Pollinators of tropical dioecious angiosperms

Dioecy is frequent in tropical forests. It has been suggested that small, unspecialized pollinators are among the factors responsible for gender separation in this habitat. The underlying assumption is that poor fliers and/or communal foragers frequently effect selfing which in turn, given sufficiently severe inbreeding depression, should favor the establishment of dioecy. At least 10% of the genera of the angiosperms includes dioecious species; in tropical flowering plants, however, pollinators are reliably known only in a few species. Whereas temperate dioecious species commonly are wind‐ or water‐pollinated, anemophily is less important in tropical forests, but occurs in at least 30 dioecious genera. Our survey of tropical dioecious zoophilous species in 29 genera (in 21 families) for which detailed pollination information is available shows that these species have specialized flowers adapted to specific pollinators rather than generalized flowers suitable for diverse insects. Known pollinators include solitary and eusocial bees, beetles, moths, flies, wasps (including fig wasps), and rarely bats and birds, and cover a wide range in animal size and locomotive capabilities. Floral rewards comprise pollen, nectar, stylar mucilage, nutritious tissues, brood‐places, and resins. About a third of the species offer no reward in the female morph, pollination by deceit apparently being common. Our data thus do not support the hypothesis that there is a broad correlation between a dioecious breeding system and unspecialized pollination, although such a correlation may be found in certain taxa. Specialized plant‐pollinator relationships seem as critical in dioecious plants in tropical forests, where individuals often grow far apart, as they are in tropical plants with other breeding systems.