Beta diversity in relation to dispersal ability for vascular plants in North America

To test the hypothesis that plant species with a higher dispersal ability have a lower beta diversity. North America north of Mexico. Propagules of pteridophytes (ferns and their allies) are more vagile than propagules of spermatophytes (gymnosperms and angiosperms), and thus pteridophytes have a higher dispersal ability than do spermatophytes. The study area was divided into 71 geographical units distributed in five latitudinal zones. Species lists of pteridophytes and spermatophytes were compiled for each geographical unit. Three measures of beta diversity were used: beta(sim), which is one minus the Simpson index of similarity, beta(slope), which is the slope of the relationship between Simpson index and geographical distance, and beta(0.5-distance), which is the distance that halves the similarity from its initial value. Average beta(sim) is higher for spermatophytes than for pteridophytes, regardless of whether the data are analysed for the whole continent or for latitudinal zones. Average beta(sim) decreases with increasing latitude for both spermatophytes and pteridophytes. The difference in average beta(sim) between the two plant groups increases with increasing latitude, indicating that beta diversity decreases with increasing latitude faster for pteridophytes than for spermatophytes. When the Simpson index is regressed against geographical distance, the regression slope (beta(slope)) is steeper for spermatophytes than for pteridophytes, and the slope decreases with increasing latitude for both plant groups. Similarly, beta(0.5-distance) was shorter for spermatophytes than for pteridophytes in each latitudinal zone and increased with increasing latitude for both plant groups. The results of the analyses using the three different measures of beta diversity are consistent. The fact that beta diversity is lower for pteridophytes with vagile propagules than for spermatophytes with less vagile propagules suggests that beta diversity is negatively related to dispersal ability.