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Wildlife plays a role in natural and agricultural ecosystems through reduction of injurious insect pests and disease vectors. In some forested ecosystems, birds, together with parasitoids and other insect predators, are effective predators of insect pests (Crawford and Jennings, 1989; see Price et al., 2000). Insects, ants, and spiders also play key roles in reducing populations of pest insects. For example, when ants were removed from maize plots in Nicaragua, two pest species increased in abundance, and damage to the maize increased (Perfecto, 1991). The annual economic value of wildlife control of insect pests has been estimated to be hundreds of billions of U.S. dollars (de Groot, 1992; Pimentel et al., 1992, 1993; Pimentel, 1997). This includes destruction of pests that are injurious to crops (estimated at US$90 billion year in the United States alone), as well as those affecting forests.
Climate change could impact many of these systems by decoupling predators from their prey (Root and Schneider, 1995). Studies in North America project reductions in the extent of distribution size of some of the species that feed on pests in forest, grassland, and agricultural ecosystems (see Price, 1995; Price et al., 2000). This could lead to an increasing need to use pesticides, with accompanying health risks (human and wildlife) and economic costs (Kirk et al., 1996; Colburn et al., 1997; Herremans, 1998).
Pollination is crucial to the reproduction of many plants, thus to maintenance of functioning ecosystems and biodiversity. Many factors influence the occurrence and density of insects (e.g., habitat conversion, excessive pesticide use). The principal mode of pollination of many plant species is by insects. Worldwide, an estimated 400 crop species are pollinated by bees and more than 30 other animal genera; possible crop loss in some species would be more than 90% in the absence of bees (Southwick, 1992; Buchmann and Nabhan, 1996). More than 100,000 different animals around the world pollinate 250,000 types of plants (Buchmann and Nabhan, 1996). The estimated annual value of wildlife pollination to commercial crops and pasture grasses is tens of billions of U.S. dollars (de Groot, 1992; Pimentel et al., 1992, 1995). If pollination of noncommercial plants were added, the figure would be significantly higher.
Seeds of some plants are dispersed by one or a few animal species. In Costa Rican tropical deciduous forests, as many as 60% of plants have their seeds dispersed by birds; the numbers are 75% in subtropical evergreen forests and 80% in montane evergreen forests (Bawa, 1995). On Samoa, the vast majority of seed dispersal in the dry season is mediated by flying foxes (Cox et al., 1991). Plants commonly disperse via seeds passing through the digestive tracts of animals or with animals that cache seeds (Robinson and Handel, 1993; Lanner, 1996). Consequently, if the ranges of the appropriate animals become disjunct from even part of the ranges of specific plants, dispersal of the plants may suffer (see Price et al., 2000, for other examples).
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