Lori Roberts, Milan Vinks

Habitat alteration and human-caused mortality have led to range reduction and fragmentation for many bears species, but conservation actions have been successful in reversing some of these trends. Natal dispersal is the primary natural mechanism leading to recolonization and population connectivity, however it represents one of the least understood of bear behaviors. We used age-specific and spatially explicit grizzly bear (Ursus arctos) data to document natal dispersal and evaluate its drivers within the Northern Continental Divide Ecosystem (Montana, USA) during several decades of population growth and range expansion. We identified mother-offspring pairs either by observation or parentage analysis and evaluated annual distance between offspring locations and their natal (i.e., mother’s) range. Using data from all pairs (n > 450 offspring aged ≥2), mean movement distance stabilized by the ages of 3 and 4 years, for female and male offspring respectively. Restricting analysis to >300 offspring known or assumed to have established their adult home range, we found that typical dispersal distances were <20 km for females and 20–60 km for males. Nevertheless, numerous cases of long-distance dispersal were documented among both sexes, with maximum distances exceeding 100 km. Most bears dispersed within the area of estimated occupied range at the time of their rearing, suggesting that range expansion was achieved by a small proportion of individuals. Some evidence suggested that dispersal was influenced by bear population density and habitat contiguity at range peripheries. These analyses demonstrate the capacity for dispersal to contribute to connectivity and gene flow among fragmented bear populations. But predicting when connectivity might be achieved remains difficult due to the individuality of the behavior and the potentially novel conditions existing within the intervening landscapes.