Predicting heterosis and inbreeding depression from population size and density to inform management efforts.
Effective population size should be positively related to census size and density, and it is expected to influence the strength of genetic drift, inbreeding and response to selection, and thus the distribution of the genetic load across populations. We examined whether census population size and density predict 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 the occurrence of self-pollination with a pollen staining experiment. The majority of populations expressed strong inbreeding depression at the seed stage (mean δID: min-max = 0.26: -0.53 to 0.51), consistent with a mainly outcrossing mating system and substantial genetic load. The effect of between-population crosses varied from strong outbreeding depression to heterosis (mean δOD: min-max = 0.05: -0.22 to 0.92), indicating varying influence of drift and selection among populations. Census population size did not significantly predict the strength of inbreeding depression, heterosis or population mean fitness. However, inbreeding depression was positively and heterosis negatively correlated with population density. The proportion of self-massulae deposition was three times higher in sparse patches compared to dense ones (41% vs. 14%). Combined effects of density-dependent pollinator behaviour and limited seed dispersal may cause stronger genetic sub-structuring in sparse populations and reduce the strength of the correlation between census and effective population size. The results point to the importance of considering population density in addition to size when evaluating the distribution of recessive deleterious alleles across populations. Synthesis and applications: Management plans for threatened species often involve crosses between populations to restore genetic variation, a process termed genetic rescue. This study indicates that such conservation efforts should be more successful if designed on the basis of population density in addition to population size, because we found population density predicted both the strength of heterosis and inbreeding depression across populations of Gymnadenia conopsea.