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, changing ecosystem processes “changes the rules of the game” in a way that influences many, if not all, of the component species. Plant invasions can also alter nutrient-cycling patterns, as illustrated by the invasion of the nitrogen-fixing tree Myrica faya on volcanic surfaces in Hawaii (Vitousek and Walker 1989). The invasion of American rangelands by Bromus tectorum (cheatgrass) has increased the frequency and intensity of fires, thereby transforming steppe once dominated by the shrub Artemisia tridentata (big sagebrush) to annual grasslands (Whisenant 1990). Similarly, the invasion of nonindigenous annual grasses into Californian chaparral has resulted in more-frequent and more-intense fires, which in turn have altered species composition (Zedler et al. 1983). Plant invasions can also alter hydrology, as illustrated by Melaleuca (Melaleuca quinquenervia), which increases soil elevations and thereby has influenced the hydrology of Florida wetlands (Schmitz et al. 1997), and by the invasion of Pinus spp. into the South African fynbos, which has radically reduced the water yield of catchments (Le Maitre et al. 1995). A recent review (Parker et al. 1999) indicates that most studies of the impacts of invaders on ecosystem processes have concentrated on the effects of the plants—through uptake of light, nutrients, or water—on other plant species. Native animals are also affected by plant invaders (Braithwaite et al. 1989), through loss of habitat and loss of food resources; these interactions have been little studied and might well be underestimated.
An invader would have substantial social or economic effects if it altered “ecosystem services” (cf. Ehrlich and Mooney 1983), such as maintaining the gaseous composition of the atmosphere, controlling regional climates, generating and maintaining soils, controlling floods, disposing of wastes, recycling nutrients, and controlling pests (Ehrlich and Wilson 1991). A potentially global change is under way through the conversion of much of the forested Amazon drainage to grasslands. Huge swaths of tropical forest continue to be cleared, burned, and sown with nonindigenous grasses. These grasses, such as Melinis minutiflora and Brachiaria spp., which were introduced primarily from Africa, are forming a variety of new communities: some appear to require continual