27 - Plant invasions in the rangelands of the isoclimatic mediterranean zone

Summary

This chapter is concerned with plant invasions in the rangelands of the isoclimatic mediterranean zone of the Mediterranean Basin, California and Chile. Australian and South African examples will not be included, although the general points to be made may apply equally well to these other two regions of mediterranean climate.

Plant invasions in rangelands usually result from disturbances of various kinds, either natural or human-induced. The former include erosion, sedimentation, flooding, wildfires, climatic variation and outbreaks of insects and/or rodents. Anthropogenic disturbances include deforestation, drainage, overstocking, human-induced fires, water and grazing management and changes in livestock husbandry. Rangelands themselves are often the result of either natural or human-induced processes. More often, however, they arise from a mixture of both kinds of process, such as deforestation and wildfires, which jointly influence vegetation dynamics and which in turn may favour the formation of grazing disclimaxes. Most of the grasslands of the world are the result of wildfires as, for instance, in tropical savannas, mediterranean shrublands, most of the American prairies and perhaps also in the origin of large tracts of the Asian steppes. In such cases the suppression of wildfires has often led to bush encroachment, as in the expanding mesquite bushlands in the rangelands of the southern United States and of various other shrubs in tropical savannas. On the other hand, the cessation of grazing in mediterranean shrublands usually leads to fuel accumulation and increased fire hazard (Le Houérou, 1974, 1977a, 1981, 1987), which may also have undesirable consequences.