Similarity of tree species in relation to distance in a rain forest in the National Forest Saracá-Taquera, Pará

The distribution of species in tropical regions is very complex. Two principal models are commonly employed toexplain species distributions: the Neutral Theory, and the Niche Theory. The former predicts that species coexistenceis the result of a balance between immigration and extinction, while the Niche Theory predicts that natural resourcesare used differently among species. Studies carried out in tropical regions have demonstrated that species similaritydecreases with geographical distance. However, few studies have been done comparing changes in species similaritywith geographical distance on a local scale, where physical variables such as topography, altitude and soil type do notvary. The objective of this study is to test differences in species similarity with geographical distance from botanical plotson an Amazonian plateau in the Sacará-Taquera National Forest Reserve, in Pará, Brazil. 179 tree plots (10 x 250 m)were systematically distributed across the entire extent of the plateau, with a distance between plots varying between200 meters and 9 km. A total of 631 species of tree were identified across all plots. The species area curve showeda clear tendency to asymptote. The estimated species richness (using first and second order Jackknife estimators)suggested 720 and 733 species respectively. The majority of species were present in low frequencies in the sampledplots, a pattern typical for the Amazon region. From the total of 631 species, 442 (70%) occurred in less than 10%of all parcels. There was a negative correlation between species similarity and plot distances. The lowest S rensensimilarity measures (ranging from 13% to 16%) were obtained from plots over 6 km apart, while the highest levels ofsimilarity (ranging from 55% to 62%) were found comparing plots less than one kilometer apart. In conclusion, thisstudy partially corroborates the Neutral Theory, where the similarity of species decreases with increasing geographicaldistances. However, the medium strength correlation between similarity and distance (r = -0.44) indicates that otherfactors are also important, and showing that the similarity of species in the tropical rain forests may be partly explainedby other biotic and abiotic factors, as well as geographical distance.