Predicting heterosis and inbreeding depression from population size and density to inform management efforts

1. Effective population size should be positively related to census size and density, and is expected to influence the strength of genetic drift, inbreeding and response to selection, and thus the distribution of the genetic load across populations.
2. We examined if census population size and density predicts the strength of inbreeding depression, heterosis and population mean fitness at the seed stage in the terrestrial orchid Gymnadenia conopsea by conducting controlled crosses (self, outcross within and between populations) in 20 populations of varying size (7-30,000 individuals) and density (1-12.8 individuals/m2). In the largest population, we also examined how local density affects occurrence of self-pollination with a pollen staining experiment.
3. The majority of populations expressed strong inbreeding depression at the seed stage (mean δID: min-max = 0.26: -0.53–0.51), consistent with a mainly outcrossing mating system and substantial genetic load. The effect of between-popula...