Mutualisms matter: pollination rate limits the distribution of oil-secreting orchids

There are at least two immediate reasons why it is important to determine the role of biotic interactions, such as pollination, in limiting species distribution ranges. Firstly, if range limits are imposed by biotic factors, current and future distribution ranges might not be constrained by climate. Secondly, if biotic interactions limit the distribution ranges of species, anthropogenic impacts on these interactions are likely to have a major effect on biodiversity. Here we test the role of pollination in limiting plant distributions by studying plant community assembly in a guild of 15 oil-secreting orchids (Coryciinae) along a pollination gradient. In all members of the guild, seed production depends on pollination by the oil-collecting bee Rediviva peringueyi (Melittidae). While the mode of aboveground reproduction is uniform across the guild, the orchid species differ widely in their capacity for belowground clonal reproduction through the formation of bulbils, and hence span a range of predicted dependence on pollination (and subsequent seed set) for population persistence. Pollination rate by R. peringueyi varied across the landscape from 0 to 98% of flowers pollinated. With decreasing pollination, species richness of the orchid guild declined, and species were lost by the successive deletion of the least clonal species. Thus, pollination is shown to act as a biotic filter, excluding non-clonal species from pollinator-poor communities. The findings are consistent with the idea that pollination mutualisms matter ecologically by limiting the distribution of non-clonal plants. Conversely, the results suggest that clonality allows some plant species to escape from the range of their pollinators.