Here's a rather contrarian viewpoint about plant diversity and natives vs. exotics. In a nutshell, Hudson argues that the decline in biodiversity is so severe ( due, essentially to humans paving the planet ) that most sources of plant diversity should be encouraged. Additionally he points out difficulties in defining "exotic" and "native" due to the way seeds spread naturally. This from a fellow who for many many years has been a source of seeds of rare and unique plants. He argues that feel-good councils that make rules about invasive and exotic species may do more harm than good. An interesting view from someone that I have a lot of respect for.

If there is even less diversity in the plant world than we thought, then I guess his argument may be even stronger. Anyway, possibly something more interesting to read ( ha! yes I know this is /. ) than the OP.

Related, excellent read, for those interested Plants, Man, and Life

Bingo.

It's hard enough --and sometimes not possible-- to connect to the greater picture without willfully ignoring it. You have to learn a lot of mathematics before it starts to connect. You're not gong to be ready for a course in real analysis until you can manage calculus pretty fluently ( and analysis illuminates why various parts of calculus actually work. ) So let's make it harder still and simply not even try.