Species diversity: from orchids to mangroves

Abstract

Why are there so many species in some plant families, e.g. Orchidaceae and Asteraceae, but much fewer in others, e.g. Rhizophoraceae? Other than the artifacts attributable to taxonomic treatment, what are the major factors that cause this divergence in species richness among plant families? Assuming that the numbers of species in angiosperm families truly reflects morphological diversity and rate of speciation, some general patterns may emerge between the number of species and factors that affect the rate of lineage diversification and hence species richness in Angiosperm families. Using data available from internet resources, I investigate the factors affecting species richness. Among numerous biological, ecological and biogeographical factors that might influence the rate of lineage diversification and speciation (including but not limited to growth habits, longevity, ability for long-distance dispersal, niche diversity/habitat richness, population size, range of geographical distribution), the range of geographical distribution and percentage of herbaceous species of each family can be quantified for statistical analysis. Preliminary regression analysis of sampled angiosperm families indicates that global geographical distribution and herbaceous growth form are two highly significant factors contributing to species richness in these families. Of course, evolutionary factors - natural selection, random genetic drift, and mutation and recombination - are underlying mechanisms of speciation, in the absence of or weakened gene flow due to vicariance and/or long-distance dispersal. The geographical range of the family could serve as a surrogate index of habitat diversity and degree of spatial/temporal isolation for selection and drift to act and accumulate genetic divergence among conspecific populations, and herbs are generally shorter-lived than shrubs/trees and have faster life cycles to accumulate the effects of mutations/selection/drift - leading to a high rate of lineage diversification and speciation. Using biological attributes and geographical distributions of Orchidaceae and Rhizophoraceae as examples, I will discuss the generality of these correlates of species diversity.