Neotropical lowland rain forests are among the most species-rich in the world (e.g. Gentry 1988; Balslev et al. 1998) and local species richness (alpha diversity) of trees and other vascular plants reaches one of its global peaks in the western and central Amazon basin with > 300 tree species of ≥ 10 cm diameter-at-breast-height (d.b.h.) per hectare. While the high alpha diversity of some Amazonian forests is well established, the larger-scale patterns of Amazonian plant diversity are much less known (e.g. Terborgh & Andresen 1998; Condit et al. 2002; Tuomisto, Ruokolainen & Yli-Halla 2003). A better understanding of these larger-scale diversity patterns, notably beta diversity (here defined as differences in species composition between local sites (transects) within a single broad habitat type, cf. Condit et al. 2002; Tuomisto, Ruokolainen & Yli-Halla 2003) is crucial not only for understanding how the high alpha diversity is maintained, but also for the development of informed conservation plans for the region (Tuomisto et al. 1995; Terborgh et al. 2002).
In the recent discussions of Amazonian plant distribution and diversity patterns three general views have emerged. The first proposes that terra firme forests in the Amazon basin are dominated by a limited set of ecologically superior species that combine high frequency with high local abundance, and form predictable oligarchies over wide areas; these dominant species are abundant and widespread because they are broadly adapted to suites of environmental conditions that extend over large areas (Pitman et al. 1999, 2001; cf. Poore 1968 for similar conclusions regarding the mixed-dipterocarp forests of Malaysia). Pitman et al. (2001) found that a limited number of common species dominated tree communities in the western Amazon basin, accounting for over 50% of trees at all scales. Low beta diversity of tree species has been found both at meso-scale (1–100 km2) in Colombian Amazonia (Duivenvoorden 1995) and over great distances in the western Amazon basin (Condit et al. 2002; but see Phillips et al. 2003; Tuomisto, Ruokolainen & Yli-Halla 2003; Tuomisto, Ruokolainen, Aquilar & Sarmiento 2003). Oligarchic forests also cover large areas in flooded or disturbed habitats in the Amazon basin (Peters et al. 1989).
Alternatively, species may be ecologically equivalent and local species composition is a result of species immigration and extinction. Species composition is unrelated to environmental factors, but exhibits strong spatial autocorrelation due to dispersal limitation (Hubbell 2001). While some support has been found for this view for tropical forest plant communities (Hubbell et al. 1999; Hubbell 2001), other recent studies have concluded that this theory alone is insufficient to explain the floristic similarity patterns observed over hundreds of kilometres (Condit et al. 2002; Phillips et al. 2003; Tuomisto, Ruokolainen & Yli-Halla 2003).
The third view is that terra firme forests in the Amazon basin grow in a fine-grained patchwork of different environments that create corresponding differences in plant species composition due to ecological differences among species (Gentry 1988; Tuomisto et al. 1995; Tuomisto, Ruokolainen & Yli-Halla 2003; Tuomisto, Ruokolainen, Aquilar & Sarmiento 2003). Several studies have reported that changes in species composition in neotropical forests correspond to variation in soil characteristics or topographic position at both local (Svenning 1999; Vormisto et al. 2000; Tuomisto, Poulsen, Ruokolainen et al. 2003) and larger scales (Tuomisto et al. 1995; Clark et al. 1995; Clark et al. 1999; Phillips et al. 2003; Tuomisto, Ruokolainen & Yli-Halla 2003; Tuomisto, Ruokolainen, Aquilar & Sarmiento 2003). The mechanisms underlying the three views are not necessarily mutually exclusive and finding evidence in favour of one of them does not therefore invalidate the others.
Here, we investigate differences in species composition (especially relevant to the latter two views) and dominance patterns (particularly relevant to the first) of palm communities in terra firme forests in the western Amazon. We assessed the relative importance of stochastic and environmental variables controlling floristic differences in palm species between local sites, and whether all palm communities in the western Amazon basin are dominated by the same, limited set of species? We used multiple linear regressions on similarity matrices to analyse the dependency of floristic composition on differences in environmental conditions and geographical distance (cf. Phillips et al. 2003; Tuomisto, Ruokolainen & Yli-Halla 2003). If floristic differences arise from neutral processes, then floristic similarity should strongly decrease with geographical distance, whereas if environmental site differences are important they should correlate strongly with the floristic data. We also analysed dominance patterns within and between two regions in the western Amazon basin (cf. Pitman et al. 2001).