Passage IV

NATURAL SCIENCE: This passage is adapted from the article “The Asphalt Jungle” by Peter Del Tredici (©2010 by Natural History Magazine, Inc.).

The ecology of the city is defined not only by the cultivated plants that require maintenance and the protected remnants of natural landscapes, but also by the spontaneous vegetation that dominates the neglected interstices. Greenery fills the vacant spaces between our roads, homes, and businesses; lines ditches and chain-link fences; sprouts in sidewalk cracks and atop neglected rooftops. Some of those plants, such as box elder, quaking aspen, and riverside grape, are native species present before humans drastically altered the land. Others, including chicory, Japanese knotweed, and Norway maple, were brought in intentionally or unintentionally by people. And still others—among them common ragweed, path rush (Juncus tenuis), and tufted lovegrass (Eragrostis pectinacea)—arrived on their own, dispersed by wind, water, or wild animals. Such species grow and reproduce in many American cities without being planted or cared for. They can provide important ecological services at very little cost to taxpayers, and if left undisturbed long enough they may even develop into mature woodlands.

There is no denying that most people consider many such plants to be “weeds.” From a utilitarian perspective, a weed is any plant that grows on its own where people do not want it to grow. From the biological perspective, weeds are opportunistic plants that are adapted to disturbance in all its myriad forms, from bulldozers to acid rain. Their pervasiveness in the urban environment is simply a reflection of the continual disruption that characterizes that habitat—they are not its cause. In an agricultural context, the competition of weeds with economic crops is the primary reason for controlling them. In an urban area, a weed is any plant growing where people are trying to cultivate something else, or keep clear of vegetation altogether. The complaints of city dwellers are usually based on aesthetics (the plants are perceived as ugly, or as signs of blight and neglect) or on security concerns (they shield human activity or provide habitat for vermin).

From a plant’s perspective, it is not the density of the human population that defines the urban environment, but the abundance of paving (affecting access to soil and moisture) and prevalence of disturbance. In other words, a sidewalk crack is a sidewalk crack whether it is in a city or a suburb. Urbanization is a process, not a place—a process that tends to leave the soil in a compacted, impoverished, and often contaminated state.

The plants that grow and survive in derelict urban wastelands are famous (or infamous) for their ability to grow under extremely harsh conditions. Through a quirk of evolutionary fate, they developed traits in their native habitats that seem to have “preadapted” them to flourish in cities. One study, by biologist Jeremy T. Lundholm of St. Mary’s University in Halifax, Nova Scotia, and his then student Ashley Marlin, concluded that many successful urban plants are native to exposed cliffs, disturbed rock outcrops, or dry grasslands, all of which are characterized by soils with a relatively high pH. Cities, with their tall, granite-faced buildings and concrete foundations, are in a sense the equivalent of the natural limestone cliff habitats where those species originated. Similarly, as the British ecologist and “lichen hunter” Oliver L. Gilbert noted in his classic book The Ecology of Urban Habitats, the increased use of deicing salts on our roads and highways has resulted in the development of microhabitats along their margins that are typically colonized by calcium-loving grass-land species adapted to limestone soils or by salt-loving plants from coastal habitats.

In general, the successful urban plant needs to be flexible in all aspects of its life history, from seed germination through flowering and fruiting; opportunistic in its ability to take advantage of locally abundant resources that may be available for only a short time; and tolerant of the stressful growing conditions caused by an abundance of pavement and a paucity of soil. The plants that grow in our cities managed to survive the transition from one land use to another as cities developed. The sequence starts with native species adapted to ecological conditions before the city was built. Those are followed, more or less in order, by species preadapted to agriculture and pasturage, to pavement and compacted soil, to lawns and landscaping, to infrastructure edges and environmental pollution—and ultimately to vacant lots and rubble.