Abstract
The literature reveals opposing views regarding the importance of intrinsic population regulation in mammals. Different models have been proposed; adding importance to contrasting life histories, body sizes and social interactions. Here we evaluate current theory based on results from two Scandinavian projects studying two ecologically different mammal species with contrasting body sizes and life history traits: the root vole Microtus oeconomus and the brown bear Ursus arctos. We emphasize four inter-linked behavioral aspects—territoriality, dispersal, social inhibition of breeding, and infanticide—that together form a density-dependent syndrome with potentially regulatory effects on population growth. We show that the two species are similar in all four behaviors and thus the overall regulatory syndrome. Females form matrilineal assemblages, female natal dispersal is negatively density dependent and breeding is suppressed in philopatric young females. In both species, male turnover due to extrinsic mortality agents cause infanticide with negative effects on population growth. The sex-biased and density-dependent dispersal patterns promote the formation of matrilineal clusters which, in turn, leads to reproductive suppression with potentially regulatory effects. Hence, we show that intrinsic population regulation interacting with extrinsic mortality agents may occur irrespective of taxon, life history and body size. Our review stresses the significance of a mechanistic approach to understanding population ecology. We also show that experimental model populations are useful to elucidate natural populations of other species with similar social systems. In particular, such experiments should be combined with methodical innovations that may unravel the effects of cryptic intrinsic mechanisms such as infanticide.
Highlights
The main aspects of condition-dependent dispersal in root voles appear to be largely similar to those documented for many other small mammal species [i.e., inversely densitydependent female dispersal rate (Le Galliard et al 2012)], the experimental studies on root voles are exceptional in terms of their ability to dissect mechanisms operating in different stages of the dispersal process (e.g., Andreassen and Ims 2001) as well as documenting the regulating effect of dispersal on population dynamics (e.g., Ims and Andreassen 2000, 2005)
Caughley and Krebs (1983) recognized that their view of size-dependent population regulation was influenced by different methodological approaches to studies of behavior and population dynamics in small and large mammals
Effects of sociality on population dynamics are difficult to detect in nature, especially among species that are large, long-lived, rare, or for other reasons difficult to observe
Summary
We examine whether the intrinsic mechanisms (territoriality, infanticide, dispersal, and social inhibition of breeding) highlighted by Krebs (2009) and Wolff (1997) occur in a small (the root vole Microtus oeconomus) and a large (the brown bear Ursus arctos) mammal, both with altricial young. Adult root voles are solitary, with larger home ranges and core areas in adult males than adult females (Andreassen et al 1998)—a result that appears to be consistent among studies of small rodents in general (Ims 1987a, Le Galliard et al 2012).
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