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Effects of Fire in the Northern Great Plains

Effects of Fire on Small Mammals

Although most research indicates limited direct mortality to rodents, several instances have been reported.

Many nests of the western harvest mouse (Reithrodontomys megalotis) in Nebraska were destroyed by fire, and an estimated 205-522 pups were killed over the entire burn (Erwin and Stasiak 1979). Of 41 mice marked in a pre-burn area by Tevis (1956), only 11 were recaptured post-burn. The rest were presumed dead.

After a fire, Chew et al (1958) found carcasses of 28 dusky-footed woodrats (Neotoma fuscipes) and four mice of three species. Few of the carcasses had been charred or singed; cause of death was asphyxiation or heat prostration.

Motobu (1978) estimated 51% mortality in mountain beaver (Aplodontia rufa) on an area completely burned and only 20% mortality on an area of patchy burn. Few of the surviving animals showed signs of burn injury.

An immediate, indirect cause of mortality from burning is predation. The lack of cover immediately after a fire produces an exposed environment and improves accessibility to avian and mammalian predators (Motobu 1978). Beck and Vogl (1972) suggested that some of the mortality associated with fire may have actually been caused by predation. Post-burn predation may be more restrictive to rodent populations than the burning itself (Lawrence 1966).

The lethal temperature tolerance of rodents is 122-145 F (50-63 C) at 22% relative humidity (Howard et al 1959); however, at 60% relative humidity, the lethal temperature drops to 120 F (49 C) (Lawrence 1966). To escape the heat of a fire many rodents take refuge in unburned islands (Motobu 1978), in rock outcroppings (Howard et al 1959), by running ahead of flames (Erwin and Stasiak 1979), or by taking refuge in burrows (Lawrence 1966; Quinn 1979). Beneath the soil surface, temperatures are reduced (Lawrence 1966) and rodents are able to survive.

Lawrence (1966) demonstrated the necessity for adequate air circulation in the burrow system. He also suggested that animals survive as long as the burrow systems allow vapor pressure below 40 mm Hg.

Fires affect population densities principally by altering habitat. The decrease of vegetative cover results in fewer microhabitats available for use by wildlife, especially rodents.

However, with the reduction of ground litter, primary production is enhanced. Within 2 to 4 years after a fire, litter gradually increases again, with a decrease in primary production (Dix 1960; Vogl 1965; McGee 1982). Based upon these habitat changes and the habitat and food preferences of rodents, major shifts in species composition and density should also occur within the first few years after a fire.

The major changes in food availability affect the type of species that will invade after a fire. Removal of the litter layer increases availability of seeds and invertebrates for granivores and omnivores (Ahlgren 1966; Stout et al 1971; Kaufman et al 1983). For the first year, these type of rodents are abundant. Species considered herbivores are limited, especially on complete burns.

As the abundance of seeds decreases, so does the population of granivores. However, by the third year new seed producing vegetation has become established and the seed eating rodent populations increase(Ahlgren 1966; Sims and Buckner 1973).

Depending upon climatic conditions, concealment vegetation will develop after 2-5 years. This allows herbivores and those rodents restricted by lack of cover to recolonize an area and reach populations similar to pre-burn levels (Gashwiler 1970; Fala 1975; McGee 1976).

Many studies show the rate of capture of deer mice (Peromyscus maniculatus) in geographically diverse post-burn habitats is significantly greater than in unburned habitats (Cook 1959; Tester 1965; Ahlgren 1966; Stout et al 1971; Beck and Vogl 1972; Sims and Buckner 1973; McGee 1976; Bock and Bock 1978, 1983).

Deer mouse populations show a positive response to the early stages of secondary succession (Beck and Vogl 1972; Kaufman et al 1983). They prefer xeric habitats with open vegetation and sparse litter cover (Kaufman et al 1983) and are restricted from areas of dense vegetation (Rickard 1960). They are opportunistic omnivores (Johnson 1961), often shifting diets according to the availability of seeds and invertebrates (Williams 1959; McGee 1976). Their food and habitat preferences make them particularly suited to exploit burned areas.

Deer mice will usually invade an area within 2-4 weeks after a fire (Cook 1959; Tevis 1956; Sims and Buckner 1973). This immigration is a response to the availability of a new food source and to the open space in which a home range may be established (Tevis 1956). Many of the colonizing mice are juveniles (Tester 1965; Stout et al 1971; Sims and Buckner 1973). Sadleir (1965) reported that although deer mice are not territorial, adults become intolerant of juveniles and will drive them out during the breeding season.

Within 3 years, deer mouse populations on a burned area will increase greatly over that of an unburned area (Cook 1959; Tevis 1956; McGee 1976; Bock and Bock 1983; Kaufman et al 1983). These increases may be caused by additional immigration or increased reproductive rates in response to favorable environmental conditions (Lawrence 1966; McGee 1976). Deer mice remain the dominant species for 2-4 years until the accumulation of vegetation becomes too dense for optimum habitat (Rickard 1960; McGee 1976).

The western harvest mouse, a granivore, will also inhabit a burn, but tends not to invade until some vegetative cover is established (Cook 1959; Kaufman et al 1983).

If western harvest mice responded favorably only to the availability of seeds, densities should peak early in the first year, as with deer mice. Therefore, habitat deficiencies must be the limiting factor in this species' response (Kaufman et al 1983). Kangaroo rats (Dipodomys spp) and pocket mice (Perognathus hispidus) also utilize burned areas (Bock and Bock 1978; Quinn 1979). Both of these species are also granivores (Johnson 1961).

Ground squirrels (Spermophilus spp) and chipmunks (Eutamias spp) are common in burn areas but are limited by the amount of remaining vegetation (Gashwiler 1970; McGee 1976). House mice (Mus musculus) also show a preference for habitat created by fire (Cook 1959). Other species may utilize a burned area depending upon the surrounding habitat types and the amount and type of vegetation that becomes established after a burn.

Not all rodent species are positively affected by fire.

Herbivores are generally absent or in low densities after a burn (Fala 1975). Voles (Microtus spp) are restricted to habitats with dense vegetative cover in which to build runways (Rickard 1960; Sims and Buckner 1973; McGee 1976). Populations of voles are usually low for the first 2-4 years following a fire, until undergrowth accumulations reach that of unburned areas (Cook 1959; McGee 1976). Tester (1965) found red-backed vole (Clethrionomys gapperi) densities to be unaltered by fire, but others have found this species to respond like Microtus species (Ahlgren 1966; Beck and Vogl 1972; Gashwiler 1970). Jumping mice (Zapus spp) are also restricted due to lack of food and cover (Sims and Buckner 1973; McGee 1976).

The small mammal response is not considered a direct response to fire but a reaction to fire-altered habitat. Fire alters the composition of rodent species from those associated with the climax community to those considered early successional species (McGee 1982).

There is a predominant shift from chaparral species (Cook 1959; Lawrence 1966) and forest species (Beck and Vogl 1972) to prairie and grassland species. Food and habitat resources are the primary factors influencing the population shifts and fluctuations. Granivores and omnivores that require little cover (deer mice, for example) are favored. As vegetative cover increases on burned areas, other rodent species also invade.

Eventually, litter accumulation, flora, and the rodent community again resemble those of an unburned area.

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