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Northern Prairie Wildlife Research Center

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Distribution and Abundance of Predators that Affect
Duck Production: Prairie Pothole Region

Factors of Abundance and Distribution

Habitat Changes

Habitat provides essential requisites for survival of species. Suitable habitat, however, may be underutilized or even unoccupied by a species for a variety of reasons such as excessive human-inflicted mortality or interspecific relations that result in one species excluding another. Although separation of influences on species abundance is difficult, habitat changes since settlement clearly have had major effects on the distribution and abundance of many predator species in the prairie pothole region.

Major habitat changes affecting predator populations in the region have been (1) conversion of grassland and wetland to annually tilled cropland; (2) expansion of the aspen parkland zone; and (3) establishment of farmsteads with associated windbreaks, food sources (e.g., refuse, grain), water, and human presence. These changes fragmented natural habitats and increased structural diversity of habitats. Changes that harmed nearly all predator species include establishment of large annually tilled fields, destruction of wetlands, and certain farming practices such as fallowing of land and application of chemicals. Changes that favored many predator species include establishment of tree plantings, farmsteads, certain crops, rock and brush piles, refuse sites, impoundments, and road grades. Both types of changes occur throughout the region.

The magnitude and type of changes determine whether changes of habitat may be beneficial or detrimental to a particular predator species. Changes from small diversified farms with livestock and several crop types probably favor nearly all predator species in all parts of the prairie pothole region. Conversely, large monotypic grain-farming operations are detrimental to nearly all predator species (see Schmutz [1989] regarding certain raptors).

In large parts of the prairie pothole region, the detriments to predator populations from habitat degradation presently outweigh the benefits from increased structural diversity of habitat. Before 1940, farming was highly diversified and farm units were small (Bird 1961; Robinson 1966; Kiel et al. 1972). Small habitat units of diversified farms were attractive to many predator and prey species. After the 1940's, farm units became less diversified and greatly increased in size (Robinson 1966). This was especially true in the prairie of southeastern Alberta, southern Saskatchewan, and eastern North Dakota where entire quarter sections and even sections are tilled annually (Fig. 4). Nevertheless, these areas still support diverse predator communities.

Mammalian predator species that clearly benefitted from habitat changes in the prairie pothole region are the coyote, red fox, raccoon, and Franklin's ground squirrel. More diverse and more stable food supplies became available to the coyote and red fox. However, major changes in abundance of these two canids can probably not be attributed primarily to habitat changes. Human-inflicted mortality and interspecific relations of canids seem to have been dominant factors of the changes in distribution and relative abundance of canid species (Sergeant 1982). For the raccoon, habitat changes provided food and shelter that previously were sufficient to maintain populations only in wooded riverine habitat in southeastern portions of the region (Bailey 1926; Cowan 1974). Habitat changes allowed raccoons to greatly expand their distribution and abundance in the region, but other factors such as incompatibility with other species (possibly coyotes) and human-inflicted mortality probably inhibited the response of raccoons to habitat changes. For the Franklin's ground squirrel, increased habitat diversity and expansion of the aspen parkland zone probably elicited an expansion of range into the prairie.

The abundance of minks in the prairie pothole region has probably not changed greatly since presettlement times. Although minks depend on wetlands (Eagle and Whitman 1987), drainage in most of the region has probably not been sufficient to cause major population changes except where wetland losses were extensive. In some areas, especially during drought years, stock dams, dugouts, impoundments, and ditches created deeper and more stable water for minks and their prey.

Striped skunks have probably also benefitted from agricultural development. They make extensive use of human habitation (e.g., often rear young and forage at farmsteads [A. B. Sargeant and R. J. Greenwood, unpublished data]). They are omnivorous and forage extensively for insects (Jones et al. 1983). Even intensively farmed areas support at least moderate densities of striped skunks.

In the prairie pothole region, several avian predator species have benefitted from habitat changes, especially expansion of the aspen parkland and the widespread planting of trees for windbreaks. The occurrence of American crows and great horned owls throughout the region and the expansion of the range of the red-tailed hawk are closely linked to the increased area of tall trees and wooded habitat (Houston and Bechard 1983). Black-billed magpies probably also benefitted from the planting of trees and availability of food from agricultural activities.

Harm from habitat change is more subtle than the benefits and relates largely to losses of habitat quality and prey. Of the mammals, the badger and long-tailed weasel were probably most severely affected. Both species feed largely on vertebrates (Jones et al. 1983) in grasslands. Neither species is suited to or allowed to make extensive use of human habitation. Conversion of grassland to cropland, which was almost complete in some of our study areas (Table 1), has greatly reduced the amount of available prey for these species.

Some avian predator species have also been harmed by habitat changes, especially by reductions in amount of grasslands and associated losses of prey. The abundance and breeding range of the ferruginous hawk have been severely reduced (Evans 1982; Schmutz 1984). Abundance of the northern harrier also declined, probably at least partially from losses of grassland and wetland habitats (Evans 1982). Abundance of the Swainson's hawk does not seem to have been reduced as much by losses of grassland habitat as by expansion of the aspen parkland.

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