Thursday, August 5, 2010

PS 74-80: Spatial variation in density-dependent dispersal in a house sparrow-metapopulation

Thor Harald Ringsby, Norwegian University for Science and Technology, Bernt-Erik Saether, Norwegian University of Science and Technology, Henrik Jensen, Norwegian University of Science and Technology, and Henrik Pärn, Norwegian University for Science and Technology.

Background/Question/Methods Dispersal may have profound influence on many ecological and evolutionary processes in spatially structured populations. Despite its importance few studies have been to able to empirically examine how the migration process is affected by fluctuations in population size. One reason for this is that migration is notoriously difficult to estimate in natural populations because it requires a large study area within which a high proportion of individuals are individually recognizeable. In this paper we examine whether changes in local population size can explain temporal variation in dispersal rates in an insular house sparrow metapopulation.  The study included 12 island-populations in a study area which covered 1600 km2 in northern Norway.

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.