Mammals (class Mammalia)
Many Mammals are introduced as food sources as in livestock, out of a sense of nostalgia, and accidentally sometimes. Being nocturnal, some Mammals can be hard to detect, but the effects from their presence can always be told --- as we will soon see in detail.
eg 1 Mice and Rats
Order: Rodentia (Rodents)
Family: Muridae (Mice and Rats)
Scientific name: \
When it comes to the most notorious pest mammals, we cannot help acknowledging that these cosmopolitan travelers are the most successful survivors in the hazardous world of humans, and consequently, those which have the most devastating effect both on human life and the native wildlife from wherever human explorers set their ominous feet upon.
Order Rodentia is the most diverse order of all mammalian groups, and over 1300 species are in the family of rats and mice. Of these however only a few are adventurous and adaptable enough to hitch a ride on explorers' vessels to be transported all over the world, and these are the few common Murids we denominate as 'vermin'.
• Brown, or Norway, Rat (Rattus norvegicus)
• Black, or Roof Rat (Rattus rattus)
• House Mouse (Mus musculus)
• Polynesian Rat (Rattus exulans)
These rodents are of Asian origin (in the case of the House Mouse the native range stretches to the Mediterranean coast), but can now be found on every continent and many Oceanic Islands, except Antarctica. The Norway Rat is the largest of the four, is rather more large-bodied than the Black Rat, which has a longer tail and is a more agile climber. Both measure about 40cm from head to tail, in contrast to the smaller House Mouse, which is only about a quarter to half the size; the Polynesian Rat is slightly larger but still is the smallest Rattus species, its native home in the humid tropical regions of South-east Asia. These venturing omnivores seldom shun potential food sources, and consume almost anything from grains, vegetables, meat, milk, confectionary, soap, milk, eggs, or less nutritious structures such as sofas and wood to grind of the tips of their ever-growing incisors, adapted originally for the consumption of plant stems and other vegetable materials. Within the vicinity of human inhabitation, they subsist equally well on carrion, refuse, offal and spoiled food, and are benefited by crop practices and horticultural developments; in newly invaded areas, in particular, the long-isolated Pacific Islands, they have rapidly requisitioned rich sources of food --- the eggs and chicks of a stunning multitude of native birds, reptiles, amphibians and plants, hustling many of them towards the brink of extinction. They also have a large appetite, and can consume food up to a third of their weight in a single day. Human constructions provide excellent shelter for these commensals, and around urban areas the population density of House Mouse can reach 10 per square metre, 1000 times of that in the wild.
Mice have a high reproductive rate, and litter sizes can easily exceed a dozen. This has led to plagues whenever food supplies for the rodents have increased, and these opportunists never fail to take advantage of the feast. They are swift runners and are adept to hiding, and this has made the control of these pests quite difficult.
Outbreaks of epidemics with Rats and Mice as vectors have broken out in times of grain ripening, along the moist areas of the temperate wheat belt, where the soil is soft and Rodents find it easier to construct nests within. Bubonic plague, the disease that managed to kill of at least a quarter of the population in Europe in the 13 th century, was carried by Rats introduced into the fertile lands of Northern Europe; though the plague is essentially long gone, these Rodents are still associated with the transmission of the salmonella and leptospirosis, which directly cause threat to human health, especially when large Rodent populations have developed around human habitation.
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eg 2 Pig
Order: Aritodactyla (Even-toed Ungulates)
Family: Suidae (Pigs and Boars)
Scientific name: Sus scrofa
A stampeding herd of pigs in the understorey may not be thrilling for humans. Yet it could be disastrous for the native life of many islands into which these merciless terminators have been introduced.
Pigs are large ungulates (hoofed mammals) that have evolved an omnivorous lifestyle. The domestication of Pigs had been intended for food and pork is still a major source of the world's protein source for humans, but it is the feral pigs that happen to be the hazard to wildlife and pristine island habitats, in particular in the Hawaiian Islands. Feral Pigs are powerful with stout legs, mobile snouts and necks for uprooting the soil in search of food, and males sport formidable tusks. They occur in dense forests, riparian shrublands and wetlands, and feed on almost everything they came across.
In the Hawaiian Islands, Pigs have been found to uproot tree seedlings, feasting on palatable stems and leaves of Ferns and Orchids. They also take both ground-nesting and burrow-nesting seabirds, devouring the eggs and young birds. As native Hawaiian plants have no protective mechanisms against this alien grazer, for example thorns, toxic chemicals and undesirable taste, Pigs feed on them preferentially and this largely puts the local flora at risk, apart from competition with other exotic plant species that compete with them for space and nutrients. Pigs are known to create mud pits in the forests, which they use in mud bathing, and these serve to accumulate water. These wallowed pools turned out to be good breeding grounds for the introduced mosquitoes that are responsible for transmitting fatal avian malaria to endangered native birds that have no resistance against the disease.
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eg 3 European Rabbit
Order: Lagomorpha (Lagomorphs)
Family: Leporidae (Rabbits and Hares)
Scientific name: Oryctolagus cuniculus
The 80 or so domestic varieties of domesticated Rabbits have all originated from this species. These medium-sized herbivores have been domesticated since 1000 years ago, when they spread throughout the Roman Empire around the Mediterranean. That has been a remarkable sprawl of range already, as the species was confined since the last Ice Age to pocketed areas in France, Northwestern Africa and Iberia only. Now it is found on virtually any continent, where suitable open areas with short grasses are available. Since the 19 th century they have been introduced to Australia and New Zealand, where colonists strove to re-create the environment of their original homeland by introducing many European mammals and birds, including Rabbits, probably the most detrimental, as a non-indigenous species, of them all. In 80 years since the initial introduction, the Canterbury Plains have become infested with Rabbits, which further denude places like Central Otago of soil-binding grasses in the summer, leading to soil erosion in winter months. Nutrients are continually lost and the land turns barren, and the impact on the environment was disastrous. The Rabbit population exploded so drastically that export of frozen rabbit meat and skin became a fast-growing industry right up into the 1940s. Local demand for Rabbit meat actually supported the growth of Rabbit populations, as there was no incentive for eradicating this pest, which managed to leave thousands of sheep to starve in some sheep farms on the South Island. Night hunters in Otago have reported sighting hundreds of Rabbits fleeing towards their burrows when they shone their spotlights on pastureland in the hilly country.
Wild Rabbits measure an average of 45cm and are exclusively herbivorous feeders on grasses, barks to roots, and their diet consist of mostly indigestible cellulose, which implies that they have the necessity to eat a great deal for sustenance. They are gregarious and territorial, and have a large breeding potential. Given the large array of predators that are present in their home territory such as Goshawks, Foxes, Weasels and other Mustelids, their natural populations are generally kept in close check. However they have a short gestation period at 30 days, and a litter can contain more than 5 kittens (young Rabbits). The high infant mortality, at 90%, indicates that many young Rabbits fall prey to natural predators within the first few months of birth, and a population explosion is less likely to occur.
In areas where natural predators are absent, scientists have suggested methods such as introducing the pathogen myxomatosis via flea carriers to Rabbit communities. Infected Rabbits die in agony, thus the measure is widely opposed. More effective and feasible methods have yet to be devised. The Australian population of Rabbits, reaching 20 million in 1900, has been dealt with by introducing foxes as predators, which in turn preyed on other native marsupials and drove many to the brink of extinction. Thereafter poison baits has been used, but the possible adverse effects this may have on other native animals can readily be foreseen.
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eg 4 Domestic Cat
Order: Carnivora (Carnivores)
Family: Felidae (Cats)
Scientific name: Felis catus
Cats are probably one of the most familiar animals of all to humans, and since their domestication over 4000 years ago in Egypt, have become glorified commensals of man. They are solitary, nocturnal predators, as is evident from sharp retractable claws for stealth compounded with efficacious lethalness at kills, acute vision in dim light and acoustic keenness. Domestic Cats come in a wide array of colours and forms but are essentially predatory and carnivorous, averaging 70cm in length and 5kg in weight; their relatively small size and agility enable them to tackle smaller prey such as Rabbits, Mice and some semi-terrestrial birds.
The comeliness and playful nature of these felines have led them to being favoured pets, and this factor probably account for many introductions, which have resulted in drastic consequences. Cats are the natural enemy of small rodents such as mice, which have become overpopulated pests in many areas, and effective controlling measures are always wanting. Some cats are introduced into households for this reason.
The very problem with domestic cats is that they are far less domesticated as they are deemed, but they are treated as so. They are well fed by their owners, thus are not vulnerable to extreme fluctuations in the prey populations and food shortage. They frequently kill wild animals in spite of feeding by their owners, and feral cats pose even more serious problems. They are responsible for driving some Island birds to scarcity or even extinction, these restricted-range species being accustomed to the predator-free environment of the islands and easily falling prey to the Cats. Such a land bird includes the dramatic Stephen Island Wren (Xenicus lyalli), which has a natural range of merely 1 square mile (2.6 square kilometers) on Stephen Island, off the Northern tip of South Island, New Zealand. The small population present on the island has been extirpated by a single lighthouse keeper's cat through direct predation, the cat bringing back dead specimens of the world's only flightless Passerine (songbird). In New Zealand alone, the introduced Cats eliminated 6 species of land birds. Similarly, the Sorocco Dove (Zenaida graysoni) has also been driven to extinction after the introduction of Cats into there island home, and many birds in the Pacific Islands share a similar fate, falling prey to these efficient, sneaky predators.
Elsewhere in the world, Cats also pose severe threats to the survival of small countryside animals. In Britain alone, 100 million small mammals and birds are estimated from studies to be consumed by Feral and Domesticated Cat. In Florida, where habitat fragmentation, pesticide pollution and various other factors have already reduced the populations of resident birds and neotropical migrants, high densities of cats in some urban parks have proved a threat to these migrants which rely on the forest remnants and parks as migration intermediate sites that supply the energy for them to complete their epic journeys.
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