Large-scale processes and the Asian bias in species diversity of temperateplants

An important issue in the study of biodiversity is the extent to which glob al patterns of species richness reflect large-scale processes and historica l contingencies(1,2). Ecological interactions in local assemblages may cons train the number of species that can coexist(3,4), but differences in diver sity in similar habitats within different regions (diversity anomalies) sug gest that this limit is not firm. Variation in rate of species production c ould influence regional and perhaps local diversity independently of the ec ological capacity of an area to support coexisting species, thereby creatin g diversity anomalies(5,6). Temperate Zone genera of plants that are disjun ct between similar environments in eastern Asia and eastern North America ( EAS-ENA) have twice as many species in Asia as in North America(7). Because lineages of these genera in Asia and North America are mostly sister pairs (8), they share a common history of adaptation and ecological relationship before disjunction. Thus, the diversity anomaly in EAS-ENA genera is not an artefact of taxon or habitat sampling but reflects differences in the net diversification (speciation-extinction) of the lineages in each of the cont inents. Here we propose that the most probable cause of the EAS-ENA anomaly in diversity is the extreme physiographical heterogeneity of temperate eas tern Asia, especially compared with eastern North America, which in conjunc tion with climate and sea-level change has provided abundant opportunities for evolutionary radiation through allopatric speciation.