Mammal Populations

Adapted from: Mac, M. J., Opler, P. A., Haecker, C. E. P. and Doran, P. D., editors. 1998. Status and Trends of the Nation's Biological Resources - Southwest. United States Geological Survey, Biological Resources Division, 986 pp. Also available at http://biology.usgs.gov/s+t/SNT/noframe/sw152.htm

Desert bighorn sheep (Ovis canadensis nelsoni). Photo courtesy H.E.McCutchen

Within temperate North America there are 37 families with 643 species of mammals. The Southwest contributes impressively to this diversity; native mammal species in southwestern states number about 120 in Texas, 138 in Arizona, 139 in New Mexico, and 163 in California (Findley et al. 1975). No other region in the country has so many mammal species–and many of these species and their named subspecies are endemic to the Southwest.

On the Colorado Plateau changes in native mammalian diversity began to be apparent by the late 1800s and early 1900s with the arrival of the railroad and the large numbers of livestock it brought to the region. With the decline of native ungulate populations (Mackie et al.1982) during and following Anglo settlement, large native carnivores such as grizzly bears, gray wolves and mountain lions began to prey on domestic livestock. The Bureau of Biological Survey trapped huge numbers of predators, eventually extirpating the gray wolf and grizzly bear from the region (Brown 1983). An effort is now underway to reintroduce the gray wolf in a few unsettled areas.

Black-footed ferret (Mustela nigripes)

The activities of the black-tailed prairie dog–grazing and construction of burrows and mounds–and, by association its chief predator, the black-footed ferret, put these two species in conflict with farmers and ranchers as well. Eradication campaigns, primarily using poison, reduced prairie dog distribution from 40,000,000 hectares to 600,000 hectares by 1960–about a 98% decline in the original geographic distribution of the species (Miller et al. 1990). The black-footed ferret nearly became extinct, and efforts are now underway to reintroduce the species into some prairie dog colonies.

20th-century management of deer and elk populations for game hunting and continued trapping of carnivores (including mountain lions) in the Southwest and have resulted in recovery of ungulates from turn-of-the-century population lows (Mackie et al.1982). Although elk were extirpated from the region, they were reintroduced by state game departments (and other game agencies) from areas to the north, such as Yellowstone National Park (Findley et al. 1975; Hoffmeister 1986). Elk numbers are now at an all-time high, and once again causing conflicts with livestock ranchers. In many areas, mule deer and white-tailed deer also have made remarkable recoveries from earlier lows, although in some areas nonindigenous deer have been transplanted (Hoffmeister 1986).

Other ungulate populations have remained at low levels. Despite considerable transplant efforts, desert bighorn sheep may only be at 2%-8% of their population levels at the time of Anglo settlement. These animals have never recovered from unregulated harvesting, habitat destruction, overgrazing of rangelands, and diseases contracted from domestic livestock (Singer 1995). Numbers of Sonoran pronghorn, listed as endangered by the U.S. Fish and Wildlife Service, are also low and continuing to decline on the Colorado Plateau.

Only recently has concern been focused on other less conspicuous mammals that differ in habits from those that were in conflict with humans during settlement. The last list of candidate and sensitive species (U.S. Fish and Wildlife Service 1994) includes a broad spectrum of mammals that are of concern. The list no longer emphasizes large carnivores, or many carnivores at all, but instead emphasizes smaller species such as insectivores (shrews), a wide array of bats, cottontails and hares, pocket gophers, tree squirrels, and a variety of mice and rats. In general, we know little of the status and trends of these species.

Several unifying trends exist among mammals on these lists, excluding bats (U.S. Fish and Wildlife Service 1994). Many of the mammals, including shrews, gophers, tree squirrels, mice, and rats, have small or restricted ranges, often on single mountaintops or other functional islands. Many mammals on the lists are isolated subspecies of wide-ranging species and often occur on different mountain ranges (Hall 1981). Also, and perhaps more importantly from an ecosystem perspective, many of the species or subspecies live in wet or moist habitats, often montane. This is clearly the case for the shrews, voles, meadow jumping mice–a unique family of mice found in North America and China–and some other mice.

A smaller number of mammals adapted to arid habitats (for example, kangaroo rats, pocket mice, and cotton rats), primarily in Arizona and Utah, are also on the sensitive species list (U.S. Fish and Wildlife Service 1994). Possible environmental threats to pocket gophers include habitat change through overgrazing, lowering of water tables, and poisoning campaigns directed against them or other rodents, such as prairie dogs. Gophers, like prairie dogs, perform ecologically important roles through their habit of turning over and aerating soils, thereby providing for percolation of water and creation of new substrates for vegetative succession.

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