Scales and Causes of Patchiness in Stream Invertebrate Assemblages: Top-down Predator Effects?

Abstract

Streams are organized with a spatial hierarchical pattern. Investigations of patterns or processes, or both, could be designed to ask questions about variance among basins, stream channels, geomorphic reaches, habitats or solely with respect to physical/biotic processes independent of spatial scale. In this paper, we present data from large- / and meso-scale experiments performed in the Taieri River, New Zealand, and in a stream in the Uinta Mountains, Utah. For each of these experiments, large-scale variance (such as land use in New Zealand) was investigated with finer-scale sources of variance such as fish predators nested within the overall design. These designs have allowed us to make inferences regarding the relative importance of native versus nonnative predators in the context of existing background variation such as riparian vegetation, geomorphic form, and land uses within the watershed. We use these hierarchically organized data to ask whether the spatial scale of variation in abundance of mayflies and caddisflies (the most numerically abundant aquatic invertebrates) is dependent on the type of predator in the stream. Specifically, we hypothesize that large, relatively mobile predators such as trout should homogenize local environments (within riffle variation), show moderate effects within streams, and 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 variation in the distribution of prey within riffles with decreasing variance as scale increases.