From several sources now I have heard people argue that bringing in new species of plant to an area must be a good thing because it increases biodiversity. To that, I propose an experiment. In a large enclosure, put deer, squirrels, groundhogs, mice, coyote, and whatever other North American fauna you like. Provide ample plants for the herbivores to eat. Now sit back and watch. There should be something akin to balance here. Hooray.
Now add tigers.
Hooray, the biodovirsity has increased! Or it has, at least, until the tigers get hungry.