lation, community and ecosystem levels. Long-term evolutionary and community effects are the least well studied and perhaps the least predictable. Furthermore, there are few widely accepted measures of the impact of invasive species.

  • Classification of organisms by functional groups might help in predicting impacts. Among invaders, grasses that produce massive amounts of combustible fuel, plants that form mutualisms with nitrogen-fixing bacteria, and trees that produce dense, light-diminishing shade can have huge impacts in ecosystems in which these functional groups had been absent or their effects have been negligible.

  • Invasive species often exert their influence by initiating a cascade of changes in the biotic and abiotic components of the ecosystem; examples are chestnut blight and gypsy moths in eastern North America.

  • Some plant invasions, such as in the Amazon watershed, are becoming so extensive that they are probably affecting global atmospheric circulation and the global carbon budget.

  • Few studies have been conducted on large temporal and spatial scales. Such studies could incorporate background variations in invasion-associated changes that are not captured by small-scale studies.

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