Implications of clonal structure for effective population size and genetic drift in a rare terrestrial orchid, Cremastra appendiculata

Effective population size (N-c) influences the degree to which random genetic drift changes allele frequencies, increases inbreeding, and decreases genetic diversity, and thus is a parameter of direct relevance to the conservation of a rare species. Few empirical data are available, however, concerning the effects of clonal structure on N-e in plant species reproducing both sexually and asexually. Using genetic markers and spatial autocorrelation analysis, we quantified the statistical significance and spatial scale of clonal spread of six populations of the rare terrestrial orchid Crematra appendiculata (D. Don) Makino in a large (180 ha), undisturbed landscape on Oenaro Island, located off of the southeastern coast of South Korea. We used this information to calculate three demographic estimators of N-e; the number of ramets within a population, the number of genets, and an estimator that incorporates information on both the number of genets and variation in the number of ramets per genet. Taking clonal structure into account results in up to a fivefold decrease in estimates of N-e relative to the ecologically apparent number of individuals within populations. Levels of standing genetic variation are in fact greater than expected given our estimates of N-e, leading us to consider historical factors resulting in N-e being greater in the past than in present-day populations. Like C. appendiculata, many terrestrial orchids occur in relatively small, spatially isolated populations and are of special concern for conservation. Our results indicate that efforts aimed at the long-term preservation of these species should be based on a sound understanding of the potential for clonal structure and its implications for the sensitivity of populations to losses of genetic diversity and fitness via random genetic drift.