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.