Measuring stream macroinvertebrate responses to gradients of vegetation cover: when is enough enough?

P>1. Management of stream biodiversity is often tightly linked with the restoration and protection of riparian and catchment vegetation. Despite that, there are no established guidelines on how much forest should be retained or replanted in riparian zones and surrounding catchments to maintain or re-establish instream ecological integrity. In this study, we assess relationships between vegetation cover at multiple spatial scales (reach, segment and catchment) and macroinvertebrate metrics that reflect community structure, ecological condition and biodiversity at 138 Waikato, New Zealand, stream sites sampled in 2006. 2. Percentage of catchment vegetation in native forest had stronger relationships with measured diversity and condition metrics than segment or reach scale measures of riparian vegetation. Functional feeding group metrics were weakly associated with upstream catchment vegetation cover. 3. Of the macroinvertebrate metrics tested, the RIVPAC O/E and an organic pollution tolerance metric based on species presence-absence (Macroinvertebrate Community Index; MCI) had the strongest relationships with percentage native riparian vegetation, followed by the quantitative MCI and measures of the richness and relative abundance of Ephemeroptera, Plecoptera and Trichoptera faunas. The O/E and MCI indicated that catchments with 80-90% in native forest or scrub (low-growing trees) were associated with faunas indicative of "clean" water quality. 4. Of the biodiversity indices considered Fisher's alpha Index of species richness had the strongest relationship with percent native riparian vegetation in the upstream catchment. There are no established thresholds for measuring biodiversity loss in New Zealand streams, but this analysis indicates that on average streams draining catchments with 40-60% upstream native vegetation cover retain 80% of the mean biodiversity present in pristine forest streams. 5. This research indicates that riparian management aimed at enhancing macroinvertebrate biodiversity and the ecological condition of streams is likely to be more successful when focused on protecting and/or restoring headwater catchments rather than short stretches of stream.