Pollination by insects is more prevalent in some types of ecosystems than in others. Pollination by animals is more common in angiosperm-dominated eco systems than in gymnosperm-dominated ecosystems, but pollination by wind is energetically efficient for dominant species in grasslands and temperate forests.
The regularity with which conspecific plants occur in close proximity to each other largely determines their pollination mechanism. Long-lived species that dominate relatively simple ecosystems (i.e., grasslands and temperate forests) are pollinated primarily by wind. These plant species do not require efficient pollination or frequent reproduction to ensure population survival. Energetically inexpensive transport of pollen by wind provides sufficient pollination (and successful reproduction) so that energy need not be diverted to production of expensive nectar rewards and floral displays to advertise availability.
Directed transport of pollen by animals is critical to reproduction of plant species that are short-lived, are sparsely distributed, or occur in habitats with restricted airflow (Appanah 1990, Moldenke 1979, Regal 1982, Somanathan et al. 2004). In contrast to long-lived plants, short-lived plants have limited opportunities for future reproduction and, therefore, tend to depend on more efficient pollination to ensure seed production. Sparsely distributed plants and plants in areas of limited airflow cannot rely on inefficient transport of pollen by wind between distant or inaccessible individuals. Such species include early successional plants dominating ephemeral communities, widely spaced plants in harsh environments (e.g., deserts), scattered forbs in grasslands, subdominant trees, shrubs and herbs in temperate forests, and all (or most) plant species in tropical forests (S. Johnson et al. 2004, Momose et al. 1998b, Regal 1982). Regal (1982) reported that fewer than 6% of desert shrub species are wind pollinated. All of the 270 plant species in a lowland diperocarp forest in Sarawak, Malaysia, were animal pollinated, 90% by insects (Momose et al. 1998b).
Insects and other animal pollinators can transport pollen over considerable distances. Kohn and Casper (1992) documented gene flow among bee-pollinated buffalo gourds, Cucurbita foetidissima, over distances up to 0.7 km in New Mexico, United States. Somanathan et al. (2004) reported that carpenter bees, Xylocopa tenuiscapa, pollinated a Neotropical tree, Heterophragma quadriloculare, isolated from pollen sources by as much as 330 m, permitting reproduction by spatially isolated trees. G. White et al. (2002) identified sources of pollen reaching isolated Swietenia humilis trees and forest fragments in Honduras. A substantial proportion of pollen (25%) was transported over distances of >1.5 km, to more than 4.5 km between fragments. By contrast, a Neotropical shrub, Lasiosiphon eriocephalus, pollinated by a weakly flying nitidulid beetle, may be particularly vulnerable to isolation or fragmentation (Somanathan et al. 2004).
Roubik (1989) reviewed studies that distinguished seasonal patterns of pollinator activity. Primary pollinators usually were most active during periods of peak flowering. Heithaus (1979) reported that megachilid and anthophorid bees were most active during the dry season in Costa Rica, halictid bees during both wet and dry seasons, and andrenid and colletid bees during the wet season or during both seasons. Social pollinators (e.g., apid bees) require a sequence of floral resources throughout the year to support long-lived colonies and visit a succession of flowering plant species, whereas more ephemeral, solitary species with short life spans can be relatively more specialized on seasonal floral resources (S. Corbet 1997, Roubik 1989).
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