The fragility of extreme specialization: Passiflora mixta and its pollinating hummingbird Ensifera ensifera

Abstract

Some of the most spectacular examples of coevolution between flowers and their pollinators are reflected in their morphologies. Tongue length of insects and bill lengths of nectar-feeding birds are some of the most significant characters in pollination studies (Kearns & Inouye 1993). As Darwin noted as early as 1862, the evolution of deep floral tubes or spurs and long tongues of flower visitors can be explained by runaway coevolution (Nilsson 1988). However, such a process has only been shown to be likely in a few cases, e.g. between Malagasy orchids and hawkmoths (Nilsson et al. 1985). The pollinator of the Malagasy orchid Angraecum sesquipedale, with a 30 cm nectar spur, is a sphingid moth with a tongue of a similar size (Darwin 1862, Nilsson 1998). However, most studies of such extreme pollination specialization also report that the interaction is asymmetrical, i.e. the pollinators interact with a guild of plants, whereas the plant often depends on only a few pollinators (Johnson & Steiner 1995).