PS 45-15
Patterns of change in population density experienced by dispersing brush mice (Peromyscus boylii)
Population density can influence all three phases of natal dispersal: departure from the place of birth, searching the landscape, and selecting a new site in which to settle. The direction of the effect of density on dispersal (positive, negative, or neither) may be affected by the relative costs and benefits of living in an area with high population density. Animals may benefit from high population density because of increased mate availability and dilution of predation risk, but may also face increased resource competition. The influence of these conflicting mechanisms should affect the pattern of change in population density experienced by dispersers between departure and settlement. We examined the influence of density on dispersal in a brush mouse (Peromyscus boylii) population in northern California. Dispersers were tracked using either radio telemetry or live-trapping. We applied a spatially explicit capture recapture model to live-trapping data to estimate density across the landscape, and within a 20m buffer surrounding each disperser’s natal and settlement sites. We used a Bayesian analysis with Markov Chain Monte Carlo methods to generate posterior distributions of change in both “neighborhood” density (around the disperser) and “landscape” density (across the trap grid) at both departure and settlement.
Results/Conclusions
Dispersers tended to settle in areas with higher population densities than where they were born: the posterior probability that there was a mean increase in neighborhood density (while accounting for landscape density change) was 0.767 for radio-tracked animals and 0.961 for trapped animals. Furthermore, during periods when the posterior median landscape density change indicated a decrease from departure to settlement, the posterior probabilities that a disperser’s neighborhood density decreased less than the landscape density were generally high among trapped dispersers; 14 of the 15 were > 0.5 and 12 were > 0.76. A similar trend was not observed when the posterior median landscape density change indicated an increase. This suggests that mice attempt to mitigate the effects of large-scale population decline by settling in regions that experience less decline, but do not mitigate for large-scale increases in population density. There are two potential explanations for these results: 1) mice are attracted to conspecifics, possibly for breeding opportunities or to dilute predation risk, or 2) some areas are rich in resources, such as food and shelter, and thus attract many mice. The observed pattern may be caused by one or both of these causes.