Scale dependent effects of productivity and disturbance on diversity in streams

The effects of productivity and disturbance on diversity vary widely with the spatial scale at which they are examined. Not only do productivity and disturbance have strong influences on diversity patterns at local and regional scales but they can affect the way in which communities assemble and in turn alter beta diversity or community dissimilarity. We assessed whether the form of both the productivity- and disturbance-diversity relationships differed between the spatial scale at which they were examined using experimental stream channels in three Hawke's Bay, New Zealand streams. In place of true local and regional richness, we used the proxies within- and between-stream richness, as well as assessing between-stream community dissimilarity (similar to beta diversity). Our results indicate that productivity and disturbance both affect diversity but at individual scales and in different forms. At the within- stream scale, richness was a u shaped function of productivity whereas at the between-stream scale richness increased monotonically with increasing productivity. Community dissimilarity on the other hand, increased monotonically with increasing rate of disturbance. Rather than a greater role of deterministic assembly with increasing disturbance, our results indicate the opposite, but it appears that communities are simply converging on those found in the surrounding streambed with time since disturbance. Specifically, communities were more similar within individual streams than within disturbance treatments and animals colonizing post-disturbance were simply a subset of taxa present at each site regardless of perceived colonizing ability, rather than a suite of specialist colonizing taxa. These results demonstrate that without a distinction between early and late colonizers, a greater rate of deterministic assembly at high disturbance will not occur.