Bird Pollination and the Mating System of Eucalyptus Stoatei.

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Abstract
Eucalyptus stoatei C. A. Gardn., a small tree endemic in south-western Australia, is unusual in the genus in being predominantly if not exclusively pollinated by birds. Many individual honeyeaters but no insects were recorded feeding on its nectar during the 1980 flowering season. The large pendulous flowers deny entry to most large insect visitors because the stamens form an impenetrable dome over the floral cup. Access to nectar is only possible through a narrow opening lined with anthers in the centre of the staminal dome. Honeyeaters visited few flowers per tree (average 1-5) and about 18% of the movements of individual birds from one flower to another were between flowers on different trees. Most intertree movements were between trees farther apart than nearest-neighbours. Hence a high rate of outcross pollination is inferred. Analysis of the mating system of E. stoatei using allozyme seed markers in one population indicated an average level of outcrossing of 82%, among the highest so far recorded for eucalypts. The study therefore supports the hypothesis that bird pollinators may effect high levels of outcrossing, although other factors such as protandry and incompatibility probably are also involved. Adaptations favouring exclusive bird pollination in E. stoatei may have evolved to promote outcrossing and heterozygosity in the face of inbreeding depression imposed by a dissected population structure. and in response to the relatively low number of seeds per plant produced by the species.