Environmental and spatial controls of palm (Arecaceae) species richness across the Americas

Aim To determine the main factors that control the distribution of palm species richness across the Americas, to understand the relative importance of climatic and other environmental factors vs. spatial variables (as substitutes for non-environmental factors such as history), and to evaluate how robust the patterns found are to changes in spatial scale. Location Tropical and subtropical America (34 degrees N-34 degrees S; 33 degrees W-120 degrees W). Methods We provide a GIS-based analysis of spatial variation in species richness at 10 grain sizes (1 degrees-10 degrees square grid cells). We use multiple regressions and variation partitioning to test patterns of species richness in relation to five environmental descriptors [mean annual temperature (degrees C), mean annual precipitation (mm year(-1)), number of wet days year(-1), topographical range (maximum-minimum elevation), number of vegetation types] and nine spatial variables (the terms of a cubic trend-surface polynomial). Results We observe peaks in palm species richness in southern Mesoamerica, the Choco area, western and eastern Amazon basin, and east Andean slopes, and a strong latitudinal richness gradient even within the tropical zone. Pure environmental and pure spatial variation components are of equal magnitude through all spatial scales analysed. Water-related variables such as annual rainfall and number of wet days are the main richness predictors. Spatial variables, notably latitude squared are also important, whereas temperature- and habitat-related variables are relatively unimportant. Main conclusions Our analysis suggests that in the Americas, palm species richness at spatial scales from 1 degrees to 10 degrees is most strongly controlled by water availability, although unknown broad-scale factors, perhaps soil, historical processes or geometric constraints, are also important.