Abstract

1. We present a case of intrapopulational dimorphism in nectar production (nectarless/nectar producers) in the honey mesquite, Prosopis glandulosa var. torreyana (Leguminosae, Mimosoidae), and use this dimorphism as a natural experiment to explore the effects of nectar production on pollinator numbers and on pod production. In our study sites, roughly half the honey mesquites produce no nectar. 2. Controlled pollinations indicate that there were no significant differences between morphs in pod production per raceme and compatibilities, and that fruit set is significantly higher in open, cross-pollinated inflorescences. 3. There is a high abortion rate in both naturally and hand-pollinated inflorescences, regardless of nectar presence. The total daily numbers of the more abundant insect pollinators, Apis mellifera and Melissodes tristis, were the same when trees with and without nectar were aggregated, but were much higher in nectar producers when trees were in isolation. The number of pollinator species was also higher in nectar-producing trees. However, although the number of pods per raceme was apparently higher in isolated, nectar-producing trees, the differences between morphs could not be shown to be significant. 4. This result could be due to high costs of nectar in the plants that produce it, resource limitation, and/or because flowers need only one or few pollinator visits to set fruit. 5. We hypothesize that nectar dimorphism in the honey mesquite is a case of automimicry.