Local resource heterogeneity drives density-dependent dispersal expression in a phoretic mite

Dispersal is favoured in highly heterogenous and ephemeral environments where it can increase fitness. However, whilst we generally assume that unfavourable local conditions cue dispersal to elsewhere, little is known about how heterogeneity in local conditions influences dispersal expression. This is an important element of dispersal, since dispersal characteristics rely on such local cues to signal their development.

We assessed the role of local environmental heterogeneity on the expression of a plastic dispersal phenotype in experimental populations of the bulb mite Rhizoglyphus robini through a multigenerational, 18-week experiment. Populations were kept under either heterogenous, homogenous, or intermediate food distribution treatments, and at low or high population density, via culling. This allowed us to test for the effect of local environmental heterogeneity and competition on dispersal expression, but also on population size and growth.

We found that heterogenous environments induce greater expression of dispersal in high density environments, but not in low-density ones. Additionally, whilst culled treatments have on average larger population sizes than non-culled treatments, non-culled and heterogenous environments have similar sizes to culled treatments. Treatments did not impact population growth rate.

Our findings that heterogenous treatments only induce greater dispersal expression under non-culled treatments suggests that heterogeneity may exacerbate competition around feeding patches, causing infrequent access to food. We also infer that culled populations become larger than non-culled, at least in the short term, due to rapid compensatory growth causing an overrepresentation of juveniles in the population. It is unclear why unculled heterogenous environments exhibit a similarly large population size to culled environment but could be due to food being hard to access but easy to locate.

Overall, we find an ecologically significant impact of local heterogeneity on population size and dispersal expression, much of which may arise due to the influences of heterogeneity on competition around foraging spaces. Such findings highlight the need to distinguish between local and regional heterogeneity when modelling the evolution of dispersal, since these two alternate levels may perform significantly different roles in influencing the expression or adaptive value of dispersal.