Tallamy makes one more point I want to mention here for comment:
Our problems are not comiong from horticulturists who want to express themselves with nonnative ornamentals. There aren't that m any serious gardenrs, adn they do not impact enough land to make a big difference. Rathere, our problems come from the millions of typical homeowners and corporate land managers who have no particular aspirations for their landscapes other than to fit in with teh neeighbors. These are the peopel who deterkmine the biotic richness of thousands of square miles of suburbia, yet they don't even know that there is an alternative to exotic ornamentals. And they are encouraged in their ignorance by nurseries that limit their selections adn by gardening publications that discuss no alternatives.