Widespread pollen limitation and the influence of geographical gradients on flowering population size and fecundity in orchids

Karl Duffy ¹, Marco G. Balducci ¹

University of Naples Federico II ¹

Biodiversity is unevenly distributed globally. Whether the outcome of mutualistic interactions, such as plant pollination by animals, also varies along geographical gradients is unclear. As animal pollinators vary in their responses to flowering plant aggregation on the population level, the extent to which plant fecundity is pollen limited may be influenced by flowering population size. Orchids have a global distribution and are generally pollen limited, hence geographical variation in flowering population size may result in geographical variation in plant fecundity. We performed literature surveys to; (i) quantify the extent of orchid pollen limitation and natural fruit set from experimental hand-pollination studies on 85 species in a phylogenetic context, according to life-form and geographical region and, (ii) quantify how both orchid flowering population size and fecundity varied according to reward strategy and life-form, in a phylogenetic context for 113 species. As the majority of species were represented by a single population, we tested whether fecundity varied along latitudinal and longitudinal gradients for 25 species represented by at least five populations controlling for reward strategy, pollinator type, habitat, and life-form. Pollen limitation was widespread with no phylogenetic signal, yet was higher in epiphytic orchids, while natural fruit set had strong phylogenetic signal. Fecundity covaried with population size and reward strategy, with fecundity increasing with population size in rewarding orchids and decreasing with population size in sexually deceptive orchids. Flowering population sizes increased at mid and high latitudes and did not vary with longitude. Fecundity varied non-linearly according to latitude while decreased with increasing longitude, indicating that orchid fecundity may vary along geographical gradients. These results indicate that interactions between population size according to reward strategy and orchid fecundity varies along geographical gradients. This supports the hypothesis that the outcome of biotic interactions varies geographically and may play a key role in explaining the limits to orchid distributions.

Main author career stage: Professor / Permament researcher

Contribution type: Poster

First choice session: 1. Systematics, phylogenetics, biogeography and evolution

Second choice session: 2. Ecology