Spatial heterogeneity and rates of spread in experimental streams

Theoretical models predict that environmental heterogeneity can decrease or potentially increase rates of spread in biological populations depending on the relationship between the scale of dispersal and the scale of heterogeneity. These effects arise from the interaction between habitat quality and the processes of dispersal, colonization and growth. Flowing water environments provide a unique opportunity to test these predictions. If advection influences dispersal, flow can alter the relative scale of dispersal to environmental heterogeneity in the upstream versus downstream direction. We explored the influence of heterogeneity on the spatial spread of a species of diatom in experimental streams. Environmental heterogeneity was created by maintaining agar diffusing substrata at different nutrient levels. Diatoms were placed at the midpoint of each stream, and spatial spread rates were determined by monitoring algal abundance non-destructively. Our results reveal that, relative to homogeneous streams, resource heterogeneity decreases spread rate in the upstream direction but increases spread rate in the downstream direction. Empirical estimates of growth rates and colonization times reveal that heterogeneity predominantly influenced colonization rates. Colonization rates estimate successful dispersal events, and thus relate to both colonization and dispersal. These results are one of the first empirical tests of general theories regarding the impact of heterogeneity on rates of spread and highlight the importance of understanding the impact of heterogeneity on colonization and dispersal in continuous habitats.