The 'diversity resistance hypothesis' suggests that com munities with high levels of diversity are less invasible by exotic species, as based on ideas of competitive exclusion and niche theory. If niches are narrower and already filled, competitive exclusion bars new species from enter ing very diverse communities. The opposite idea is also claimed, that is, that communities with high levels of diversity are more invasible by exotic species. Studies support both claims. However, studies of the pattern of species invasion at large regional scales suggest that spe cies rich landscapes are more invaded by invasive species than species poor landscapes.
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