Habitat fragmentation can have multiple negative effects on the long-term viability of plant species. Smaller populations that result from fragmentation are predicted to lead to increases in the potential for inbreeding relative to larger populations in predominantly outcrossing species. Because the degree to which the Darwinian fitness of individuals is influenced by inbreeding varies among species, the effect of increased selfing or biparental inbreeding rates on long term population viability depends on the species and the environmental context. As tallgrass prairie has been converted to cropland a species of fringed orchid (Platanthera praeclara) has suffered a dramatic reduction in numbers and an increase in isolation of small populations. The objectives of our study were to 1) examine the likelihood of self pollination in this species and 20 measure the degree of inbreeding depression associated with self-pollination. We conducted a series of hand pollination experiments, measured fruit and seed characters of natural populations of plants and observed pollinator behavior.
By comparing the seed quality from outcross versus self pollinations in plants from two field populations under field conditions we determined that self pollination reduces seed quality by a factor of 1.7 while having no detectable effect on fruit set. In addition, we found that native hawkmoth pollinators commonly visit multiple flowers in an inflorescence and thus increase the probability of geitenogamy (i.e. the transfer of pollen between flowers within a single plant). Long term field censuses indicate that fruit set is unusually low in this species and that mature fruits may well contain inbred seeds. These results suggest that small fragmented populations are at risk from multiple obstacles to successful sexual reproduction and recruitment.