Evolutionary Demography of Orchids: Intensity and Frequency of Pollination and the Cost of Fruiting

Abstract

Life history theory assumes that there is a trade—off between current reproduction and future growth or reproduction or both; therefore, natural selection is expected to result in the maximization of reproduction within the limits imposed by the trade—off. Accordingly, it has been predicted that fruit and seed production in perennial plants should be resource limited. Many orchid species seem to be pollinator limited, but this hypothesis has been recently challenged by experimental studies that show a cost of increased fruit set in some orchids. In this study, I combined the results of a 2—yr pollination experiment and a 3—yr demographic assessment of a population of the orchid Tolumnia variegata in a matrix model of population dynamics, and by means of simulations I evaluated the effect of pollination intensity and frequency, fruit production, and cost of fruiting on the asymptotic population growth rate. Mean natural fruit set in this population was <1%, whereas intermediate and high pollination intensity resulted in mean fruit set of 35 and 72%, respectively. In any given year, °98% of all flowering individuals fail to set fruit under natural conditions. Despite the dramatic increase in fruit set after hand—pollination, only plants in the high pollination intensity treatment showed a statistically significant reduction in future growth and flowering. The simulation showed that a small seedling production per fruit would be enough to overcome the cost of fruiting; therefore, plants in this population should experience strong selection for increased pollination. This conclusion does not seem to explain the widespread occurrence of low fruit set and high proportion of fruiting failure among nonautogamous orchids. The results of this study suggest an alternative explanation: pollinator limitation could be evolutionarily stable if the correlation between fruit or seed production and seedling recruitment is sufficiently low. If an increase in fruit or seed production does not translate into an increase in fitness, then selection for increased pollination would not occur or would be too weak. The evaluate this alternative, quantitative studies on the transition from seed to seedling in natural orchid populations are required.