The effect of population structure, plant size, herbivory and reproductive potential on effective population size in the temperate epiphytic orchid, Sarcochilus australis

Distribution of plant size and reproductive success is investigated in the temperate epiphytic orchid Sarcochilus australis (Lindl.) Rchb. f. at Kinglake National Park, Victoria, in south-eastern Australia, and applied to estimating the effective population size. Plant size distribution (leaf number, length of longest leaf and number of flowers) was not normally distributed. Most individuals were vegetative and it is estimated that more than half of all individuals are too small to flower, however exceptionally large individuals even though rare are able to have more than one active inflorescence. Flowering probability is plant size dependent and follows a sigmoid curve. The minimum observed leaf size of a flowering individual was 26 mm, however these small individuals have a low probability of flowering ( 80 mm) have a much higher probability of flowering (90%). The effective population size (Ne) of the Kinglake population of Sarcochilus australis was estimated from the distribution of flower production, and shown to be small (Ne = 10–19%) and comparatively similar to some of the other published estimates of effective populations size in orchids. From this basic survey of size distribution in Sarcochilus australis it is predicted that genetic diversity is low