Theoretical consequences of heterogeneous transport conditions for pollen dispersal by animals

Pollination governs a plant's mating options by establishing the diversity and intensity of male-gamete exchange between plants. For animal-pollinated plants, the pattern of pollen dispersal arises from the pollinator's inter actions with floral organs during each flower visit and with the pollen it carries during transport. These interactions typically cause some portions of a flower's pollen to experience different transport conditions than othe r portions. We model three classes of heterogeneous transport conditions to assess thei r effects on pollen dispersal. Two classes occur when the pollen carried by individual pollinators becomes spatially structured. "Horizontal" structur e arises when areas of a pollinator's body differ in their frequency or int ensity of contact with anthers and/or stigmas. Alternatively, "vertical" st ructure develops when pollinators do not disturb their pollen load, causing pollen to accumulate on the pollinator's body in layers, so that much poll en is temporarily inaccessible to stigmas. The third class of heterogeneity occurs when the pollinators that visit a flower differ consistently with r espect to pollen transfer from pollinator to stigmas. These classes of hete rogeneous transport produce similar mean patterns of pollen dispersal, whic h differ from that expected if transported pollen acts as a homogeneous pop ulation. In particular, near and far recipient flowers receive proportionat ely more of the donor pollen reaching stigmas than predicted for a single p opulation. Reanalysis of data in the literature illustrates that prediction s based on heterogeneous transport conditions are more representative of ob served dispersal patterns than predictions of the traditional characterizat ion of pollen dispersal. We also demonstrate that stochastic variation betw een flowers in pollen donation and receipt does not affect the arithmetic m ean pattern of pollen dispersal, although it does reduce the geometric mean . Although the different types of heterogeneous transport conditions produce similar mean patterns of pollen dispersal, the influences of specific flora l and pollinator characteristics on dispersal can depend on the type(s) of heterogeneity involved. In particular, pistil characteristics should affect dispersal independently of stamen characteristics when horizontal heteroge neity governs transport dynamics, but not when pollen accumulates in layers . This relation is likely reflected in many aspects of floral divergence am ong related taxa pollinated by different vectors and convergent design amon g unrelated taxa served by similar vectors.