Publication Date:
2011-07-05
Description:
Background: A territory as a prerequisite for breeding limits the maximum number of breeders in a given area, and thus lowers the proportion of breeders if population size increases. Meanwhile, some territorially breeding animals can have dramatic density fluctuations and little is known about the change from density-dependent processes to density-independendence of breeding during a population increase or an outbreak. We suggest that territoriality, breeding suppression and its break-down can be understood with an incomplete-control model, developed for social breeders and social suppression. Results: We studied density dependence in a microtine species, the bank vole, known as a territorial breeder with cyclic and non-cyclic density fluctuations and periodically high densities in different parts of its range. Our long-term data base from 38 experimental populations in large enclosures in boreal grassland confirms that breeding rates are density-regulated at moderate densities, probably by social suppression of subordinate potential breeders. We conducted an experiment, were we doubled and tripled this moderate density under otherwise the same conditions and measured space use, mortality, reproduction and faecal stress hormone levels (FGM) of adult females. We found that mortality did not differ among the densities, but the regulation of the breeding rate broke down: at double and triple densities all females were breeding, while at the low density the breeding rate was regulated as observed before. Spatial overlap among females increased with density, while a minimum territory size was maintained. Mean stress hormone levels were higher in double and triple densities than at moderate density. Conclusions: At low and moderate densities, breeding suppression was possible by the dominant breeders. But above a density-threshold (similar to a 'competition point'), the dominance of breeders could not be sustained because of the sheer number of non-breeders to be suppressed ('incomplete control'). In our experiment, this point was reached after territories could not shrink any further, but the number of intruders continued to increase. In wild populations, crossing this threshold, possibly by migration or additional cohorts an early onset of breeding in spring, would allow territorial species to escape density-dependency.
Electronic ISSN:
1472-6785
Topics:
Biology
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