Joint analysis of microsatellites and flanking sequences shows shared demographic response of coastal sand dune plant communities to past environmental changes.

Understanding recent change in effective population size is crucial to evaluate how specie respond to environmental modifications and their ability to adapt to global changes. However, the impact of past environmental change on the demography history of species communities has rarely been studied across multiple species, members of the same community sharing the same habitat. In this study, we compare historical change in effective population size across eight species of coastal sand dune plant communities along the coastal dune in Southwest France. We developed and sequenced species-specific microsatellite markers to genotype 3116 individuals. In an attempt to increase temporal resolution, we integrate both fast (microsatellite repeat number) and slow (substitution in flanking sequence) mutating polymorphisms to infer demographic parameters of a simple model of a single population size change in the past using coalescent simulations and approximate Bayesian computations. Recent and ancient effective population sizes were well inferred and informed by allelic richness at microsatellites and heterozygosity at flanking sequence respectively. The timing of event was more difficult to estimate, but benefit from summary statistics combining microsatellite variation and flanking substitutions. Most species showed a synchronous and strong decline dating back from 135 to 450 years ago, overlapping with the Little Ice Age during which high storminess caused sand drift that probably wiped out most of the open sand dune vegetation, which is now restricted along the shoreline. Historical habitat loss led to community level decline in effective population size that will constrain future sand dune plant responses.