The introduction of animal taxa to areas where they do not naturally occur has the potential to damage severely the native fauna and flora. Introductions, both accidental and intentional, to Australia, New Zealand, Marion Island and other oceanic islands provide spectacular examples of this. Non-native mammalian herbivores often become invasive in the absence of their natural predators and their impact on vegetation, which may include alterations to plant species composition, structure and diversity, is exaggerated, especially if the vegetation has evolved in the absence of similar herbivores. This influence is not limited to the direct consequence for the vegetation and there may be a cascade effect on ecosystem functioning through, for example, a decline in the amount of available forage for indigenous herbivores, a reduction in the breeding efficiency of birds that rely on the vegetation and a negative effect on carbon storage by transforming stands of dense vegetative cover to open savanna like systems. Nor are these outcomes restricted to non-native herbivores; the re-introduction of a species, such as the elephant (Loxodonta africana), to areas from which it has been absent for many years may have similar consequences. Additional problems associated with the uncontrolled movement of large mammals include the transmission of disease, such as brucellosis in the United States and a threat to the genetic integrity of a species through hybridization. It is thus clear that deliberate introductions of herbivores to areas where they do not naturally occur may not be sound conservation practice.