaiian Island birds.
In the Hawaiian Islands, about half the land bird species predating human arrival have vanished. Direct human impacts from hunting and gathering and indirect human impacts are strongly implicated in the decline or extinction of native species, particularly flightless birds and ground-nesting winged species. The magnitude of ancient human impacts on specific species is still the subject of vigorous debate and inquiry. However, there is little doubt that the rate of worldwide ecological change and species extinctions directly and indirectly attributable to human beings began increasing in recent centuries.
The European ships of the eighteenth and nineteenth centuries brought exotic mammals from around the world to the Hawaiian Islands, including new rat species, European pig genotypes, cattle, goats, sheep, the house mouse, and the mongoose. Goats and cattle trampled and grazed native plants and generally degraded habitats. Hawaii's native bird species suffered additionally when early nineteenth century whalers introduced the first mosquitoes and avian malaria. New diseases like smallpox and syphilis were transferred from the European arrivals to the Polynesian population. Clearly, as the top mammal species, human impacts increased as human populations increased and spread.
Globalization and the expansion of ship and airplane commerce in recent centuries accelerated the rate of new species
introductions. When all the newly introduced plant and animal species were added up, the rate of new species introductions into the Hawaiian Islands was estimated to have accelerated to several dozen species per year in the twentieth century. Ecological upsets and pest problems from introduced mammals became more noticeable in the Hawaiian Islands during the nineteenth and twentieth centuries.
Black rats, also known as roof rats (Rattus rattus), and Norway rats (Rattus norvegicus) disembarked on Pacific islands as stowaways aboard European sailing ships and spread rapidly in the nineteenth century. Norway rats and black rats are omnivorous, attacking agricultural crops and feasting on the young and eggs of seabirds like petrels, shearwaters, gulls, terns, and tropicbirds.
Birds on isolated islands like New Zealand and Hawaii evolved in pre-human times when there was no need for defenses against mammalian predators like rats. Bird depredations by rats are less common nearer the equator. One untested hypothesis is that birds nearer the equator developed better predator defenses useful against rats, because of land crabs preying on eggs and chicks.
Black rats are suspected in the demise of many native Hawaiian birds during the nineteenth century. But the biological documentation from that period is not considered conclusive by modern standards. Very recent technological advances like night vision videos provide more conclusive evidence. For example, night vision videos revealed beyond doubt that black rats were a major predator of New Zealand's endangered Rarotonga flycatcher (Pomarea dimidiata). Consequently, rat control became part of the program to save that endangered forest bird species.
Mongooses for rat control
The Indian mongoose, Herpestes auropunctatus, was deliberately introduced into Pacific and Caribbean islands in the late nineteenth century for biological control of rats in plantation crops like sugarcane. This was a rare attempt at biological control by introducing a mammal to prey on another introduced mammal. One of the more colorful stories from the Hawaiian Islands is that a man known as "Mongoose" Forbes sold mongooses to sugar plantations as rat catchers in the 1870s. More scientific accounts suggest that the Indian mongoose was introduced to the Hawaiian Islands via the West Indies in the early 1880s. Whatever the story, the deliberate introduction of the mongoose for rat control was a badly flawed idea.
Modern twentieth century biological control programs screen potential introductions to make sure that desirable flora and fauna are not destroyed. This was not part of the scientific protocol in the nineteenth century when the mongoose was introduced for rat control. Even though mongooses eat rodents in sugarcane fields, they do not provide adequate rat control. One problem is that the mongoose is a diurnal hunter, whereas the rats they were introduced to control are nocturnal. Also, mongooses do not stay put in agricultural plantations. So there have been serious consequences for native species in native ecosystems.
From the islands of Fiji to the Caribbean, mongooses are predators of a wide range of native wildlife and a potential reservoir for diseases like rabies and leptospirosis. The ground-nesting quail dove (Geotrygon mystacea) was nearly eliminated from the Virgin Islands by mongoose predation. Hawaii's endangered state bird, the nene or Hawaiian goose (Nesochen sandvicensis) is attacked by the mongoose, as are the endangered Hawaiian dark-rumped petrel (Pterodroma phaeopygia), Newell's shearwater (Puffinus newelli), and the Hawaiian crow (Corvus hawaiiensis). Turtles, native lizards, snakes, and poultry are also mongoose prey. Live traps, hunting, and poison baits are among the mongoose control methods available.
The bad experience with mongooses did not extinguish Hawaii's interest in biological control of rats. In the late 1950s, Hawaii introduced barn owls (Tyto alba) for rodent biocontrol. Even dogs have been used to hunt rodents in sugarcane fields. Trapping tends to be too labor intensive for large outdoor areas with rats. Early in the twentieth century almost 150,000 rats were trapped annually in Hawaii's sugarcane plantations with no noticeable effect on rat populations or crop damage. Shooting is also of questionable value for rat control. So, poison baits have been the fallback for keeping rat populations under control.
Feral European pigs, a late eighteenth century introduction, may be the largest mammalian threat to native forest ecosystems in the Hawaiian Islands. Feral pigs dig up young trees to eat the roots and spread weed seeds. Native plants and crops like sugarcane are attacked. From New Zealand to the Galápagos Islands, feral pigs also have a reputation for digging into burrows to consume seabirds like petrels. Feral pigs also feast on the eggs and young of surface-nesting seabirds like boobies, shags, and albatrosses.
The original pot-bellied pigs introduced from Polynesia into the Hawaiian Islands in the fourth century are smaller, more docile animals and a different genotype than the larger, much more aggressive European pigs. Polynesian pigs are also less inclined to roam and go feral, and are less of a threat to the native ecosystems of the Hawaiian Islands.
Most popular accounts date the arrival of the European pig genotype in the Hawaiian Islands to the arrival of British ships under the command of Captain James Cook in 1778. Apparently the British were disappointed by the small size and tougher texture of the meat of the Polynesian pig, and so
introduced the larger, more succulent European pig to the Hawaiian Islands.
In the nineteenth century, the introduction of new species was viewed as a positive and encouraged. Indeed, mid-nineteenth century advertisements exhorted sea captains to import and release their favorite songbirds in the Hawaiian Islands. Even in the mid-1800s, settlers in the Hawaiian Islands noticed forest habitat destruction and native bird losses, and felt that the introduction of their favorite European species would help compensate. Never mind that previous introductions like European pigs and goats were among the culprits destroying the habitat. It was an age when feral European species were welcomed by hunters, albeit opposed by agricultural interests suffering crop damage.
By the early twentieth century European pigs had completely displaced Polynesian pigs in the wild, and an eradication program was started because of feral pig damage to the Hawaiian Islands' native rainforests. By the mid-twentieth century, 170,000 feral pigs were killed. But feral pig populations
were still trampling, uprooting, eating, and otherwise destroying the Hawaiian Islands' native ecosystems.
Though damage by goats, feral pigs, and other grazing mammals seemed obvious to observers, there was little quantitative baseline data against which to measure the ecological impacts. Near Thurston Lava Tube in Hawaii Volcanoes National Park, the National Park Service set up an experiment in which some forest plots were fenced to keep out feral pigs. Compared to unfenced plots where pigs roamed freely, fenced plots had fewer exotic plant species and more native plant species. Fenced plots also had less pig damage, as measured by fewer exposed plant roots and less exposed soil. Not surprisingly, fencing is used as an integrated control measure to keep pigs out of sensitive areas.
The National Park Service has also been following the spread of feral pigs from lower to higher elevations on the Hawaiian island of Maui. Feral pigs were removed from Maui's Kipahulu Valley in the 1980s. Removing the pigs allowed the forest understory to recover and slowed the invasion of exotic plant species, a positive ecological outcome. Nonetheless, there has been opposition to using snares and hunting to remove the pigs. Indeed, one person's invasive mammal pest may have redeeming positive qualities for another person. Hunting groups want the pigs to remain as game, and some indigenous groups oppose eliminating the pigs for cultural reasons.
Pigs threaten foxes
The case for removing feral pigs (Sus scrofa), has less opposition in the Channel Islands National Park and other islands off the coast of southern California. This is a case where one invasive exotic mammal species, the feral pig, has changed the ecological relationships among several native predators. On four islands, feral pigs introduced into the ecosystem are indirectly leading to the extinction of four subspecies of island fox (Urocyon littoralis), a tiny animal smaller in size than a house cat. The smallest member of the family Canidae in North America, island foxes show little fear of humans and were probably once kept as pets by Native Americans.
There are no island foxes left in the wild on San Miguel and Santa Rosa Islands, where captive breeding programs are aiming to save the island fox subspecies from extinction. Fewer than 200 island foxes are left on Santa Catalina Island, in part because of canine distemper virus vectored by domestic dogs. The island fox subspecies on Santa Cruz Island has seen its population decline from 1,300 to fewer than 100. The U. S. Fish and Wildlife Service proposed the four rare island fox subspecies for protection under the Endangered Species Act, and eliminating feral pigs from the Channel Islands is part of the plan to save the island foxes.
Feral pigs and other introduced grazing mammals like rabbits, sheep, goats, cattle, deer, elk, and horses contribute to degradation of the island habitat. The presence of feral pigs also introduces an indirect ecosystem food chain effect contributing to the demise of the island fox. Feral pigs serve as prey, allowing golden eagle (Aquila chrysaetos) populations to flourish. Historically, golden eagles are a mainland species that has neither bred nor overwintered on the islands. The islands have historically been nesting grounds for the bald eagle (Haliaeetus leucocephalus), which preys mostly on marine mammals and fish.
Basically, the introduction of feral pigs allowed the golden eagle, a mainland species, to become established on the islands. Unlike bald eagles, golden eagles prey on island foxes. One study found that with 90% golden eagle predation, island fox numbers declined to zero. Released from competition with island foxes, island spotted skunk (Spilogale gracilis amphiala) populations also increase as an indirect consequence of feral pigs supporting golden eagle populations. In other words, feral pigs feed the golden eagles which eat the foxes which frees up space for the skunks.
Golden eagle removal and relocation is part of the plan to save the island fox and restore the ecosystem. However, satellite telemetry studies indicate that golden eagles relocated to the mainland will try to return to the islands as long as feral pigs are available as a food source. Reintroduction of the bald eagle is also being considered to help restore the ecosystem. The bald eagle is very territorial, and may deter the golden eagle from nesting. However, removing feral pigs as a food source is the more important factor in preventing reestablishment of golden eagle populations, restoring the ecosystem, and saving the island fox.
There are many more stories of invasive mammals to tell, and more details untold about feral pigs, foxes, rabbits, cats, squirrels, rats, mice, horses, burros, and other invasive mammals than can easily fit on the printed page. But this overview of mammalian invasions contains many of the basic principles needed to better comprehend the plethora of media stories on invasive mammals.
Many of the invasive mammal examples have been from islands, as scientists prefer to study the simplest possible finite systems before venturing forth to tackle larger continental problems. In other words, the island serves as a laboratory for studying invasions at their simplest. Even in Australia, many rabbit control solutions were first tested on small islands to perfect them before introducing them to the mainland continent.
The invasive mammal problems in some areas are likely to grow worse before they get better. But knowing that many of these invasive species are so successful in their new homes because they left behind the predators, parasites, diseases, and other natural control factors that kept populations under control in their native lands suggests one avenue of control. Namely searching the native lands of the pestiferous mammals for natural control factors that can be safely introduced elsewhere, like the myxomatosis virus introduced successfully into Australia for rabbit control.
However, if nothing else is learned, it is that great care must be taken so that the cures introduced are not worse than the original invasive mammal problems, as was the case with the introduction of the mongoose for rat control. Since humans created most of the invasive mammal problems, it might be reasonable to expect that humans can collectively atone by researching new solutions that minimize the possibility of ecological harm.
In the future, look for clever ecological manipulations of populations, like on the Channel Islands, as well as more molecular, biotechnology solutions like immunocontraception for rabbits. But rather than expecting the newest technological solution to be the ultimate answer, remember that organisms can and often do adapt, just as the rabbits in Australia built up immunity to the myxomatosis virus and the virus attenuated over time.
If nothing else, the half century of experience with rabbits in Australia points to the need for a control strategy integrating multiple techniques (like the myxomatosis virus plus ripping rabbit warrens) to achieve the best and longest-lasting results. Hopefully, the magic bullet approach of the pesticide era will be replaced with this more comprehensive integrated pest management approach.
However, the human side of the equation must never be forgotten when dealing with invasive mammals, as many of these animals in other non-pest contexts are highly valued. Hence, pestiferous feral cats, rabbits, wild horses, burros, pigs, and other mammals causing problems cannot be treated as the object of extermination like cockroaches or termites. Every mammal seems to be loved by some group, be it hunters and indigenous people who favor wild pigs or animal rights groups who champion freedom for minks. Right or wrong, good or bad, these varied human sensibilities need to be taken into account in designing any integrated pest management program to control invasive mammals. For example, in the western United States, capturing wild horses and letting people adopt them has replaced the old practice of herding the horses into canyons and shooting them. This type of solution may have more to do with politics or social science and consensus building than with biological or ecological principles, but ignoring the human species behind the invasive mammal problems is to invite failure.
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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.
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.
Joel H. Grossman
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