Effects of disturbance frequency, intensity, and area on assemblages of stream macroinvertebrates

Disturbance frequency, intensity, and areal extent may influence the effect s of disturbance on biological communities. Furthermore, these three factor s may have interacting effects on biological diversity. We manipulated the frequency, intensity, and area of disturbance in a full-factorial design on artificial substrates and measured responses of benthic macroinvertebrates in a northern Vermont stream. Macroinvertebrate abundance was lower in all disturbance treatments than in the undisturbed control. As in most other s tudies in streams, species density (number of species/sample) was lower in disturbed treatments than in undisturbed controls. However, species density is very sensitive to total abundance of a sample, which is usually reduced by disturbance. We used a rarefaction method to compare species richness b ased on an equivalent number of individuals. In rarefied samples, species r ichness was higher in all eight disturbed treatments than in the undisturbe d control, with significant increases in species richness for larger areas and greater intensities of disturbance. Increases in species richness in re sponse to disturbance were consistent within patches, among patches with si milar disturbance histories, and among patches with differing disturbance h istories. These results provide some support for Huston's dynamic-equilibri um model but do not support the intermediate-disturbance hypothesis. Our an alyses demonstrate that species richness and species density can generate o pposite patterns of community response to disturbance. The interplay of abu ndance, species richness, and species density has been neglected in previou s tests of disturbance models.