Epiphytism and pollinator specialization: drivers for orchid diversity?

Abstract

Epiphytes are a characteristic component of tropical rainforests. Out of the 25 000 orchid species currently known to science, more than 70% live in tree canopies. Understanding when and how these orchids diversified is vital to understanding the history of epiphytic biomes. We investigated whether orchids managed to radiate so explosively owing to their predominantly epiphytic habit and/or their specialized pollinator systems by testing these hypotheses from a statistical and phylogenetic standpoint. For the first approach, species numbers of 100 randomly chosen epiphytic and terrestrial genera were compared. Furthermore, the mean number of pollinators per orchid species within the five subfamilies was calculated and correlated with their time of diversification and species richness. In the second approach, molecular epiphytic orchid phylogenies were screened for clades with specific suites of epiphytic adaptations. Epiphytic genera were found to be significantly richer in species than terrestrial genera both for orchids and non–orchids. No evidence was found for a positive association between pollinator specialization and orchid species richness. Repeated associations between a small body size, short life cycle and specialized clinging roots of twig epiphytes in Bulbophyllinae and Oncidiinae were discovered. The development of twig epiphytism in the first group seems repeatedly correlated with speciation bursts.