Population viability of the orchid Gymnadenia conopsea increases with population size but is not related to genetic diversity

Population size is a main indicator of conservation potential, thought to predict both current and long-term population viability. However, few studies have directly examined the links between the size and the genetic and demographic properties of populations, using metrics that integrate effects across the whole life cycle. In this study, we combined 6 years of demographic data with SNP-based estimates of genetic diversity from 18 Swedish populations of the orchid Gymnadenia conopsea. We assessed whether stochastic growth rate increases with population size and genetic diversity, and used stochastic life table response experiment (SLTRE) analysis to evaluate how underlying vital rates contribute to among-population variation in growth rate. For each population, we also estimated the probability of quasi-extinction (shrinking below a threshold size) and of a severe (90%) decline in population size, within the next 30 years. Estimates of stochastic growth rate indicated that 10 populations are declining, seven increasing and one population is approximately stable. SLTRE decomposition showed that low mean adult survival and growth characterized strongly declining populations, whereas high mean fecundity characterized strongly increasing populations. Stochastic growth rate increased with population size, mainly due to higher survival in larger populations, but was not related to genetic diversity. One third of the populations were predicted to go extinct and eight populations to undergo a 90% decrease in population size in the coming 30 years. Low survival in small populations most likely reflects a positive association between local environmental conditions and population size. Synthesis: The association between G. conopsea population size and viability was driven by variation in survival, and there was no sign that ongoing declines are due to genetic erosion. This suggests that large populations occur in favourable habitats that buffer effects of climatic variation. The results also illustrate that demographic metrics can be more informative than genetic metrics, regarding conservation priority.