Invasive species may drive local native species to extinction via competitive exclusion, niche displacement, or hybridisation with related native species. Therefore, besides their economic ramifications, alien invasions may result in extensive changes in the structure, composition and global distribution of the biota at sites of introduction, leading ultimately to the homogenisation of the world's fauna and flora and the loss of biodiversity. It is difficult to unequivocally attribute extinctions to a species invasion. Although evidence is strong that the recent extinction of about 90 amphibian species can be traced to the chytrid fungus spread by international trade, most scientific research has focused on animal invaders. Concern over the impacts of invasive species on biodiversity typically weighs the actual evidence (either ecological or economic) in relation to the potential risk.