Significance to humans

Many species of rodents are considered nuisance animals. Burrowing activities of nutria and muskrats can undermine water retention structures like earthen dams and levies. Woodchucks (or ground hogs), eastern chipmunks, prairie dogs, and ground squirrels also burrow, resulting in potential damage to vegetable gardens and structures. Tree squirrels are incredible pests in urban areas. They are true artisans at exploiting "squirrel-proof" bird feeders. Upon invading houses, tree squirrels can damage attics, especially insulation and wiring. Woodrats (genus Neotoma), sometimes known as packrats, frequent barns and other buildings associated with humans. These rodents are notorious for decorating their nests with objects taken from various parts of their territory, including human artifacts. North American pocket gophers of the family Geomyidae are both a pest and a benefit. These subterranean rodents are beneficial in terms of increasing soil fertility, aeration, and water infiltration. Their rotation of soil also reduces compaction. At the same time, pocket gophers cause serious damage to underground cables and irrigation lines by their gnawing activities. Underground telephone cables are vulnerable unless either protected by a gopher retardation device or buried below the soil's A zone. The common house mouse, Mus musculus, is a fixture in many homes, warehouses, and other human-made structures. In some circumstances, house mice can contaminate stored food, damage materials maintained in warehouses, and cause structural damage through their nest building and habit of gnawing.

Some rodent species cause serious damage to commercial forest operations. The mountain beaver, Aplodontia rufa, a species occurring from southern British Columbia to northern California, is a serious pest that feeds on conifers. Primary damage occurs as a result of girdling trees, damaging seedlings, and gnawing roots. It has been estimated that this rodent has extensively damaged over 300,000 acres (120,000 ha) of coniferous trees in parts of Washington state and Oregon. At a value of $10,000 per acre, this species has caused millions of dollars in damage. Voles also eat conifer seedlings at sites of reforestation and Christmas tree farms. These small rodents require high grass for cover. Therefore, prevention of damage to seedlings can be accomplished by minimizing the amount of grass cover. Although less damaging, nutrias are known to girdle trees, and in forest plantations and fruit orchards, the North American porcupine can cause damage. Like the pocket gopher, the porcupine also is beneficial by creating diverse habitat for many forms of wildlife, especially birds. Cotton rats (Sigmodon) occur throughout regions of the southern and western United States. These rodents are herbivorous and prolific breeders. When cotton rat populations are high, these rodents can cause damage to alfalfa and other crops. Although prairie dogs generally prefer overgrazed or disturbed habitat, they can impact rangelands by reducing the amount of forage available to livestock. At the same time, the burrow of prairie dogs supports many species of vertebrates and invertebrates, and the prairie dog is a major prey item of both the endangered black-footed ferret and birds of prey.

Rodents cause considerable damage to cash crops in several countries. Studies have shown that rodents consume from 1 to 20% of crops in industrialized nations and as much as 50% in less developed countries. In parts of Southeast Asia, where rodents destroy large portions of the rice crop, reduction of rodent populations resulted in a 200% increase in rice crops. The reproductive ability of many species of the family Muridae make these rodents especially harmful to crops in many parts of the world. For instance, rodent outbreaks can dramatically increase population densities of rodents, with numbers being in the thousands per 400 acres (1 ha), thus posing a serious threat to crops. For instance, a population of the European hamster (Cricetus cricetus) in Hungary resulted in nealy 988,422 acres (400,000 ha) being impacted. Several genera in the murid subfamily Gerbillinae (e.g., Meriones, Tat-era, and Rhombomys) are considered major pests of cultivated crops from northern Africa to regions in India. These gerbils can severely damage cereal as well as vegetables, olive saplings, and other economically important crops.

A North American red squirrel (Tamiasciurus hudsonicus) carries a pine cone, from which it may extract the seeds. (Photo by Bob & Clara Calhoun. Bruce Coleman, Inc. Reproduced by permission.)

North America's largest rodent, the beaver (Castor canaden-sis), has some positive and negative attributes. Historically, the beaver has been prized for its pelt, and beaver trapping had a severely negative impact on natural populations throughout portions of its range in North America. As "nature's engineers," beavers are capable of modifying habitat that favors wetlands. Through their dam building activities, beavers create habitat suitable for waterfowl and other wetland species. However, alteration of habitat and harvesting trees for food and support materials also threatens both agricultural and forest interests. Dams can initiate flooding in some areas resulting in the loss of trees that are intolerant of high water levels. In addition, beavers can pose a threat to human health. They are known to carry Giarda, an intestinal parasite, transmitted to humans through drinking water, and exposure to beaver lodges can result in humans contracting Gilchrist's disease, which causes pneumonia-like symptoms.

Many species of rodent endanger human health. Bubonic plague or the "Black Death" is a dreaded disease that created havoc during the sixth and fourteenth centuries in Europe by killing almost one third of its population. The total number of deaths from bubonic plague was approximately 137 million people. Thousands died as a result of this rodent-borne bacterial disease. Plague is transmitted by the bite of an infected flea as well as through inhalation of the bacteria or direct contact with body fluids, and during the European epidemic, the primary rodent host was the black rat, Rattus rattus, of the family Muridae. Even today, several species of rodents carry bubonic plague, even though the frequency of human infections has declined. Squirrels and chipmunks are the major rodent hosts of plague in California, and humans most at risk live in the more rural areas of the state. Prairie dogs are also bubonic plague carriers, and although they rarely transmit the

Tunney's rat, or pale field rat (Rattus tunneyi). (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)

disease to humans, whole prairie towns have ceased to exist as a result of plague epidemics.

Lyme disease is another bacterial infection transmitted indirectly from rodent reservoirs to humans through tick bites. This disease occurs worldwide and has become endemic throughout the United States. The symptoms are flu-like and can become chronic. Rodents, especially the white-footed mouse (Peromyscus leucopus), act as reservoirs by supporting the larval and nymphal stages of tick species known to transmit the disease. In the northeastern United States, the cycle of Lyme disease involves a rodent intermediate host followed by the adult stage of the tick infesting white-tailed deer. Humans catch the disease through incidental bites from infected ticks.

Hantavirus causes pulmonary distress in humans and is contracted through exposure primarily to the urine and droppings of several species of mice. This exposure generally occurs as a result of breathing the virus during contact with dust in areas of heavy rodent infestation. In North America, there are several rodent hosts of the subfamily Sigmodontinae (family Muridae) including the deer mouse (Peromyscus manicula-tus), the white-footed mouse (Peromyscus leucopus), the cotton rat (Sigmodon hispidus), and the rice rat (Oryzomys palustris). The most recent and famous hantavirus outbreak in North America occurred in the "Four Corners" of the southwestern United States. In South America, outbreaks of viruses related to hantavirus have occurred in Chile. Again, the primary rodent hosts are sigmodontines, such as the genus Oligoryzomys. Even in parts of Europe and Asia, several species of rodents harbor similar viruses that cause pulmonary disorders and he-morrhagic fever in humans. In many cases virus outbreaks are cyclical and are more virulent during population increases in response to increased rainfall and plant production.

Prairie dogs watch for predators. (Photo by © Nancy Stanford/Corbis. Reproduced by permission.)

Rodents provide both indirect and direct benefits to humans. Many species serve as the primary prey of many vertebrate predators, and through their burrowing, dam building, seed hoarding, and other activities, rodents can have a positive impact on wildlife habitat. In terms of human health, rodents provide excellent animal models for studying human disease. In fact, approximately 95% of animals used in research are either rats or mice. The house mouse, in particular, provides a very useful animal model for biomedical research. Inbred strains, first developed by pet fanciers, of rats and mice are widely used to study human diseases. Transgenic and knock-out mice represent genetically manipulated strains of the house mouse that are used in research on a variety of human diseases (e.g., Parkinson's diseases, cancer, heart disease, etc.). Similar rodents are being used to study Alzheimer's disease, aging, and cystic fibrosis, and they are also useful for testing potentially beneficial cancer treatments and other drugs used to combat human disease. Research supported by the National Aeronautics and Space Administration uses rodent models to study bone loss in response to being in an environment without gravity.

In addition to serving as research models, many rodents are popular pets, including rats, mice, gerbils, hamsters, and guinea pigs. Rodents are also eaten in many parts of the world. For example, roasted, fried, or stewed cuy (guinea pig) is popular in Ecuador, Peru, and other South American countries.

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