Increasing density enhances pollination and fecundity in the rewarding orchid Gymnadenia conopsea

Abstract

Conspecific density is an important factor in determining the fecundity of pollinator-dependent plants. However, the outcome of increased conspecific density can range from reduced per capita pollination rates and fecundity (competition) to increased per capita pollination rates and fecundity (facilitation). I tested whether local conspecific density influenced pollination and fecundity in the rewarding orchid, Gymnadenia conopsea. I measured the distance to each of the nearest five plants in sixty patches of varying density in a large population in the Italian Apennine mountains. I examined whether pollen removal, pollen deposition, fruit, and seed set varied according to mean distance from conspecifics and plant height. After controlling for plant position in the population, the proportion of pollen removal, pollen deposition, and fruit set decreased with increasing distance from conspecifics, while pollen removal and deposition, but not fruit set, increased with increasing plant height. The proportion of viable seeds per fruit did not vary according to conspecific distance. Decreases in pollen removal, pollen deposition, and fruit set occurred within an average of 1 m distance from conspecific plants, indicating that local aggregation is an important determinant of both pollination success and fecundity. These results suggest that despite being a geographically widespread orchid, G. conopsea may suffer reduced fecundity when it occurs in small, isolated populations. Such field investigations can potentially lead to better predictions for successful re-establishment of populations of orchids under conservation-aided restoration efforts.