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Status of Pollinators in North America
higher pollination fees that eventually would be seen in higher prices at the supermarket.
Over the long term, the indirect effects of honey bee shortages will depend on how markets and technology respond. If the pollination markets adjust to a new, higher price equilibrium, then higher costs could undermine the profitability of crop production in marginal areas. Increased food prices also are possible, although whether they occur would depend on competition from fruit, nut, and vegetable producers outside North America.
POLLINATORS IN NATURAL AREAS
Pollinator Limitation of Seed Production
As with agricultural systems, pollinator limitation in natural plant populations occurs through a variety of mechanisms that decrease pollinator abundance below that required by plants for full reproduction, and pollinator shortages are expected to depress fruit and seed set in local plants. Low seed set can be caused by other factors as well, including severe weather, inadequate concentrations of soil nutrients, disease, damage caused by herbivores, partial sterility, or shortages of neighboring plants with compatible pollen. Thus, in plant populations where reproduction is low or appears to be declining, explanations other than inadequate pollination should first be ruled out (Figure 4-4). It is difficult to study the efficacy of pollination services because it is usually impossible to augment native pollinator populations artificially in controlled experiments (Thompson, 2001). Therefore, many studies rely on indirect evidence, such as correlations between pollinator abundance and seed set or the effects of hand-pollination treatments on seed set (Box 4-2).
In the most extreme examples of pollinator limitation, seed set fails because a plant’s primary pollinators are extinct or in precipitous decline. The best known cases in the United States involve endemic Hawaiian plant species that depend on pollinating birds or moths that are now extinct on one or more islands (Nabhan and Buchmann, 1996); some of them are maintained only by hand pollination. Two species of native lobeliads, Brighamia insignis and B. rockii (Shepherd et al., 2003), are examples whose principal sphingid moth pollinator is believed to be extinct (Kearns et al., 1998). In other cases, broadly applied insecticides have killed some native pollinators, resulting in low seed production in two species of wild plant (Thomson et al., 1985).
A more subtle form of pollinator limitation occurs when a plant species loses its pollinators to competition with other plants that have overlapping flowering seasons and share the same habitat (Kephart, 2005). Pollinators often prefer one plant species over another because of differences in floral