Spatial and environmental determinants of vascular plant species richness distribution in the Iberian Peninsula and Balearic Islands

Using an exhaustive data compilation, Iberian vascular plant species richne ss in 50 x 50 UTM grid cells was regressed against 24 explanatory variables (spatial, geographical, topographical, geological, climatic, Wand use and environmental diversity variables) using Generalized. Linear Models and par tial regression analysis in order to ascertain the relative contribution of primary, heterogeneous and spatially structured variables. The species ric hness variation accounted for by these variables is reasonably high (65% of total deviance). Little less than half of this variation is accounted for spatially structured variables. A purely spatial component of variation is hardly significant. The most significant variables are those related to alt itude, and particularly maximum altitude, whose cubic response reflects the occurrence of the maximum number of species at the highest altitudes. This result highlighted the importance of Iberian mountains as hotspots of dive rsity and the relevance of large and small scale historical factors in cont emporary plant distribution patterns. Climatic or energy-related variables contributed little, whereas geological (calcareous and acid rocks) and, to a lesser extent, environmental heterogeneity variables (land use diversity and altitude range) seem to be more important. (C) 2001 The Linnean Society of London.