Plant diversity, biogeography and environment in Iberia: Patterns and possible causal factors

Abstract

We associated patterns of plant diversity with pos- sible causal factors by considering 93 local regions in the Iberian Peninsula and Balearic Islands with respect to biogeo- graphy, environmental favourability, and environmental hetero- geneity, and their relationship with measured species diversity at four different scales: mean local species richness standard- ized at a grain of 100 m 2 , total species richness in a community type within a region (regional community richness), mean compositional similarity, and mosaic diversity. Local regions in biogeographic transition zones to the North African and Atlantic floras had higher regional commu- nity richness and greater mosaic diversity than did non-transi- tional regions, whereas no differences existed in mean local species richness or mean compositional similarity. Mean local species richness was positively related to environmental fa- vourability as measured by actual evapotranspiration, but negatively related to total precipitation and temporal heteroge- neity in precipitation. Mean local species richness was great- est in annual grassland and dwarf shrubland communities, and on calcareous bedrock types. Regional community richness was similarly related to actual evapotranspiration and total precipitation, but in addition was positively related to spatial heterogeneity in topography and soil water holding capacity. Mean compositional similarity decreased with increasing spa- tial heterogeneity and temperature seasonality. Mosaic diver- sity, a measure of complexity, increased with increasing local and regional richness. We hypothesize that these relationships can be explained by four ecological and evolutionary classes of causal factors: nu mbers of individuals, intermediate environments, limits to adaptation, and niche variation. These factors operate at various scales and manifest themselves in various ways. For example, at the site level, apparently processes that increase the number of individuals increase mean local species richness, but at the level of the entire region no such effects were found