Consequences of pollination by chance in the pink ladys̕-slipper, Cypripedium acaule

Pollination is potentially a critical step in the reproductive cycle of angiosperms. Pollinators may act as a mechanism for selection in pollen or pollinator limited plants. The connection between life history, pollination syndrome and long-term fitness was studied in the pink lady's-slipper, Cvoripedium acaule. This orchid species has several traits which make it an interesting subject for such a study. These include: a specialist pollinator; no free nectar or other apparent reward; low pollinator visitation; low fruit set; and clonal growth.

Three types of data were collected. A single population of approximately 2000 individuals was monitored from 1982 to 1985. Recruitment, mortality, individual size, flowering and fruit set was recorded each year. Regional variation in fruit set was assessed by sampling populations in Tennessee, North Carolina, South Carolina and Georgia in 1984 and 1985. Finally, the relative intensities of pollinator limitation and resource limitation and their impact on flowering and fruit set were studied by experimentally manipulating populations in the field. Hand pollinations were performed in 1984 and 1985. Defoliation experiments were conducted in 1984 and nectar additions were performed in 1985.

Only a small fraction of either the mature ramets or mature genets flower in a given year. The probability that a ramet will produce a flower increases with ramet leaf area. Pollinator visitation which results in pollinia removal is low, generally including about 20% of the flowers produced. Fruit set is also low, occurring in about 10% of the flowers produced, and is variable from year to year. Reproduction is limited by pollinators, resources and nectar, but pollinator limitation has the greatest long-term influence on reproductive success.

A game-theoretic model of the effect of nectar on pollinator attraction and fruit set was developed. This model was used to examine how different pollinator foraging schemes may influence the reward offered by plants. In particular, the model predicts that under some foraging schemes, the plant species should produce no reward.