Recent Plant Invasions in the Arid and Semi-Arid Southwest of the United States

Several plant communities in the Southwest have been drastically altered within a century by the rapid spread of a small number of woody species. The habitats principally affected have been the plateaus and plains at intermediate elevations, which formerly supported grassland and have now been invaded on a massive scale by mesquite and other native shrubs, and the stream courses, which have been extensively occupied by tamarisk, an alien species from Eurasia. Historical evidence and field observations suggest that these invasions have resulted primarily from occupation of the Southwest by American settlers. The development of commercial livestock ranching led to increased seed dispersal, overgrazing, and the suppression of grass fires, the combined effects of which favored the invasion of grassland by woody plants. Short-term climatic fluctuations towards greater aridity have tended to accentuate rather than to initiate the processes of invasion. The spread of tamarisk is owing mainly to changes in stream regime resulting from the creation of reservoirs. The history of recent plant invasions in the Southwest illustrates how man may unintentionally bring about rapid and profound ecological changes in dry areas by the introduction of new systems of land use.