Bootstrap Analysis has an extraordinary post this week on finding the urban prairie in blighted Detroit. The vacant lots, rank with weeds and rubble, that the author explores as an urban ecologist are astounding examples both of the catastrophic collapse of human communities and the habitat adaptations that wildlife can sometimes make in the face of massive, unnatural disturbance. The author provides a contemporary satellite image of one such area, along with an aerial view of the same neighborhood from 1961, and the contrast could not be more striking. The elms that once shaded children at play and neighbors chatting on front steps are long gone, and race riots, poverty, drugs and chronically under-resourced city budgets have left a desolate, ruined landscape.
Yet life remains. Our urban environments, even the most neglected and inhospitable, still provide habitat for more than rats and roaches. Coyotes are able to negotiate urban spacial gaps to occupy Manhattan's Central Park. Peregrines nest on the ledges of skyscraper windows. Migrating birds pause in the feral green of abandoned lots and neglected places across the urban landscape. It is a small consolation, perhaps, and hardly mitigates the horrendous decline of the inner city and the plight of its residents. Still, there is more of conservation value beyond city parks and wealthy suburbs, when there are eyes to see.
My friend and former colleague on the Massachusetts Invasive Plant Advisory Group, Dr. Peter Del Tredici of Boston's Arnold Arboretum, has written recently about urban ecology and the value of plants in urban habitats for wildlife and people alike. Writing in the February edition of Landscape Architecture, Del Tredici identifies a disconnect between the advocates of ecological restoration using native plants to replace invasive ones and the nature of urban environments:
"Regardless of how we feel about the unique assemblages of plants that populate our sprawling cities, they have become the de facto vegetation of the urban environment. As such, we need to acknowledge that they are actually performing significant ecological functions, including water and air filtration, heat absorption, mineral cycling and carbon storage...From a functional perspective, the presence of Pragmites in disturbed landscapes is a symptom of environmental degradation rather than its cause. Research has shown that a number of invasive plants have a similar kind of 'Jeykll and Hyde' impact on the local ecology, pushing out some native native plants while providing food and shelter for a wide variety of native animals, especially pollinating insects and migrating birds. Like so much else in the modern world, the designation of a species as invasive is a relative concept that depends on the context in which the plant is growing."
In other words, conservation is all about the values we place on certain qualities and attributes of the environment over other aspects and land uses. While some might choose to challenge Del Tredici on the relative value of Phragmites over the full, potential native expression of biodiversity in the habitats where it invades, he is absolutely right to ask that we take plants growing in urban environments on their own terms and assess their impacts and benefits locally as well as their potential to become invasive in minimally-managed habitats.
The urban prairie of desolate Detroit is far from the unplowed soils and seed banks of its presettlement history. One would not attempt to successfully to restore full ecosystem function and a tallgrass prairie system amid the ruins (although some pioneer cemeteries retain the soil horizons and seeds that the plowed lands did not spare and now form remnants of native habitat). Del Tredici recommends that the primary consideration of landscape architecture in urban environments should be sustainability. Many invasive species will tolerate the extreme and harsh conditions of the city better than native plants. There is certainly the risk of their becoming a source for introductions of invasive material to uninvaded habitats, but they may be less of an ecological sink than we who manage natural areas may have suspected.