Impacts of land use at the catchment scale constrain the habitat benefits of stream riparian buffers

Stream conservation and restoration strategies commonly focus on preserving extant riparian forest and restoring riparian habitat. Though these practices are beneficial to stream habitat, they may not suffice to restore stream condition in catchments heavily influenced by intensive land use. To evaluate this hypothesis, we measured invertebrate populations in 12 streams affected by different land-use types (cultivated, developed, forested and grassland). Within each stream, we compared reaches with and without riparian buffers of remnant mature forest. Within each reach, we calculated the relative abundance of individuals from the insect orders Ephemeroptera, Plecoptera and Trichoptera (%EPT) and assessed specific conductance, dissolved oxygen (DO), temperature and substratum composition. Forested catchments had the highest %EPT (average 23.8) followed by grassland (16.1), cultivated (1.96) and developed (0.31). Reaches with forested buffers present were not associated with higher %EPT in forested, cultivated or developed catchments. In grassland catchments, however, %EPT was about eightfold higher in reaches with forested buffers than those without. High values of %EPT were associated with large stream substratum and low specific conductance and temperature, which were most common in forested catchments. Developed and cultivated catchments did not follow this pattern, %EPT being uniformly low, even where abiotic conditions were similar to forested catchments. These results confirm that intensive urban and cultivated land uses at the catchment scale degrade stream biological communities. They also indicate that patchy forested riparian corridors, as found in many restoration projects, are probably insufficient to mitigate severe, large-scale biological degradation.