Drivers of plant community (in)stability along a sea–inland gradient

1. Global change pressures are highlighting the need to better understand the mechanisms driving the temporal stability of natural communities under different environmental conditions. There is ample evidence that species richness helps communities to withstand environmental fluctuations and stabilise over time. However, it is still debated whether richness promotes stability through the diversity of species functional traits, phylogenetic lineages and ecological strategies in the community or because of the likelihood of including stable species. Furthermore, it is unclear whether the positive effect of diversity on stability is maintained in conditions of strong environmental fluctuations (e.g. frequent disturbances and stress).2. To address these questions, we analysed long-term monitoring data of 84 permanent plots in coastal dune plant communities distributed along a gradient of natural stress and disturbance, with communities closer to the sea subject to greater stress and more frequent disturbances. Specifically, we used structural equation models to disentangle the relative influence of the environmental gradient and the different diversity components (species richness, functional and phylogenetic diversity; SR, FD and PD), as well as of the dominant ecological strategy (captured by species lifespan) on community stability, through their effect on two key stability mechanisms (population stability and species asynchrony).3. We found that the sea-inland environmental gradient was the main driver of stability mechanisms. Stress and disturbance decreased both population stability and species asynchrony, but also reduced species richness, which thus exerted a stabilising effect only on the communities in more favourable environmental conditions.4. Surprisingly, we did not find an effect of FD and PD on community stability, neither directly nor via asynchrony. However, the dominance of perennial species mitigated the instability generated by stress and disturbance. Perennial species were on average more stable than annuals and displayed a wider range of species fluctuations, including compensatory dynamics among species (i.e. asynchrony).5. Synthesis: Overall, our results highlight the importance of accounting for the environmental context when examining mechanisms of community stability. Species richness remains a useful direct predictor of community stability. Species ecological strategies, like the acquisitive-conservative trade-off connected to lifespan, however, should also be routinely considered as drivers of both population stability and compensatory dynamics.