The four questions: What does the introduction of exotic species do to diversity?

The breakdown of isolating barriers between biogeographical provinces will not have much effect on species diversity. In the short term, it will reduce global diversity, but increase local diversity. At steady state, the effects on global diversity disappear, but local increases remain. The real damage to diversity will come from shrinking the areas of the Earth that harbour wild species. The considerable damage exotic species have been known to do comes primarily from direct effects of particular introductions.