Experimental demonstration of an Allee effect in American ginseng

Harvesting of wild American ginseng (Panax quinquefolius) for the herbal tr ade has lowered natural population sizes. We tested for reproductive limita tion due to small population size (a form of the Allee effect) by experimen tally planting "natural" populations numbering 4, 16, and 64 using 4-year-o ld cultivated plants. Plant size traits and reproductive traits (bud, flowe r, green fruit, and mature fruit) were recorded through the ensuing summer. Fruit production per flower and per plant increased in proportion to flowe ring population size (p = 0.0063 and p = 0.0017, respectively), strongly su ggesting that an Allee effect occurs in very small populations. The increas e in fruit production was not explained by either plant or inflorescence si ze differences. Although population size-dependent pollination, through ins ufficient pollinator visitation rate or pollen transfer rate, seems the mos t likely cause of the observed effects, our limited observations of pollina tors were not sufficient to demonstrate a change in pollination rates as a function of population size. Knowledge of the presence as well as the mecha nism underlying this Allee effect may be especially useful for management a nd determination of minimum viable population size of the species in the wi ld.