Distance decay of similarity in stream communities: Is everything everywhere?

Compositional similarity between two communities often decreases as the geographical distance between them increases. Explanations for this pattern have typically relied on niche theory, emphasizing environmental control of species distributions, or neutral theory, invoking limited dispersal and speciation processes. We examined the relationship between community similarity, environmental similarity and geographic distance (1-1000 km) in three stream taxa (bryophytes, insects, diatoms), using partial Mantel test. As these groups exhibit very different dispersal abilities (highest for diatoms, lowest for invertebrates), we expected this to be reflected in the relative importance of environmental vs. spatial control of their community composition. Community similarity was strongly related to environmental similarity in all three groups, particularly diatoms (Mantel r: diatoms 0.53, bryophytes 0.38, invertebrates 0.33), whereas relationships with geographic distance were much weaker (0.13, 0.03 and 0.10, respectively). Thus, diatoms do not seem to deviate strongly from the other two groups, although they do exhibit the highest degree of environmental control. Similarly, the fact that bryophyte communities exhibited no relation to spatial distance partly supports the classical view for unicellular organisms: “everything is everywhere, environment selects”. Overall, our results provide more support for the niche-based explanations of community control.