Quantifying Disturbance in Streams: Alternative Measures of Disturbance in Relation to Macroinvertebrate Species Traits and Species Richness

Appropriate tests of disturbance theory require that disturbance is defined and measured in an organism-related sense. We quantified 7 measures of disturbance, 3 dealing with the pattern of disturbance of the stream bed (assessed using painted tracer particles), 3 dealing with aspects of discharge variation and 1 being a subjective composite measure of stream channel stability (Pfankuch's index). Hypotheses relating to invertebrate taxon richness (maximal at intermediate levels of disturbance) and the representation of particular insect species traits (assemblages in more disturbed sites contain higher percentages of individuals possessing high adult mobility and streamlined-flattened larval morphology) were both supported when disturbance was measured in terms of bed movement but generally not when measured in terms of discharge variation. When disturbance was estimated by Pfankuch's index, species trait predictions were supported but the taxon richness prediction was not. A co-inertia analysis, searching for a co-structure between our environmental and faunistic data sets, allowed us to distinguish taxa that apparently are resistant to bed movement from taxa that appear to be resistant to flow variations; this more detailed analysis indicates that the most appropriate measure of disturbance may vary even among quite closely related taxa. The construction of a comprehensive theory of disturbance in streams requires that disturbance is quantified in a way that allows both multi-site and multi-study comparisons. The painted-particle approach, described here, has the potential to permit such standarized comparative studies.