The ecological and biophysical environments across the four mountain valley systems in this study are essentially identical (Medinger & Pojar 1991
). Because the control valley is larger than the test valleys, it should show the largest genetic separation. However it has the smallest genetic distance (a compete mixing), suggesting that factors present in the test valleys are mediating fragmentation. The terrain within each test valley would be considered continuous grizzly bear habitat, just as in the Flathead valley, were it not for the presence of the transportation corridor and associated human settlements. The distance across each test corridor is well within the average daily movement distance of a grizzly bear (2.4
; B. McLellan, unpublished data). Because of the short spatial distances in each system, the wide-ranging movement capability of grizzly bears and the very low genetic distance with thoroughly integrated assignment plots of the control area (d
), we expected to see similar connectivity in all river valleys sampled. Instead, we found much larger genetic distances across the transportation corridors. In the undisturbed Canadian north, these same genetic distances (Purcells DLR
=2.04; Rockies DLR
=2.97) correspond to groups of bears separated by 650 and 1000
km, respectively (Paetkau et al. 1998b
). Our evidence strongly suggests that the transportation corridor and associated human settlements are fragmenting grizzly bears.
The power to detect the number and sex of migrants moving between geographic areas varies between systems. In the control area we have no power to detect individual movements across the landscape because no genetic separation has occurred. Many animals move across this valley, as determined by 28 years of radio-telemetry data (B. McLellan, unpublished data). Conversely, in the Selkirk system we have excellent power to detect migrants (a
) because of distinct genetic separation; no migrants were detected in this study. In the Rocky and Purcell Mountains we had limited but adequate power to detect individual migrants as genetic separation had occurred, but to a lesser degree than in the Selkirks. This power is demonstrated by the ability of STRUCTURE to separate the groups into relatively distinct groups based on the dynamic iterative assignment cluster analysis and to separate migrants from residents so clearly with bimodal migrant probability distributions ( and ). We found that the allele frequencies in the Purcells and Rockies were sufficiently distinct to detect more individuals in the tails of the migrant probability distributions than can be explained by chance (GeneClass
2) and these migrant choices were corroborated by STRUCTURE. We chose the GeneClass
2 method because of its use of an improved simulation routine (Paetkau et al. 2004
) that more closely mimics natural population processes by developing individuals through united gametes. We found that this process results in accurate type I error rates as theoretically demonstrated in Paetkau et al. (2004)
. STRUCTURE may be more sensitive than GeneClass
2 in detecting migrants between areas that share recent ancestry and have only moderate genetic structure, as suggested by the bimodal probability distribution evident in the Purcell system ( and ).
All putative migrants are heavily skewed toward males and no female migrants were detected by either method, other than the translocated female in the Rockies. Sex-biased dispersal is widespread in mammals (Greenwood 1980
; Pusey 1987
), has been demonstrated in grizzly bears in North America by comparing mtDNA and nDNA (Paetkau et al. 1998a
) and is documented in our study area (McLellan & Hovey 2001
; Proctor et al. 2004
). The lesser dispersing sex may be more affected by human influence and this is evident in our results. Because each test valley is <3
km wide and in these areas females disperse on average 10–14
km (McLellan & Hovey 2001
; Proctor et al. 2004
), the limited female movement observed across the corridors is unexpected. In contrast, both males and females in the unpopulated control valley were moving across the valley (B. McLellan, unpublished data). While male movement in the Purcells and Rockies is mediating genetic connectivity, the limited female movement is cause for demographic concern. This disruption of the female dispersal process diminishes the possibility of natural population augmentation or re-colonization (Lande 1988
) and may have serious implications for the small south Purcell and Selkirk areas along the Canada–USA border. The south Purcell area has an estimated 40–50 bears (Kasworm et al. 2000
; M. Proctor, unpublished data) and is declining at approximately 3.7% per year (Wakkinen & Kasworm 2004
). The predominance of movement from north to south in the Purcells suggests that grizzly persistence to the south of Highway 3 into the USA may depend on connectivity to the north. The demographically and genetically isolated south Selkirk population has an estimated 70–100 bears (Wielgus et al. 1994
) and although the population is reported to be stable (Wakkinen & Kasworm 2004
), it should be considered threatened.
The threat of local grizzly extirpation is primarily driven by demographic forces in the form of human-induced mortality (McLellan et al. 1999
). Without demographic connectivity, the two small demographically isolated populations revealed in this study are vulnerable to stochastic events and are reliant on positive fecundity rates, a challenge in a context of negative human–bear interactions. Loss of these two border populations would leave only one US border population tenuously linked to Canadian populations.
While the Rocky Mountain population south of Highway 3 is experiencing demographic fragmentation, it is still genetically connected (male mediated) to bears in the north and is relatively large (>400 animals; B. McClellan, unpublished data); as such it is not under immediate conservation risk due to fragmentation. However, consideration should be given to the paucity of female connectivity across the Highway 3 corridor where monitoring and enhanced connectivity management may be necessary.
The mechanism leading to limited movement across the transportation and settlement corridor is probably a combination of some bears avoiding human activity centres (Mattson et al. 1987
) and increased grizzly bear mortality in these areas, due to bear attractants such as garbage, human food and perceived threats to human safety (McLellan et al. 1999
). Dispersal of male and female grizzly bears in the southeastern BC region requires several years, resulting in newly established home ranges that usually overlap or are adjacent to the maternal home range (McLellan & Hovey 2001
). When this gradual dispersal process requires bears to spend time in human-dominated landscapes, mortality rates can increase; out of the three natural migrants detected in the Rockies by both GeneClass
2 and STRUCTURE, all are dead, killed either by hunters or as ‘problem wildlife’. During the past 10 years, 60 grizzly bears were removed from the Rocky Mountain study area by conservation officers because of conflicts with people, and over the past 25 years an additional 500 were harvested legally (BC Ministry of Water, Land and Air Protection files). These mortalities may also decrease density-dependent movement within the system (Swenson et al. 1998
; McLellan & Hovey 2001
). We suspect that human-caused mortality is a contributory factor to decreased migration (Proctor 2003
) and recommend management to reduce human–bear conflict and ultimately mortality in the transportation/settlement corridors. Further, we recommend research to locate areas with landscape attributes that foster successful bear movement through these corridors. These identified linkage areas can then be managed accordingly. We also recommend further work to determine the benefits and feasibility of population augmentation in the south Purcell and Selkirk populations.
Demographic processes appear to be the dominant influence over grizzly bear persistence within North America (McLellan et al. 1999
). Excessive mortality and isolation played a primary role in the extirpation of approximately 31 small isolated grizzly bear populations between 1922 and 1970 within the conterminous USA (Mattson & Merrill 2002
). There is evidence that the deleterious effects of reduced genetic variation are minimal in grizzly bears. For example, Kodiak Island grizzly bears have 33% reduction in heterozygosity (HE
) and have been thriving for centuries (Paetkau et al. 1998a
). However, this population is large (ca
3000; L. Van Daele, personal communication) relative to the isolated southern Selkirk population of 70–100 bears (20% reduction in HE
; ), and the effects of inbreeding depression tend to be more detrimental for small populations (Frankham et al. 2002
There is a recent but growing body of evidence that anthropogenic fragmentation is influencing carnivore populations in North America, but no studies documenting sex-specific differences in fragmentation. Schwartz et al. (2002)
found little genetic structure in lynx (L. canadensis
) populations in western North America and recommended maintenance of connectivity. At a finer scale, Campbell (2002)
found genetic structure in lynx across a major highway in Alberta, Canada, suggesting that human disturbance may be influencing connectivity. Kyle & Strobeck (2002)
found increased genetic structure in southern peripheral populations relative to northern core populations in the North American wolverine (Gulo gulo
), although they did not measure immediately adjacent populations within the dispersal distance of a wolverine. Cegelski et al. (2003)
also found wolverines to be fragmented in the southern periphery of their western North America distribution.
Our methods demonstrate the importance of using individual-based analyses in addition to traditional population genetics techniques (e.g. FST and genetic distance) to provide insight into gender-specific processes of immigration and emigration in recently disturbed systems. Our results underscore the need for connectivity management and highlight the importance of international co-operation for the management of highly vagile animals.