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Status of Pollinators in North America
MAINTAINING WILD POLLINATORS
Wild pollinators are mobile organisms that often use many resources in noncontiguous habitats. Some butterflies visit suburban gardens for nectar but oviposit on the foliage of tree species in forest habitats. Many species of hummingbirds that breed in the United States and Canada overwinter in Mexico (Nabhan et al., 2004; Shepherd et al., 2003). Maintaining diverse communities of wild pollinators therefore requires an understanding of various habitat needs and of managing habitats and landscapes to provide necessary resources (Table 6-2). Populations of pollinators can be supported if habitats are managed to provide food, and areas for nesting, overwintering, and breeding (Dover, 1991; Erickson and West, 2003; Evelyn et al., 2004; Fenton, 1997; Schultz and Dlugosch, 1999; Scott, 1986). Because pollinators are mobile, the area over which they forage, disperse, and migrate must be considered in strategies to maintain populations. Adequate resources must be available within foraging and dispersal areas (Westrich, 1996) and along migratory routes (Nabhan et al., 2004).
Managing pollinator populations and communities requires planning and action locally, regionally, and across continents. Because of their ecological and economic significance and because they are in some respects better known than are many other wild pollinators, bees can serve as a paradigm group to illustrate how multiscale approaches can be implemented in habitat restoration, conservation, and management.
Resource Requirements for Bee Species
All native and introduced bee species, whether solitary or social, require the correct balance of water, floral hosts that offer sufficient pollen and nectar of the correct types (Roulston and Cane, 2000; Roulston et al., 2000), nest-building materials (leaves, resins, sap, gums, floral oils, essential oils, bark, plant trichomes, old mouse nests, snail shells, mud, sand, pebbles), and nesting substrates (O’Toole and Raw, 1991; Roubik, 1989; Shepherd et al., 2003) to survive as adults and rear their larval broods (Michener, 2000).
Michener (2000) provided a comprehensive review of floral resource requirements for bees. Bees obtain pollen and nectar from cultivated and wild plants. Pollen (usually moistened with nectar or floral oil) is used to feed larval bees, and nectar is used to fuel the flight of adults. Many solitary bees are active above ground as adults for only a few weeks or months. Oligolectic bees specialize on one or a few closely related species within a genus of flowering plants; polylectic bees collect pollen from an array of unrelated plants. Species with long flight seasons are usually polylectic and include the long-lived carpenter bees and euglossine orchid bees, those that produce multiple generations within a season, and highly social bees with annual or permanent colonies (honey bees, bumble bees, stingless bees).