Spatial patterns of invertebrate species richness in a river: the relationship between riffles and microhabitats

This study describes the pattern of invertebrate species richness in a rive r reach with large differences in habitat complexity at two, hierarchically nested, spatial scales. The aim was to determine whether the mass effect w as likely to be increasing invertebrate species richness in epilithic micro habitats;in this river. The mass effect is the process by which the species richness of a patch is increased when it acts as a 'sink' for species gene rated by 'source' patches. Microhabitat patch types in Mountain River, Tasm ania, were distinguished on the basis of physical structure and orientation on the river bed. They were nested within two types of riffle with contras ting structural complexity: bedrock and boulder-cobble riffles. It was hypo thesized that microhabitats with high species richness would act as source patches, contributing species to other microhabitats (sinks) and thereby in creasing their species richness. Microhabitat sampling was carried out in f our consecutive seasons and rarefaction was used to estimate riffle-scale s pecies richness. Analysis of variance (ANOVA) was used to compare the ident ical microhabitats present in the contrasting riffle types, to detect evide nce of the mass effect in either riffle type. The more structurally complex boulder-cobble riffles had higher species richness than did bedrock riffle s. Amongst the microhabitats, the spaces beneath the cobbles had the most s pecies. Microhabitats accounted for a higher percentage of the variation in species richness than did differences between riffles of the same type. No evidence was found for the operation of the mass effect in either riffle t ype. The majority of species found only in boulder-cobble riffles were uniq ue to the beneath-cobble microhabitat and appeared to be unable to colonize other microhabitats, even as transients. In Mountain River, small-scale ha bitat characteristics appeared to be more important than larger-scale effec ts in determining microhabitat species richness.