AN ANALYSIS OF CHANGES IN THE INVERTEBRATE COMMUNITY ALONG A SOUTHERN NEW-ZEALAND MONTANE STREAM

Changes in benthic community structure at 4 sites along a flood-prone montane stream continuum in the South Island of New Zealand were analyzed using rank correlation (Kendall''s Tau and Spearman''s rs) and a community similarity index (Sc) developed initially for stream pollution assessment. Species richness was highest in a forested headwater stream, which is attributed to its relatively greater physical stability and habitat heterogeneity. Community similarity decreased with distance apart, and pairs of open and forested sites showed the greatest similarity. Species composition and relative abundance along the continuum changed least in the numerically dominant Ephemeroptera and Plecoptera, groups in which several species had broad habitat requirements. A number of dipteran and trichopteran species had more restricted distributions. The more specialized niche requirements of some species in these orders are attributed to their generally small size, their ability to spin silk and their holometabolous development. Physical rather than biological factors appeared to limit the distributions of most invertebrates; there was little evidence of ecological segregation by either food resource partitioning or differences in life history patterns among many closely related species. Theories that stream communities are well ordered entities maximizing the efficiency of energy use cannot realistically be applied to such unstable stream systems.