Native Nazis?

What constitutes a "native" plant? My ecologist friends despise species they consider non-native, e.g., alfalfa, horses, certain kinds of birds and plants. The idea, I gather is to maintain a pristine environment little influenced by external forces. I have always wondered what period of time should be considered the baseline. A footnote in a fascinating book called Rats by Robert Sullivan sheds some light on the subject.

Sullivan quotes from an article entitled "The Mania for Native Plants in Nazi Germany," in a book edited by Joachim Wolschke-Bulman. The editor comments, "The missionary zeal with which so-called foreign plants are condemned as aggressive is significant. Such characterizations do not contribute to a rational discussion about the future development of our natural and cultural environment, but possibly promote xenophobia." He points out that many of the plants we now consider "native" were most likely brought over via the land bridge from Siberia. Advocates of preserving a native America had close ties to the Nazis before the war and a desire to rid the country of anything foreign. A Jewish writer responded to one of them, "If this kind of garden-owning barbarian became the rule, then neither a gillyflower nor a rosemary, neither a peach tree nor a myrtle sapling, not a tea-rose would ever have crossed the Alps. Gardens connect people, time and latitudes . . . . The garden of humanity is a huge democracy. It is not the only democracy, which such clumsy advocates threaten to dehumanize.”