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

• Published in: African journal of ecology 2006-03, Vol.44 (1), p.47-60
• Main Authors: Tattersfield, Peter, Seddon, Mary B., Ngereza, Christine, Rowson, Ben
• Format: Article
• Language: English
• Subjects: Abundance; Altitude; Biodiversity; Eastern Arc; elevational variation; forest; Forests; land-snail; Mollusca; Mollusks; Tanzania; Udzungwa
• ISSN: 0141-6707; 1365-2028
• DOI: 10.1111/j.1365-2028.2006.00612.x

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 (DP), species richness (D20) and observed (SO) and interpolated (SI) alpha diversity – were measured; beta diversity and abundance were also investigated. Mean elevational range was 470 m. DP, D20, alpha diversity and mollusc abundance increased with elevation, but alpha diversity peaked at 1695 m (SO) or 1500 m (SI) and declined at higher elevations. Maximum beta diversity was at 1000 m. Soil pH was negatively correlated with elevation and DP. 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. Résumé Nous avons échantillonné des mollusques terrestres à 15 élévations entre 400 m et 2000 m dans la Réserve Forestière de Mwanihana en Tanzanie et enregistré 84 taxons. Quatre mesures de diversité– la densité des espèces (DP), l'abondance des espèces (D20) et la diversité alpha, la diversité bêta et l'abondance observées (SO) et interpolées (SI) furent aussi enquêtés. La portée d’élèvation moyenne fut 470 m. DP, D20, la diversité alpha et l'abondance des mollusques augmentèrent avec l’élèvation, mais la diversité alpha culmina à 1695 m (SO) ou 1500 m (SI) et diminua aux élévations plus hautes. Le niveau maximum de diversité bêta fut à 1000 m. Le pH du sol fit une corrélation négative avec le niveau d’élèvation et DP. Les sites furent repartis en groupes de basse altitude (400–900 m) et haute altitude (>1000 m) à travers une analyse par faisceau. Le premier axe d'une analyse canonique des correspondances (CCA) fut associéà l'altitude et impliqua une discontinuité de la faune à 1000 m. Des fluctuations dans les groupes de faune de haute altitude (>1000 m) et de basse altitude furent liées à l’élèvation et la stru