People routinely move species, such as crops and ornamental plants, across natural barriers such as mountain ranges or oceans that would otherwise limit their spread. These plants may carry with them small, unrecognized infestations of pest insects. In some cases the plants themselves may spread and become invasive. Both invasive plants and insects often escape their specialized natural enemies when they cross geographic barriers and establish in new locations. Local natural enemies, unfavorable habitats or climates, or, for insects only, lack of a susceptible local plant, prevent many invaders from establishing or reaching high densities. However, for some invasive species, local climates and hosts are favorable and local natural enemies are gen-eralists that have limited impact. Those species increase to densities from 10 to 10 000 times greater than their numbers in the native range. Such high-density invasive species cause great economic and ecological damage and are the targets of classical biological control. By introducing more effective natural enemies, classical biological control seeks to lower the invader's density, which then allows the invaded community to return fully or partially back to its preinvasion condition. Two examples follow that illustrate the process and how such projects produce economic and ecological benefits.
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