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

Patterns of point, alpha-, and beta -diversity, and of an index of range si ze were studied for Acanthaceae, Araceae, Bromeliaceae, Melastomataceae, Pa lmae, and Pteridophyta in 204 vegetation plots of 400 m(2) along an elevati onal gradient in humid montane forest at 220-3950 m in central Bolivia. Zon al forest point-diversity (mean species number per plot) and alpha -diversi ty (extrapolated total species number) showed either hump-shaped curves or constant values from lowlands to mid-elevations, followed by a steep declin e. These patterns correspond to the hypothesis of maximum diversity at inte rmediate levels of productivity, while for pteridophytes and epiphytic Brom eliaceae they also fitt a null-model of random distribution within bounded geographical ranges. The ratio of point to alpha -diversity was surprisingl y 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 elev ation) either increased with elevation, showed hump-shaped patterns, or rem ained 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 Pter idophyta, but not for Acanthaceae, Araceae, and epiphytic taxa. With few ex ceptions, endemism was most pronounced in zonal forests. These results impl y that conservation measures should focus on zonal forests in a few large r eserves at low and mid-elevations and in many smaller reserves at higher el evations.