Effect of nectar-robbing birds on fruit set of Fuchsia magellanica in Tierra Del Fuego: A disrupted mutualism

Abstract

1. This paper investigates spatial habitat variation in flower visitation by avian pollinators (mainly Hummingbirds) and nectar robbers to Fuchsia magellanica at the southern limit of its range, Tierra del Fuego, and their effects on plant reproductive success. 2. The plant-pollination mutualism appears to be disrupted as Hummingbirds (Sephanoides galeritus) are rare at this high latitude and the frequencies of a nectarrobbing bird (Phrygilus patagonicus) are high, especially in open areas where F. magellanica is most abundant. 3. Nectar robbery by P patagonicus reduced fruit set about 20%, mainly because this fringillid can damage the ovary when piercing a flower. This bird was more abundant in open areas than inside the forest and, as expected, flower piercing was also more frequent in the former habitat. 4. Spatial variation was also found in the frequency of flower visitation by Hummingbirds; these birds foraged chiefly in the forest presumably because of the greater probability of finding intact (non-robbed) flowers there. This suggests that cross-pollinations in this self-compatible plant are more likely to occur in the forest. Nectar robbers therefore may influence not only the quantity of seeds produced but also, indirectly, their quality. 5. Fuchsia magellanica is polymorphic in flower colour. Robbery was significantly lower in the pale than in the normal (red) morph, although fruit set was similar between morphs.