books, etc.: Wild Urban Plants of the Northeast: A Field Guide / Peter Del Tredici -- Ithaca: Cornell University Press, 2010
For at least a century, American botanists have raised the alarm over the invasion of non-native plant species. The immigration to North America of people from all over the world has brought with it food crops, ornamental plants, and stow-away seeds that have found a congenial new habitat in the Americas. The primary concern is that these non-native species are sometimes "invasive," meaning they exist without natural predators and therefore spread out of control. Their success crowds out many native species, thus reducing the diversity of the native ecosystem and damaging the health of the environment.
Despite the apparent threats, Peter Del Tredici, Senior Research Scientist at the Arnold Arboretum at Harvard University, is not so alarmed about non-native invasives. Starting from the premise that our native ecosystem has been irreparably transformed by urban development, native species are now as alien to the new urban ecosystem as non-natives. For Del Tredici, the important question in the urban setting is not whether the plant thrived in a by-gone local ecosystem, but whether the plant provides "ecosystem services" that would not otherwise be provided to the city.