ANGIOSPERM GROWTH HABIT, DISPERSAL AND DIVERSIFICATION RECONSIDERED

Previous studies have sought to elucidate the relationship between dis persal mode (biotic versus abiotic) and the taxonomic diversification of angiosperm families, but with ambiguous results. In this study, we propose the hypothesis that the combination of (1) the large seed size required of plants germinating in closed, light-poor environments and (2) the necessity to move disseminules away from the maternal plant i n order to avoid intraspecific competition, predation and pathogens sh ould favour biotically-dispersed relative to abiotically-dispersed woo dy arborescent angiosperms, resulting in higher diversification of the former. In this paper, we seek patterns of diversification that suppo rt this hypothesis. We examine the association between dispersal mode, growth habit and taxonomic richness of monocotyledon and dicotyledon families using (1) contingency table analyses to detect the effect of dispersal mode on the relative abundances and diversification of woody versus herbaceous taxa and (2) non-parametric analyses of variance to detect the statistical effect of dispersal mode on taxonomic diversif ication (mean number of species per genus, genera per family and speci es per family) in monocot and dicot families dominated by biotic or ab iotic dispersal. We found a clear statistical effect of dispersal mode on diversification. Among families of woody dicots, dispersal by vert ebrates is associated with significantly higher levels of species per genus, genera per family and species per family than is abiotic disper sal. The same pattern is observed among woody monocots, but is not sig nificant at the 0.05 level. Among families of herbaceous monocots and dicots, the situation is reversed, with abiotically-dispersed families exhibiting higher levels of diversification than vertebrate-dispersed families. When woody and herbaceous families are pooled, there is no association between dispersal mode and diversification. These data coi ncide with evidence from the fossil record to suggest vertebrate dispe rsal has positively contributed to the diversification of woody angios perms. We suggest that vertebrate dispersal may have promoted the dive rsity of extant taxa by reducing the probability of extinction over ev olutionary time, rather than by elevating speciation rates. Our result s suggest vertebrate dispersal has contributed to, but does not explai n in toto, the diversity of living angiosperms.