Impacts of land use on the structure of river macroinvertebrate communities across Tasmania, Australia: spatial scales and thresholds

The formulation of scientifically justified guidelines for management of anthropogenic impacts on river health requires better understanding of the quantitative linkages among river-system parameters. The present study examines relationships between land use and biological metrics of river health in Tasmania, in the context of a variety of environmental drivers. An extensive dataset (103 sites) of macroinvertebrate assemblages was collected between 1999 and 2006. We hypothesised that grazing by domestic livestock would have the greatest impact on community structure of the land-use types investigated because grazing is a dominant land-use type in Tasmania (and can cover a large proportion of catchment area), because land clearance for grazing is rarely followed by regeneration and because historically riparian vegetation has not been protected. Multivariate and correlation analysis showed that community structure responded strongly to land use and confirmed that the strongest relationships were observed for grazing land use and environmental variables associated with grazing, such as e.g. water abstraction and/or regulation and riparian vegetation. Analyses accounting for hydrological region and location confirmed the generality of this relationship. We conclude that catchment-wide management actions would be required to mitigate these impacts of grazing because land use and riparian vegetation condition were generally stronger determinants of community structure at catchment rather than local scales.