Fifty Years of Brown Bear Population Expansion: Effects of Sex-Biased Dispersal on Rate of Expansion and Population Structure

Abstract

Although the brown bear (Ursus arctos) is generally classified as an endangered species, the size and range of several bear populations are increasing in different parts of the world. Understanding how this occurs is essential for the species’ management. Using reliable signs of bear presence and models, we analyzed multiple aspects of the spatial expansion of the brown bear population in Slovenia from the Core Bear Protective Area (an area established in 1966 for the conservation of brown bears; hereafter, Core Area) toward the Alps and other parts of the country in 1945‐1995. Bear population densities increased between 1945 and 1995, but densities decreased with distance from the Core Area. The observed proportion of females increased overall during the study period from 6% to 20%, but decreased with distance from the Core Area from 27% (0‐10 km from the Core Area) to 5% (.70 km from the Core Area). This pattern likely is a consequence of male-biased dispersal, which can cause substantial changes in the sex structure of peripheral parts of populations in space and time. The population showed a net annual growth rate of 1.7%, and expanded spatially at an average rate of 1.6‐1.9 km/year. Some females were recorded far from the Core Area (.80 km), suggesting that unlike in stable or declining populations, females in expanding populations can exhibit long-distance dispersal. However, the frequency of females that dispersed far was so small that it probably had little impact on the dynamics of the population expansion.