The ability of an alien species to overcome various bar riers in the new environment is affected, positively or negatively, by the presence of other species, native or alien, already resident in the area. Such interactions may counter or even override any inherent biotic resistance. Simberloff and Von Holle introduced the term 'invasional meltdown' to describe synergistic interactions among invaders that accelerate invasions and/or amplify their effects on native communities. Soil biotas have potentially facilitative effects for invading plants. Some plants were reported to switch from negative plant-soil community feedback in native ranges to positive plant-soil commu nity feedback in the invasive ranges.
Many invasive plant species qualify as ecosystem engi neers, that is, they affect resource availability, directly or indirectly, by altering abiotic or biotic features of an ecosystem. The best example of this is the 'grass fire cycle' in which invasive alien grasses change the distribu tion and abundance of fine fuels, resulting in more frequent fires (and in some cases introducing regular fires to non fire prone ecosystems). This profound altera tion of ecosystem functioning, which often favors further invasion of fire tolerant alien species, has had radical effects on biodiversity in many semiarid systems.
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