A number of exotic plants species have been introduced to the savannas around the world. Amongst the woody plant species are serious environmental weeds such as Prickly Acacia ("Acacia nilotica"), Rubbervine ("Cryptostegia grandiflora"), Mesquite ("Prosopis" spp.), Lantana ("Lantana camara" and "L. montevidensis") and Prickly Pear ("Opuntia" spp.) A range of herbaceous species have also been introduced to these woodlands, either deliberately or accidentally including Rhodes grass and other "Chloris" species, Buffel grass ("Cenchrus ciliaris"), Giant rat's tail grass ("Sporobolus pyramidalis") parthenium ("Parthenium hysterophorus") and stylos ("Stylosanthes" spp.) and other legumes. These introductions have the potential to significantly alter the structure and composition of savannas worldwide, and have already done so in many areas through a number of processes including altering the fire regime, increasing grazing pressure, competing with native vegetation and occupying previously vacant ecological niches. Other plant species include: white sage, spotted cactus, cotton seed, rosemary. Human induced climate change resulting from the greenhouse effect may result in an alteration of the structure and function of savannas. Some authors have suggested that savannas and grasslands may become even more susceptible to woody plant encroachment as a result of greenhouse induced climate change. However, a recent case described a savanna increasing its range at the expense of forest in response to climate variation, and potential exists for similar rapid, dramatic shifts in vegetation distribution as a result of global climate change, particularly at ecotones such as savannas so often represent. Savanna ecoregions are of several different types: