Environmental patterns and ecological correlates of range size among bromeliad communities of Andean forests in Bolivia

The abiotic, historical, and autecological factors determining the range sizes of tropical plant species and the distribution of endemism are still poorly understood. In this study, the variation of range-size rarity was analyzed among the bromeliad communities of 74 forest sites in the Bolivian Andes and adjacent lowlands with respect to 14 environmental factors reflecting mostly climatic conditions and to species attributes such as life-form, ecophysiological type, pollination mode, and fruit type. The global ranges of all 192 recorded bromeliad species were mapped on a 1degrees grid, quantified as the number of P grids occupied by a species, and range-size rarity indices were calculated as the mean inverse range size of all species at a given study site. At the community level, range-size rarity increased with elevation, most notably among epiphytic taxa. Ran.-e-size rarity of terrestrial forest species increased with decreasing habitat area, presumably reflecting the agglomeration of endemic species in isolated dry forest valleys with restricted area. Epiphytes showed higher range-size rarity in the most humid areas, which are also geographically isolated. At the species level, range size revealed a limited relationship to pollination mode or ecophysiological type but differed significantly between epiphytic species (large ranges) and terrestrial and saxicolous taxa (small ranges). However, this pattern was outweighed by differences among fruit types, with berries corresponding to large ranges, wind-dispersed seeds with flight appendages to intermediate ranges, and wind-dispersed seeds without appendages to small ranges. It is hypothesized that the tendency toward larger ranges among epiphytes (of any plant group) is due at least partly to the prevalence of taxa with adaptations to long-distance dispersal, ensuring efficient colonization of canopy habitats while preventing the differentiation of populations.