Lately I have been digging further into an amazing vegetation dataset of Banks Peninsula (New Zealand), trying to understand at what minimum abundance alien species may start having an impact. This is a question to which we still have very few answers, since most impact studies to date have compared heavily invaded sites to uninvaded ones. This type of binary impact assessment has been very useful to detect impact size, but does not allow us to know at what abundance we need to start worrying about an invader. The little we do know suggests that relationships between invader abundance and impact are likely to be non-linear, with threshold effects which are invader-specific. This is especially important to understand invader impact at the landscape scale. For a given invader, the abundance threshold of impact will determine the spatial spread and the size of potential impact in the landscape. Taking into account the differences in thresholds across alien species would provide a more accurate ranking of invader risks across a region, and could potentially provide better restoration targets for managers.
Last September, I presented some preliminary results at the EMAPi conference in Hawaii. You can find the talk here.