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 interactions 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 other portions. We model three classes of heterogeneous transport conditions to assess their effects on pollen dispersal. Two classes occur when the pollen carried by individual pollinators becomes spatially structured. 'Horizontal' structure arises when areas of a pollinator's body differ in their frequency or intensity of contact with anthers and/or stigmas. Alternatively, 'vertical' structure develops when pollinators do not disturb their pollen load, causing pollen to accumulate on the pollinator's body in layers, so that much pollen is temporarily inaccessible to stigmas. The third class of heterogeneity occurs when the pollinators that visit a flower differ consistently with respect to pollen transfer from pollinator to stigmas. These classes of heterogeneous transport produce similar mean patterns of pollen dispersal, which differ from that expected if transported pollen acts as a homogeneous population. In particular, near and far recipient flowers receive proportionately more of the donor pollen reaching stigmas than predicted for a single population. Reanalysis of data in the literature illustrates that predictions based on heterogeneous transport conditions are more representative of observed dispersal patterns than predictions of the traditional characterization of pollen dispersal. We also demonstrate that stochastic variation between flowers in pollen donation and receipt does not affect the arithmetic mean 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 floral 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 heterogeneity governs transport dynamics, but not when pollen accumulates in layers. This relation is likely reflected in many aspects of floral divergence among related taxa pollinated by different vectors and convergent design among unrelated taxa served by similar vectors.
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