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.
In place of the native/non-native distinction, Del Tredici distinguishes plants that grow spontaneously in the urban environment versus plants that require cultivation. In the concrete jungle, Ailanthus altissima (the tree-of-heaven) is able to grow without any help from us, and according to Del Tredici, without this species, our urban tree canopy would be greatly impoverished. Replacing them with "native" species and maintaining natives would require an enormous effort for little or no increased benefit. Among the ecological benefits that spontaneous plants can provide to the city are temperature reduction, food and habitat for wildlife, erosion control, riparian stabilization, nutrient absorption, soil building, and phytoremediation or the ability to absorb and store heavy metals thereby cleaning contaminated sites.
Del Tredici's argument is persuasive, but only as long as we accept that our urban environments will be built without caring that they be hospitable to formerly native species. Careful urban planning can increase the areas that are compatible with native plants and thereby preserve the local biological heritage. Del Tredici would, however, point out that this reflects a cultural value judgement and can not be defended on the basis of biodiversity or ecological services. On the other hand, invasives are often able to spread rapidly outside of the urban environment and into relatively undisturbed ecosystems. Such escapees from the city likely pose all the threats to native species about which botanists warned us.
Del Tredici's argument in favor of spontaneous plants makes up just 23 of the book's 374 pages. The rest of the book is, as the subtitle states, a field guide in which spontaneous urban plants are cataloged and described in a manner traditional to such field guides. Each entry includes, however, information on the ecological functions and cultural significance of the plant described. This is consistent with Del Tredici's two primary goals, "to teach people how to identify the plants that are growing in urban areas, and to counter the widespread perception that these plants are ecologically harmful or useless and should be eliminated from the landscape." Each entry also includes five or six useful color photographs of the plant or its near cousins.