Elevational variation in diversity and composition of land-snail faunas in a Tanzanian forest

We sampled terrestrial molluscs at fifteen elevations between 400 and 2000 m in Mwanihana Forest Reserve, Tanzania and recorded 84 taxa. Four diversity measures - species density (D-P), species richness (D-20) and observed (S-O) and interpolated (S-I) alpha diversity - were measured; beta diversity and abundance were also investigated. Mean elevational range was 470 m. D-P, D-20, alpha diversity and mollusc abundance increased with elevation, but alpha diversity peaked at 1695 m (S-O) or 1500 m (S-I) and declined at higher elevations. Maximum beta diversity was at 1000 m. Soil pH was negatively correlated with elevation and D-P. Cluster analysis divided the sites into lowland (400-900 m) and highland (> 1000 m) groups. Axis 1 of a canonical correspondence analysis was associated with altitude and suggested a faunal discontinuity at 1000 m. Variation within the highland (> 1000 m) and lowland faunal sets was related to elevation and forest structure respectively. The findings indicate that mollusc diversity peaks at intermediate elevations. This may be related to the combined effects of low rainfall conditions at low elevations and increasing effects of soil leaching at high elevations, both of which may limit mollusc diversity and abundance. Diversity at intermediate altitudes may be further elevated because of faunal mixing of lowland and highland groups.