Patterns of diversity and range size of selected plant groups along an elevational transect in the Bolivian Andes

Abstract

Patterns of point, α-, and β-diversity, and of an index of range size were studied for Acanthaceae, Araceae, Bromeliaceae, Melastomataceae, Palmae, and Pteridophyta in 204 vegetation plots of 400 m2 along an elevational gradient in humid montane forest at 220-3950 m in central Bolivia. Zonal forest point-diversity (mean species number per plot) and α-diversity (extrapolated total species number) showed either hump-shaped curves or constant values from lowlands to mid-elevations, followed by a steep decline. These patterns correspond to the hypothesis of maximum diversity at intermediate levels of productivity, while for pteridophytes and epiphytic Bromeliaceae they also fitt a null-model of random distribution within bounded geographical ranges. The ratio of point to α-diversity was surprisingly constant across study groups and elevation with values of 0.2-0.4. Range size index (mean inverse range size of all species recorded at a given elevation) either increased with elevation, showed hump-shaped patterns, or remained roughly constant. Non-zonal habitats (ravines, ridge-tops, rock faces, roadsides, pastures) contributed significantly to overall species number for terrestrial herbs and shrubs of Bromeliaceae, Melastomataceae, and Pteridophyta, but not for Acanthaceae, Araceae, and epiphytic taxa. With few exceptions, endemism was most pronounced in zonal forests. These results imply that conservation measures should focus on zonal forests in a few large reserves at low and mid-elevations and in many smaller reserves at higher elevations.