Composition and diversity of northwestern Amazonian rainforests in a geoecological context

The northwestern Amazonian landscape includes most of the representative landscape units that characterize Amazonia, and for this reason it constitutes an excellent place to investigate relationships between the abiotic environment (geology, geomorphology, soils) and biodiversity. In this review we assess these relationships on three temporal/spatial scales. At the macroscale, the geology of this area includes the sub-Andean zone in the west, the Guiana Shield to the north, and the varied Neogene substrate and river valleys. These geological units account for much of the habitat variation that can be linked to species distributions. For instance, observations of different tree species communities in Ecuadorian or Peruvian Amazonia are explained by variations in soils and geological substrate. Mesoscale abiotic heterogeneity is reflected in the spatial configuration of different land units, which create conditions for habitat-specific forest types. Experimental field studies arc needed to compare effects of the abiotic environment' (water and soil nutrients) with those related to dispersal and species interactions. At micro scales, processes in the biotic environment probably contribute most strongly to pattern formation in diversity and composition of floras or plant communities.