Plants exhibit a diversity of reproductive mechanisms. Many reproduce vegeta-tively, but this mechanism is limited largely to local reproduction. Genetic heterozygosity and colonization ability are increased by outcrossing. Although many plant species are capable of self-fertilization, a large percentage (a vast majority in some ecosystems) are self-incompatible, and many are dioecious (e.g., 20-30% of tropical tree species), with male and female floral structures separated among individual plants to preclude inbreeding (Bawa 1990, Momose et al. 1998a). Mechanisms for transporting pollen between individuals becomes increasingly critical for reproduction with increasing separation of male and female structures and increasing isolation of individual plants (Ghazoul and McLeish 2001, Regal 1982, Steffen-Dewenter and Tscharntke 1999).
Several mechanisms move pollen among flowering individuals. Pollen can be transferred between plants through abiotic and biotic mechanisms (Regal 1982). Pollen is transported abiotically by wind. Biotic transport involves insects (Fig. 13.1), birds, and bats. Insects are the major pollinators for a vast majority of plant species in the tropics (Bawa 1990), but the proportion of wind-pollinated plants increases toward the poles, reaching 80-100% at northernmost latitudes (Regal 1982). These mechanisms provide varying degrees of fertilization efficiency, depending on ecosystem conditions.
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