Spatio-ecological niche segregation of two sympatric species of<Emphasis Type="Italic">Clidemia (Melastomataceae)</Emphasis> in Western Amazonian non-flooded rainforests

The frequent occurrence of sympatric series of closely related plant species in tropical rainforests has evoked claims for and against the application of the competitive exclusion principle in these ecosystems. Narrow niche limits defined by biotic as well as abiotic specialization have been reported for sympatric species of the same genus or family. In Amazonian lowland rainforests this question deserves renewed attention because: (1) the existence of edaphically defined community types has recently been well established, and (2) spatio-ecological niche segregation of congeneric species may help explain not only the maintenance of the high Amazonian alpha-diversity, but also its origin through sympatric ecological speciation. In this study, the morphology, ecology, and distribution patterns of two species,Clidemia epiphytica andC. longifolia (Melastomataceae), from western Amazonia, were analyzed. The aims were to find out whether they really are two distinct taxonomic species and if so, whether they also can be considered biological species; if the species are sympatric; and if they are ecologically specialized. The results showed that the morphological variation of the species seems continuous, but that they exhibit opposite morphological responses to variation in soil cation concentration, which suggests that they also are separate biological species. Furthermore, the species occur sympatrically but in different habitats. It is suggested that a part of the enigma of sympatric congeners in rainforests may be explainable by spatial segregation stemming from ecological specialization in relation to subtle environmental variation. It is hypothesized that the studied species are a good candidate case of sympatric speciation driven by ecological specialization.