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Predicting Invasions of Nonindigenous Plants and Plant Pests
tions have only recently become established, there was no association between abundances of small mammalian predators and gypsy moth dynamics (Grusheky et al. 1998).
The composition and structure of North American forests have always been in flux as they are continuously affected by biotic and abiotic agents. Human activities associated with settlement over the last 400 years have been some of the most pervasive forces in shaping the forests that exist today (Franklin et al. 1987, Merrill 1996). Among those activities have been logging and agricultural practices and the fires that were used to aid hunting and the preparation of land for farming. As the primeval forests were felled, their diversity, composition, and complexity were greatly altered. The forests that have regenerated represent the cumulative effects of the various disturbance agents and are the forests into which most invasive organisms have been introduced (Cronon 1983).
Insects and pathogens historically were viewed as two of the most important damaging agents with respect to forests (Hepting and Jemison 1958). Attempts to eradicate or suppress their outbreaks were initiated when pests produced effects that were in conflict with wood and fiber production, destroyed wildlife habitat, or interfered with natural resources that have been aesthetically valued by humans. More contemporary views have recognized insects and pathogens as normal forest ecosystem components. Their roles as recyclers of carbon and other nutrients, as pollinators and plant symbionts, as food sources for vertebrates, invertebrates, and other microorganisms, and as creators of habitat for wildlife are well summarized (Haack and Byler 1993, Gilbert and Hubbell 1996). In contrast, introduced insects and pathogens are not normal components of the ecosystems that they have come to influence. The general view is that invasive insects and microorganisms are not regulated by the co-evolved resistance mechanisms in their hosts or by the parasites, predators, and diseases that regulate them in their native ecosystems (von Broembsen 1989). All too often, the impact of an invading species has been viewed narrowly as causing extensive mortality and growth loss in the affected species. In reality, the ecological changes that have resulted from their damage typically set off a cascading chain of events that has resulted in rapid ecosystem changes.
A brief examination is warranted of how chestnut blight and gypsy moths have created cascading events to alter forest organization rapidly. Chestnut blight resulted in the most profound set of changes ever recorded in a North American forest ecosystem. The causal fungus, Cryphonectria parasitica is native to East Asia; after its early-1900s discovery in North America, it proceeded to infect and kill American chestnut trees, which once made up 25% of the eastern hardwood forest (Liebhold et al. 1995). As the chestnuts died, the newly available space was occupied by middle-story and understory species, including oak. Fire disturbances before the blight had given oaks an early advantage over light-seeded, less-fire-tolerant, thinner-barked species such as maple and yellow poplar (Stephenson 1986, Oak 1998). As the forests with emergent oaks have