Comparisons of benthic communities adjacent to riparian native eucalypt and introduced willow vegetation

1. Willows, Salix spp., have been widely introduced as a riparian species in temperate Australia and New Zealand. The present study was a broad-scale observational survey to document the differences between reaches of river lined with willows and native vegetation in the community structure of benthic invertebrates and the resources which these plants use.
2. Nine rivers in south-eastern Tasmania were examined on three occasions in autumn, spring and summer. Taxa were identified to family level, with the exception of Oligochaeta and Acarina, and benthic organic matter (CPOM and FPOM) and epilithic biomass were measured for each reach.
3. Taxon diversity and evenness were lower in willowed reaches in autumn, and total macroinvertebrate density and number of taxa were lowest in willowed reaches in summer. No differences in the fauna between willowed and native reaches were observed in spring. Measures of community similarity of the fauna in willowed and native reaches were significantly different in autumn and summer, but not in spring.
4. The taxa responsible for the significant differences seemed to be responding to differences in food availability and habitat quality in reaches of each vegetation type. Organic matter standing stock was higher in willowed reaches in autumn although the influence of these litter inputs on the fauna were not marked. Epilithon biomass was highest in autumn and spring in willowed reaches when shading in these reaches was least.
5.  The most marked differences between willowed and native reaches were during summer low flows, when the instream fauna appeared to be responding to changes to shading, water quality and the quality of the habitat.