Ambophily, not entomophily: the reproduction of the perennial Discaria chacaye (Rhamnaceae: Colletieae) along a rainfall gradient in Patagonia, Argentina

Abstract

Geographical gradients are useful tools for studying how changing environmental conditions affect communities’ structure and function, and any perceived general pattern can be tested by examining the response of different populations of a given species widely distributed along the focal gradient. We studied populations of the perennial, putatively entomophilous plant Discaria chacaye located along a strong east–west rainfall gradient in the north-west of Patagonia, Argentina, where a significant community-level eastwards replacement of Diptera by Hymenoptera as main flower visitors takes place. Discaria chacaye showed an invariant set of traits (flowers rewarding with nectar and pollen a fly-dominated pollinator assemblage, combined with protandry, incomplete dichogamy and self-incompatibility) which were modulated by site-dependent factors (availability of pollinators, wind strength, and perhaps abundance of seed predators). The interplay between all factors resulted in an increase in fruit set from the wet to the dry end of the gradient. Unexpectedly, wind was found to be an important pollen transporter, while (a) the plant’s use of pollinator groups was mostly uncorrelated with their local abundances along the gradient, and (b) both pre-and postzygotic fecundity losses occurred, mostly attributable to geitonogamy. The finding of ambophily in D. chacaye suggests that this pollination strategy could be more frequent than previously suspected along the studied gradient.