Results/Conclusions We found large spatial heterogeneity within the metapopulation in the relationships between dispersal rates and local population sizes. At the inner islands where house sparrows live in colonies on diary farms, both the number of dispersing recruits per breeding pair (the specific dispersal rate) and the probability of dispersal was significantly negatively related to population size. In contrast, at the outer islands, where there were no farms and the house sparrows live outdoor, there was a significantly positive relationship between the probability of dispersal and local population size, whereas the specific dispersal rate showed no density dependent relationship.
These contrasting patterns in density dependent dispersal probability can be interpreted in two alternative ways. At the outer islands where environmental conditions were harsh, large population sizes may indicate increased intraspecific competition for food, which again may imply that a higher proportion of juveniles were “forced” to leave at high population densities. In contrast, at the inner islands it is plausible that large population sizes indicated that food availability in farm colonies was high, with negligible intraspecific competition which resulted in lower dispersal rates at high densities.
Whatever underlying mechanism, this study demonstrates that the dispersal process in house sparrows was affected by fluctuations in local population sizes, and that the density-dependent relationship showed contrasting patterns within a restricted study area. Thus, a proper understanding of the dynamics of spatially distributed populations requires that the ecological basis for such differences in density-dependent relationships is further explored and identified. Furthermore, our results encourage to include spatial heterogeneity in dispersal processes in future development of metapopulation models, in order to obtain a reliable description of the spatial dynamics that is correlated in space.
