Biogeographic affinity helps explain the productivity-richness relationship at regional and local scales

The unresolved question of what causes the observed positive relationship between large-scale productivity and species richness has long interested ecologists and evolutionists. Here we examine a potential explanation that we call the biogeographic affinity hypothesis, which proposes that the productivity-richness relationship is a function of species’ climatic tolerances, which in turn are shaped by the earth’s climatic history combined with evolutionary niche conservatism (sensu Wiens and Donoghue 2004). Using botanical data from regions and sites across California, we find support for a key prediction of this hypothesis, namely that the productivity-species richness relationship differs strongly and predictably among groups of higher taxa based on their biogeographic affinities (i.e., between families or genera primarily associated with north-temperate, semiarid, or desert zones). We also show that a consideration of biogeographic affinity can yield new insights on how productivity-richness patterns at large geographic scales filter down to affect patterns of species richness and composition within local communities.