DISTURBANCE AND PATCH SPECIFIC RESPONSES - THE INTERACTIVE EFFECTS OFWOODY DEBRIS AND FLOODS ON LOTIC INVERTEBRATES

Disturbance may play an important role in generating patterns of abund ance and distribution of biotic assemblages, particularly if its impac t differs among habitat patches. Despite much speculation concerning t he probable importance of spatial variation in the response of stream fauna to flooding, empirical work on patch-specific responses to spate s is largely lacking. Floods typically reduce the abundance of lotic i nvertebrates dramatically in open-channel areas. We conducted a set of experiments to determine if faunal abundances are less affected in pa tches more sheltered due to the presence of woody debris dams. Specifi cally, we tested two hypotheses using chironomids and copepods living in a warmwater, 4th order stream: (1) the effect of flooding on the fa una varies between patches associated with debris dams versus the open channel, and (2) the absence of woody debris in a stream impedes faun al recovery throughout the channel following hoods. We tested the firs t hypothesis by quantifying faunal abundances prior to, during, and fo llowing two floods in four patch types: mid-channel sandy patches dist ant from dams, coarse sediments associated with dams, fine sediments a ssociated with dams, and leafy debris in dams. The second hypothesis w as tested by removing all of the woody debris from two stretches of th e stream and comparing the impact of a flood on fauna in debris-remove d versus control stretches. Across all of the eight study dams, there were patch-specific faunal responses to two floods. Removal of woody d ebris from the stream did not prevent faunal recovery throughout the c hannel; however, the presence of woody debris darns did confer greater resistance of fauna to floods (as measured by no decrease in abundanc e during flooding) in two patch types. Abundances of chironomids and, to a lesser extent, copepods in the leafy debris of darns and in fine sediment patches associated with some darns either did not change or i ncreased during floods, despite the fact that abundances in the domina nt patch type of the stream (the sandy mid-channel) were reduced by 75 -95%. All instances of faunal increase were limited to fine sediment p atches associated with dams, thus entire dams cannot be labeled as flo w refugia per se. Statistically, we distinguished fine patches which a ccumulated animals during floods from the other fine patches based on two physical attributes. Patches accumulating animals were all charact erized by low water flux and nearbed flow, which likely contributed to the retention and/or passive deposition of animals. Whole dam attribu tes (e.g. dam size or complexity) were not useful in predicting which of the dams would accumulate animals in their fine sediments during fl ooding. Although structural complexity - here in the form of wood and leafy debris - is clearly important in generating biotic pattern in ma ny ecosystems, our work underscores the need to understand what proces ses are responsible for the link between physical structure and biotic pattern.