Background/Question/Methods
Streams are a unique environment to study population dynamics. Compared to most other types of spatial structure, they have a preferred direction of dispersal. For many species, movement is restricted to the stream itself, limiting dispersal to a single dimension. Permanent spatial variation is inherently found in streams, which combines with species-specific environmental preferences to lead to two quantifiable coexistence mechanisms- the spatial storage effect and the scale transition. We examine the fitness consequences at the regional level for species inhabiting streams and compare it to species with a more traditional habitat structure, focusing on the scale transition. Scale transition theory provides a method for scaling up local population dynamics to the regional scale and to quantify how regional dynamics differ from local dynamics. The nature of the scale transition is inherently dependent on the nature of nonlinearities at the local scale, but is also dependent on the mechanisms that concentrate organisms and alter the covariance between fitness and density. Advection and 1-dimensional dispersal leads to very different concentration mechanisms in streams compared to most terrestrial systems, implying that the magnitude, and possibly sign, of this coexistence mechanism will be different in the two types of systems.
Results/Conclusions
Analytical and numerical modeling accounting for both advection and random diffusion shows that streams cause species to be distributed differently across the landscape than in two- dimensional diffusion only models. This changes the nature of the covariance between fitness and density between the two types of model. The exact nature of the difference depends on the magnitude of movement relative to the scale of spatial variation and the relationship between advection and diffusion. However, if either total dispersal or diffusion is held constant, the advection term always has a negative effect on invader fitness at the regional scale, because it pushes individuals away from areas of high fitness. This indicates the scale transition term is able to overcome a smaller range of fitness differences in streams than in systems without advection, making stable coexistence less likely. However, this indicates the possibility of coexistence via a tradeoff between competitive ability and sensitivity to advection.