Community development of epilithic invertebrates in streams: independent and interactive effects of substratum properties

Abstract

The composition of macroinvertebrate communities living on rocks in streams depends on arrivals at a particular rock, available microhabitats, and biotic interactions among colonizers. We manipulated the color (dark, light), size (82 – 1320 cm2 surface area), and shape (sphere, rectangle, cube) of individual rocks to assess their independent and interactive effects on the development of the epilithic community and the physical displacement (i.e., disturbance) of the particles. Disturbance depended on the size and shape of the rocks. Smaller particles were more disturbed, while rectangular rocks of a given size had more frequent disturbance events than spheres or cubes. Many interactions among the manipulated factors were detected in their effects on taxonomic richness and total invertebrate abundance and density. Although several commonly observed effects were found (e.g., increased richness with greater substratum complexity and size), these patterns depended in many cases on the particular set of shapes and colors examined. Additionally, the site (5 m2) within the experimental area where a rock was located had strong effects on both disturbance and the invertebrate community. The two most common taxa also differed in their relationship with disturbance on different particle shapes: Chironomidae were relatively common on more frequently disturbed rectangular and cubic shapes, while Simulium were more common on the less disturbed spheres. Generalizations about the effects of substratum characteristics (e.g., size, "complexity") should be made with caution. Different features of the rock itself, qualities of the rock's location at a larger spatial scale, seasonal influences, and variation in response among taxa may all influence the development of the epilithic community in an interactive fashion.