Extensive genetic differentiation at a small geographical scale: reduced seed dispersal in a narrow endemic marsh orchid, Anacamptis robusta

The endangered orchid Anacamptis robusta is a narrowly endemic species restricted to fragmented marsh habitats on Majorca, Spain. To investigate the effects of habitat fragmentation, we quantified genetic diversity and levels of seed exchange among all living metapopulation units of this species. A hypervariable plastid minisatellite was analysed in 1882 individuals and used to estimate genetic diversity, structure, and levels of seed dispersion. High levels of genetic isolation were detected, indicated by low effective migration values between metapopulation units and high genetic differentiation. Bayesian inferences of population growth confirmed previous results of overall population reduction. Comparison of haplotypes found in adult and juvenile plants confirmed reduced seed dispersal among patches. Given the small effective population size and strong population structuring with low exchange of migrants, demographic stochasticity is likely to be the greatest threat to the long-term persistence of this species. However, high values of genetic diversity were observed in almost all metapopulation units, suggesting that the initial colonization process probably involved seed immigration from multiple sources. The genetic survey presented here provides vital information for the future effective management of this rare orchid.