SCALES AND CAUSES OF PATCHINESS IN STREAM INVERTEBRATE ASSEMBLAGES - TOP-DOWN PREDATOR EFFECTS

Streams are organized with a spatial hierarchical pattern. Investigati ons of patterns or processes, or both, could be designed to ask questi ons about Variance among basins, stream channels, geomorphic reaches, habitats or solely with respect to physical/biotic processes independe nt of spatial scale. In this paper, we present data from large-/and me so-scale experiments performed in the Taieri River, New Zealand, and i n a stream in the Uinta Mountains, Utah. For each of these experiments , large-scale variance (such as land use in New Zealand) was investiga ted with finer-scale sources of Variance such as fish predators nested within the overall design. These designs have allowed us to make infe rences regarding the relative importance of native Versus nonnative pr edators in the context of existing background variation such as ripari an vegetation, geomorphic form, and land uses within the watershed. We use these hierarchically organized data to ask whether the spatial sc ale of variation in abundance of mayflies and caddisflies (the most nu merically abundant aquatic invertebrates) is dependent on the type of predator in the stream. Specifically, we hypothesize that large, relat ively mobile predators such as trout should homogenize local environme nts (within riffle variation), show moderate effects within streams, a nd result in highly variable distributions among streams. Benthic fish , because of their moderate mobility should cause the highest Variance at intermediate scales. Finally, in the absence of fish, stoneflies, which show localized predator effects, should cause a high level of: v ariation in the distribution of prey within riffles with decreasing Va riance as scale increases.