Understanding the effect of habitat structure on population dynamics and diversity is one of the most challenging and active domains in ecology. Until recently, studies on connectivity, dispersal or invasions have only considered minimally the specific structure of landscapes, or were done in landscapes with a simplified spatial structure. Furthermore, dispersal among ecological communities has usually been assumed to be random in direction, or to vary in distance or frequency among species. However, the habitat structure of a variety of natural systems is more complex. Also, in habitats with environmental gradients, dispersal and invasions of organisms are often biased in direction. This is especially the case for organisms in dendritic river ecosystems. Rivers are structured in a linear and hierarchical way. Relative to their area, freshwater rivers are among the most diverse habitats on earth. Currently, however the diversity and species composition in many river systems is rapidly declining, often because of species invasions and habitat changes. Understanding how the directionality of dispersal and invasions and network structure affect community composition is thus a high priority. We developed a general model for competing species in linear metacommunities to evaluate the role of directionally-biased dispersal on species diversity, abundance and traits. In parallel, we experimentally tested how directionally-biased dispersal affects diversity in linear and dendritic metacommunities, using microcosm experiments with protists and rotifers.
Results/Conclusions
The model and experiment independently demonstrated that diversity in local communities was reduced by directionally-biased dispersal, especially when dispersal was biased away from disturbed patches. In linear systems, abundance of species (and composition) in local communities was a product of disturbance intensity but not dispersal directionality. Furthermore, experiments on dispersal in 2D versus dendritic networks showed that community composition and diversity patterns on the landscape level depend on the network structure and specific species traits. Overall, our conclusions about the key role of dispersal directionality in linear and dendritic metacommunities seem robust and general, since they were supported both by the model, which was set in a general framework and not parameterized to fit a specific system, and by specific experimental tests with microcosms.