Quantifying the effect of stream networks on species coexistence

Background/Question/Methods

Stream systems exhibit environmental heterogeneity at multiple scales, from large-scale trends in temperature and discharge to very small scale variation in microhabitats within pools or riffles. In addition, streams have a dendritic branching pattern that has clear effects on the dispersal of aquatic species. These characteristics affect the distribution of organisms with respect to the environment, with the potential for effects on species interactions and stream community structure. Here we use a simulation model to quantify the effect of stream networks on species coexistence using scale transition theory.  Coexistence is promoted when species experience greater intraspecific than interspecific competition at the regional scale. One way this can occur is if competing species have different environmental preferences, and each species tends to concentrate in the habitats where it performs best. Such an outcome depends on the network structure of streams as well as the pattern of environmental heterogeneity, and defines the fitness-density covariance coexistence mechanism. Quantification of the fitness-density covariance allows us to quantify the effects of environmental heterogeneity and network pattern on coexistence in streams.

Results/Conclusions

Stream systems do not inherently alter coexistence conditions. If the environment is variable on a very short scale, a dendritic branching pattern does not alter the strength of coexistence. However, a dendritic topology can interact with other parameters to affect coexistence. When compared to a linear environment, the strengths of coexistence mechanisms decrease as branching increases if the environment has consistent trends from headwaters to the mouth. This effect is amplified as dispersal distances increase because increased branching effectively decreases the distance between each species’ favored environments. In contrast, when compared to a rectangular lattice, the branched network increases effective distance between sites and therefore the strength of coexistence. When stream parameters interact to increase the concentration of species in good environments as quantified by the fitness-density covariance, coexistence is promoted. When they have no effect or a negative effect, coexistence is unaffected or inhibited. Other parameters such as temporal variation likely interact with network topology to alter the strengths of coexistence mechanisms in streams.  The scale transition approach allows further quantification of these effects.