Inversely density-dependent natal dispersal in brown bears Ursus arctos

Abstract

There is considerable controversy in the literature about the presence of density dependence in dispersal. In this study, we exploit a data series from a long-term study (>18 years) on radio-marked brown bears (Ursus arctos L.) in two study areas in Scandinavia to investigate how individual-based densities influence the probability of natal dispersal and natal dispersal distances. Cumulatively, 32% and 46% of the females and 81% and 92% of the males dispersed before reaching 5 years of age in the northern and southern study area, respectively. Density had a negative effect on both the probability of dispersal and dispersal distances for the dispersing animals, when controlling for study area, sex and age, making this the first study to show that natal dispersal probability and distances are inversely density dependent in a large carnivore. We suggest that female-female competition for space caused females in higher density areas to settle closer to their natal area. For males, however, merging of demes, resulting in decreased relatedness and increased heterozygosity in an expanding population, might be the reason for shorter dispersal distances in males living at higher densities. This has been hypothesised for small mammals. The high proportion of dispersing female brown bears in Scandinavian compared with North American studies might be due to lower densities in Scandinavia and recent population expansion, with unoccupied areas available at the edges of the population. The longer dispersal distances in female Scandinavian brown bears suggest less social constraints on movements than for North American females. The longer dispersal distances by Scandinavian males may be due to increased searching for potential mates in peripheral areas with lower densities of females. These results, in addition to results of other brown bear studies, suggest that brown bears might be more territorial than previously thought, and that density is regulated by social interactions.

Survival and divergence in a small group: The extraordinary genomic history of the endangered Apennine brown bear stragglers

“…This pattern was not observed in the nuclear genomes, and it implies a strong genetic barrier in Sweden and a strict affinity between Apennine and Alpine bears. The discrepancy between nuclear and mitochondrial data is likely explained by male-biased dispersal (43)(44)(45)(46), and the pattern of low genetic structure we observed at the Y-chromosome further supports this view (47, see SI Appendix, section S7) Until recent times, bear populations were geographically homogenized by males, but females philopatry resulted in some level of mtDNA structuring. Interestingly, habitat destruction and fragmentation has been suggested as a general factor that favors the increase of female philopatry (48,49).…”

Decline and recovery of a large carnivore: environmental change and long-term trends in an endangered brown bear population

“…This mechanism was suggested by spatial analyses indicating that, in northern Europe, neighbouring females have the ability to influence the likelihood of reproduction among each other [22]. This effect might be enhanced at high densities and by behavioural syndromes favouring spatial aggregation, like the repeated use of specific reproductive areas [34] and philopatric behaviour favouring that females remain close to their natal home range [35].…”

The role of seasonality and non‐lethal carry‐over effects on density‐dependent dispersal

“…A common expectation regarding movement is that dispersal rates increase as environmental conditions within a population deteriorate (Travis et al 1999, Innocent et al 2010, which leads to the prediction that dispersal should increase with intraspecific density (positive density-dependent dispersal; Travis et al 1999, Innocent et al 2010. Although this prediction has been supported by several studies (Matthysen 2005, Clobert et al 2009, Meester and Bonte 2010, it has also been shown that dispersal can decrease with density (negative density-dependent dispersal; Herzig 1995, Matthysen 2005, Støen et al 2006, Meylan et al 2007). Interestingly, empirical observations suggest that, in seasonal environments, where density can operate in more than one season (Fretwell 1972, Ratikainen et al 2008, dispersal is often negatively related to density Andreassen 2005, Matthysen 2005), suggesting that temporal variation in resources (e.g., seasonality) is a key factor to understand densitydependent dispersal (Wahlströ m and Liberg 1995, Morris and Diffendorfer 2004, Matthysen 2005, Rodrigues and Johnstone 2014.…”