Diversity and dominance in palm (Arecaceae) communities in terra firme forests in the western Amazon basin

1 Current discussions about the structure and functioning of tropical plant communities revolve around whether tropical forests are dominated by sets of widespread and locally abundant species and the relative importance of environmental variation and geographical distance as determinants of variation in species composition. We explored these questions by examining diversity and dominance patterns of palm communities in lowland rain forests in the western Amazon basin (Yasuni National Park, Ecuador and Iquitos-Pebas region, Peru). 2 We used multiple regression to analyse floristic similarity between sites as a function of environmental differences (in topographic position, elevation difference, amount of exchangeable cations and soil texture) and geographical distance. We studied dominance patterns by quantifying the correlations between local abundance and landscape frequency of species within and between the regions. 3 We found floristic composition to be highly variable and more strongly related to geographical distance than to environmental differences. Together, geographical distance and environmental differences explained 70-85% of the variation in floristic similarity. 4 Species abundant in one region were also often abundant in the other, although some dominant species in Iquitos-Pebas did not occur in Yasuni. Local dominance was more pronounced in Yasuni, where the dominant species formed a larger and less variable proportion of all individuals within transects. 5 Limited dispersal and, to a lesser extent, local environmental variation appear to act as determinants of floristic variation in the western Amazon basin. Oligarchic dominance patterns were observed, particularly in Yasuni. 6 Our results lend support to the hypothesis that dispersal across broader spatial scales plays a strong role in the assembly of local ecological communities, but indicate that such limitation supplements, rather than replaces, local ecological determinism.