NECTARLESS HONEY MESQUITES

1. We present a case of intrapopulational dimorphism in nectar product ion (nectarless/nectar producers) in the honey mesquite, Prosopis glan dulosa var. torreyana (Leguminosae, Mimosoidae), and use this dimorphi sm as a natural experiment to explore the effects of nectar production on pollinator numbers and on pod production. In our study sites, roug hly half the honey mesquites produce no nectar. 2. Controlled pollinat ions indicate that there were no significant differences between morph s 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 in florescences, regardless of nectar presence. The total daily numbers o f the more abundant insect pollinators, Apis mellifera and Melissodes tristis, were the same when trees with and without nectar were aggrega ted, but were much higher in nectar producers when trees were in isola tion. The number of pollinator species was also higher in nectar-produ cing trees. However, although the number of pods per raceme was appare ntly higher in isolated, nectar-producing trees, the differences betwe en morphs could not be shown to be significant. 4. This result could b e due to high costs of nectar in the plants that produce it, resource limitation, and/or because flowers need only one or few pollinator vis its to set fruit. 5. We hypothesize that nectar dimorphism in the hone y mesquite is a case of automimicry.