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ensure accuracy. Predicting Invasions of Nonindigenous Plants
and Plant Pests in their floras (Vitousek et al. 1996).
Deliberately introduced species can play a role in the
maintenance and management of ecosystem processes. Examples of
such species are natural enemies of pests for biological
control; aesthetically pleasing, fast-growing, pollution-
resistant horticultural plants; fish communities in reservoirs;
and grasses that can reclaim strip-mined land in arid regions.
The danger arises from nonindigenous species that either play no
constructive role or play unexpected roles in their new ranges.
Invaders that affect ecosystem processes—such as productivity,
nutrient cycling, or disturbance regimes—have been viewed as the
most difficult to quantify and verify (Vitousek and Walker 1989,
Mack and D’Antonio 1998). In a sense, chan