Resource- and pollinator-mediated selection on floral traits

Abstract

Female reproductive success is predicted to be simultaneously limited by the availability of pollen and resources. Selection on floral traits results from both factors, but their relative importance and interaction is poorly understood. We increased nutrient and pollen availability of the orchid Dactylorhiza lapponica in a factorial experiment to quantify resource- and pollinator-mediated selection on floral traits. Hand-pollination increased female fitness (number of fruits × mean fruit mass) by 74% in both nutrient treatments, whereas nutrient addition did not significantly affect female fitness. There was selection for more flowers and longer spurs, and selection on spur length was significantly pollinator-mediated and of similar strength across nutrient treatments (Δβpoll= 0·54 and Δβpoll_NPK = 0·59). There was no statistically significant resource-mediated selection. Nutrient addition increased flower size the following year, but did not affect flower or fruit production, or selection on any trait. The results demonstrate that D. lapponica does not increase flower production in response to nutrient addition, that the increase in female fitness in response to hand-pollination is not resource limited, and suggest that natural resource variation does not influence selection on floral traits. The study illustrates that crossed manipulations of pollen and resources can clarify their relative importance for selection on floral traits. A Lay Summary is available for this article.