USGS Home
Contact USGS
Search USGS

Discussion

The minimum distances these wolves had to have traveled are great, yet consistent with known dispersal distances. Van Camp and Gluckie (1979) reported that a young male wolf traveled at least 670 km before it was killed. A minimum dispersal of 732 km by a group of two to four wolves occurred in Alaska (Ballard et al., 1983). Ream et al. (1991) reported a 840 km dispersal by a female yearling. A 886 km dispersal of a young adult male from Minnesota to Saskatchewan is the longest documented dispersal for a wolf (Fritts, 1983). Actual distance traveled by the wolves described in this paper was no doubt greater than reported because (1) the origin of the dispersing wolves may have been well within the area of current wolf distribution, and (2) the distances reported are straight-line distances.

The fact that most animals were killed in winter suggests they had dispersed then. Dispersal peaks in the breeding season (late winter) or autumn to early winter but can occur any time of the year (Gese and Mech, 1991). The only summer mortality occurred in the county with the lowest human density (0.3/km²) and road density (0.18 km/km²). This animal may have dispersed into the county during the prior winter, as local citizens reported it had been present for several months. If the wolf had indeed settled there, some degree of contentment with the local area can be inferred, since lone wolves and dispersers usually continue traveling until they find a mate and/or an area to their liking (Fritts and Mech, 1981; Gese and Mech, 1991; Rothman and Mech, 1979).

The forest-prairie interface generally defines the present southern edge of the gray wolf range in central North America (Carbyn, 1983). Each of the wolves we report on was far from the nearest forest in distinctly different surroundings than where they apparently originated. The counties where the wolves were killed averaged 1% woodland, 93% cropland and pastureland, 0.71 km roads/km², and 3.5 humans/km². In contrast, the primary wolf range in adjacent Minnesota is 77% forested with a maximum of 5% cropland and pastureland (10% is water), whereas the peripheral range segments average 62% forested with roughly 27% cropland and pastureland (Mech et al., 1988; G. L. Radde, pers. comm.). In Minnesota, wolves generally survive only where both road densities and human densities are low; 88% of packs and 81% of single wolves were in townships with <0.70 km roads/km² and <4 humans/km² or with <0.50 km roads/km² and <8 humans/km² (Fuller et al., 1992). Mech et al. (1988) described the primary wolf range in Minnesota as having a mean road density of 0.36 km/km², and the peripheral and disjunct parts of the range having a road density of 0.54 km/km². Overall, data from Minnesota and Wisconsin indicate that wolves have difficulty surviving where road densities exceed about 0.58 km/km² (Mech et al., 1988; Thiel, 1985). The effect of roads would seemingly be exacerbated in the absence of forest cover. Interestingly, at least seven of the wolves we report on must have crossed 4-lane interstate highways during dispersal.

Selection of any particular habitat type in the Dakotas by the wolves described in this paper seems unlikely. Habitat characteristics of the counties where the wolves were killed were not appreciably different from the mean for the Dakotas, except for wolf 3 which was killed in a county with very low human and road density and a high percentage of rangeland (Table 1).

The concentration of four wolf mortalities in a 1174 km² area in the east-central region of the Dakotas that was approximately 250 km from the nearest known breeding population may be more than coincidence. A tendency for wolves to disperse along the same travel routes or to recolonize recently-vacated areas has been implied by others. In Minnesota, Fritts (1983) reported that a dispersing wolf appeared to generally follow the route of an earlier dispersing wolf described by Berg and Kuehn (1982). A wolf in Montana dispersed 200 km from its natal pack and recolonized an area outside contiguous wolf range that had previously been recolonized by other wolves that were no longer present; since the time between recolonizations was over a year it seems unlikely that pheromones were involved (J. A. Fontaine, pers. comm.).

If the four wolves that occurred in the east-central region of the Dakotas had survived it seems likely that they would have found one another. The home range of a solitary wolf in unoccupied habitat in British Columbia was 816 km² (Ream et al., 1985), which is not appreciably less than the 1175 km² area in which the four mortalities occurred.

Studies of wolves outside the Dakotas have not documented dispersal into the Dakotas or even any long distance movements in that direction, although wolves in northeastern Minnesota did show a slight tendency to disperse southwestward (Gese and Mech, 1991). Studies in the southern (Berg and Kuehn, 1982) and western portions of the Minnesota wolf range (Fritts and Mech, 1981) have reported that many of the wolves that moved long distances tended to travel northwestward or southeastward and thus paralleled the edge of the species' range (one notable exception reported by Berg and Kuehn [1982] moved southwestward). Translocated problem wolves also avoided leaving the main species range (Fritts et al., 1984). Minnesota wolves seem to strongly prefer the cover of a forested environment (Fritts and Mech, 1981). Most are reluctant to traverse large areas of open cropland and pastureland, and when they have been known to do so, movement was rapid (Fritts, pers. obs.). Occupied habitat at the edge of the species' range consists of "islands" of forest (Berg and Kuehn, 1982; Fritts, pers. obs.; Fuller et al., 1992; W. J. Paul, pers. comm.). Minnesota wolves that traveled as far as the Dakotas therefore were very atypical dispersers in the sense that large expanses of open landscape did not deter them.

Our data indicate that radically different habitat types are not necessarily a hindrance to dispersal and gene flow in wolves. This is consistent with the newest subspecific arrangement that places wolves from the Great Lakes to the west coast of the U.S. in the same subspecies (Nowak, in press--compare with Hall, 1981) and with the view that significant local differentiation of characteristics in wolf populations is unlikely because of the high mobility of the species, at least historically (Brewster and Fritts, in press; Wayne et al., 1992).

These data, when combined with other records of wolves taken outside forested habitat in the northern U.S. and southern Canada in the last half-century, demonstrate that wolves have considerable plasticity in dispersal strategies to adjust to new environmental opportunities (Gese and Mech, 1991). If not for killing by humans, wolves would eventually recolonize the prairie regions of the Dakotas. Realistically however, the high road and human densities in the eastern portion of the Dakotas suggests that the rate of human-caused wolf mortality will remain high. The western portion of the Dakotas provides low road and human densities; however, the nonforested habitat throughout makes wolves highly vulnerable to humans. In addition, the high cattle densities create an environment with a high potential for conflicts (Fritts et al., 1992), further exacerbating human-caused wolf mortality. For these reasons, pack formation and colonization are unlikely throughout the prairie regions of the Dakotas despite the regular occurrence of wolves dispersing into the region from adjoining states and provinces.