Is beta diversity higher in seed plants with larger body sizes?

Beta diversity is a scalar linking local (alpha) with regional (gamma) species diversity. Understanding patterns of beta diversity is central to ecology and biogeography. It is suggested that beta diversity should be higher for organisms with larger body sizes, which is called the beta diversity-body size hypothesis, because organisms with smaller body sizes are considered to be better dispersers and thus would be expected to have lower rates of species turnover between areas, compared with organisms with larger body sizes. Using a comprehensive database of seed plants in North America, I test this hypothesis. Beta diversity was measured by Simpson's dissimilarity index. Seed plants were divided into three categories (trees, shrubs and herbs), which represent large, intermediate and small body sizes, respectively. While beta diversity in shrubs is higher than that in herbs, supporting the hypothesis, beta diversity in trees is lower than that in shrubs and herbs, contrary to the hypothesis. These findings hold true regardless of whether North America is considered as a whole or different latitudinal zones are considered separately. Thus, the overall finding of this study only partially supports the hypothesis. This study also shows that in all the three groups of plants, beta diversity decreases with increasing latitude, and is more strongly related with geographic distance than with climatic distance.