Reduced fecundity and genetic diversity in small populations of rewarding versus deceptive orchid species: a meta-analysis

Background and aims - Understanding how plant traits mediate the relationship between plant population size, fitness and genetic diversity is fundamental to conservation. Many orchid species are threatened, possibly due to their susceptibility to the consequences of habitat fragmentation and the associated decreasing population size. One third of all orchids do not produce nectar to entice pollinators, and this is believed to promote gene flow and outcrossing. Therefore, we hypothesised that deceptive orchids are less susceptible to reductions in population size than rewarding species. Methods - We reviewed the available literature and used traditional and phylogenetically independent meta-analyses to explore relationships between population size, fitness (fruit set) and genetic diversity. Key results - Results showed that orchids in general are highly susceptible to the consequences of small population size. Fruit set significantly decreased in small populations of rewarding, but not in deceptive species. Deceptive orchids were, however, as likely to lose genetic diversity in small populations, even though significantly less genetic differentiation (Fst) was present between populations of deceptive orchids. Finally, we found that homozygosity did not relate with population size, which may indicate a selection process against less fit homozygotes in small orchid populations. Conclusions - We conclude that reductions in population size most likely resulting from habitat loss and fragmentation represent major threats to the population viability of orchids. Fitness of rewarding orchids declines through ecological mechanisms, whereas deceptive orchids may be more susceptible to genetic loss as they are more susceptible to decreased gene flow.