animal pollination improves production in 75 percent of the crops studied. Most cultivars of another 10 percent of the crops require animal pollination. Another 8.5 percent of the crops do not benefit from animal pollination and its role in production of the remaining 6.5 percent crops is not known. Many crops, however—notably the staple grains that form the foundation of most human diets (rice, wheat, maize, sorghums, millets, rye, barley)—are self-pollinating or pollinated by the wind. Together, species that do not rely on pollinators account for most of the world’s food supply by weight (FAO, 2005).
Pollinator declines, therefore, do not fundamentally threaten the world’s caloric supplies. However, fruits and vegetables, which add diversity to the human diet and provide essential nutrients, tend to depend heavily on pollinators (Prescott-Allen and Prescott-Allen, 1990; Roubik, 1995). Seven of the nine crops that provide at least 50 percent of the vitamin C available to the human diet globally depend partially or entirely on animal pollination for the production of fruits or seeds (oranges, cabbages, green peppers, tomatoes, melons, tangerines, watermelon) (FAO, 2005; Free, 1993; McGregor, 1976; USDA-NASS, 2006b).
Animal-pollinated crops also tend to have greater economic value than do crops that are not animal-pollinated, and they provide relatively more income to farmers per unit of land (USDA-NASS, 2006b). Moreover, several oilseed crops depend on pollinators, and bee pollination is required to produce the seeds of major forage and hay crops, such as alfalfa and clover, that feed the animals that supply meat and dairy products. Thus, despite the fact that they do not provide the bulk of the human diet, animal-pollinated plants contribute importantly to the quality of the human diet. Although estimates of the proportion of the human diet that is attributable to animal pollination are occasionally attempted and frequently cited (for example, McGregor’s 1976 estimate that one-third of the human diet can be traced directly or indirectly to animal pollination), the proportion likely varies among countries and regions and depends on dietary preferences, seasonal availability, cultural practices, and economic status of consumers.
Plant species grown as agricultural commodities display a wide variety of breeding systems. Some readily self-pollinate; others are dioecious, monoecious, or self-incompatible and require animal pollination (Free, 1993; McGregor, 1976; Table 4-1). Dependence on pollinators also varies greatly among crops. In the United States, about 130 agricultural crops benefit from insect pollination (McGregor 1976; see examples in Table 4-1). Most North American crops can produce some fruits and seeds without animal pollination, but pollination often increases their value through higher yields or im-