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.

The genome sequence of the orchid Phalaenopsis equestris

“…In contrast to all vascular plants, of which about 10% are estimated to be epiphytes 39 , most orchid species (72%) are epiphytes, with the majority being restricted to tropical regions 40,41 . Many orchids use the CAM pathway for photosynthesis rather than the C 3 pathway, which is considered to be an adaptation to their epiphytic lifestyle that limits water supply.…”

Introduction and synthesis: plant phylogeny and the origin of major biomes

“…The papers in this volume by Lavin et al (2004;Leguminosae), Renner (2004; Melastomataceae sensu stricto), Plana (2004;Begonia), Linder & Hardy (2004;Restoniaceae), Richardson et al (2004; Annonaceae and Rhamnaceae), Gravendeel et al (2004;Orchidaceae) attest to the much wider value of such systematic work, and to the urgency of its continuation and completion. These groups are ecologically important as different life forms in different biomes and geographical areas.…”

“…Elucidating the evolutionary history of these epiphytes, and the factors that have driven their diversification is an important element of understanding the historical assembly of these species-rich habitats. Gravendeel et al (2004) use phylogenies of orchids and some other major epiphytic groups to elucidate this. However, they also turn the question on its head and ask whether the epiphytic habit promotes diversification.…”

Evolution along the crassulacean acid metabolism continuum

“…The Orchidaceae is the largest family of flowering plants with >800 genera and~26 000 species worldwide, of which~75% are estimated to be epiphytic (Atwood 1986;Dressler 1993;Gravendeel et al 2004). Orchids exhibit a large number of morphological, anatomical, ecological and physiological characteristics that allow them to exist within diverse ecosystems and ecological niches with the greatest diversity in mountainous regions of the tropics (Cribb and Govaerts 2005).…”