WEAK VS. STRONG INVADERS OF NATURAL PLANT COMMUNITIES: ASSESSING INVASIBILITY AND IMPACT

In response to the profound threat of exotic species to natural systems, much attention has been focused on the biotic resistance hypothesis, which predicts that diverse communities should better resist invasions. While studies of natural communities generally refute this hypothesis, reporting positive relationships between native species diversity and invasibility, some local-scale studies have instead obtained negative relationships. Most treatments of the topic have failed to recognize that all exotic invaders do not behave alike: while “weak” invaders become minor components of communities, “strong” invaders become community dominants at the expense of native species. At the same time, the specific impacts of strong invaders on communities are poorly documented yet critical to understanding implications of diversity loss. With these shortfalls in mind, we examined local-scale relationships between native and exotic plant taxa in bunchgrass communities of western Montana, USA. We found that measures of...

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