Demographic and genetic consequences of dispersal in house sparrows

Abstract

The importance of dispersal in population dynamics is widely acknowledged. Key to understand population consequences of dispersal is by quantifying the demographic and genetic contribution from dispersers. The influence of dispersers on population growth and genetic composition may depend on habitat characteristics interacting with phenotypic traits.

In this thesis we first examined how dispersal rates and numbers were affected by spatial variation in mean population size, and temporal variation in annual population size, aiming to separate these effects on both emigration and immigration within a natural metapopulation. We found that the influence of mean population size depended on habitat type, resulting in dispersal driven by population numbers in one habitat, whereas by active attraction towards large populations in the other habitat. Temporal variation in population size affected dispersal differently among populations independent of habitat type.

Moreover, to examine the demographic and genetic contribution from novel individuals, we translocated conspecific individuals into an existing population of house sparrows. First we compared the demographic contribution from translocated and resident individuals. We found no differences in annual individual fitness, or in the contribution to population demographic variance. Thus individuals showed similar contribution to population long-run growth rate independent of origin. However, the genetic examination revealed only a small contribution from translocated individuals, in terms of a few novel alleles successfully introduced. The small contribution was explained by low establishment and mating success, in combination with increased genetic drift, and low genetic diversity among the translocated individuals. Finally, we studied the role of badge size in dispersal. Dispersers had significantly larger badge size than resident individuals, however, dependent on habitat type. Because the badge size is a fitness related trait, habitat dependent differences in badge size between dispersers and residents might result in variation in fitness for dispersers across populations differing in habitat quality.

This thesis provides knowledge about underlying mechanisms of inter-island dispersal related to habitat quality, population density and individual characteristics such as sex and morphological traits. Increased understanding of dispersal consequences on local population dynamics and genetic diversity in natural populations, should also aid conservation of small isolated populations.