In ecology an ecosystem service is defined as the flow of energy and materials through the arrangement of biotic and abiotic components of an ecosystem, including net primary production, trophic transfer from plants to animals, nutrient cycling, water dynamics, and heat transfer, among others. When such processes provide benefits and value to humans they are defined as ecosystem services (medicinal use, edibility, phytoremediation, carbon capture, erosion control, etc).

Many plant species play an important role in providing such services. In recent decades ecologists have identified plant traits that are crucial for providing such processes and services. The following is a list of performative attributes that have been identified for spontaneous urban plants.

Disturbance Adapted

A disturbance is a temporary change in average environmental conditions that causes a pronounced change in an ecosystem. Disturbances often act quickly and with great effect, sometimes resulting in the removal of large amounts of biomass. Ecological disturbances may include fires, flooding, windstorms, plague outbreaks, earthquakes and even climate change. Human impact (anthropogenic disturbance) is evident within cities, constantly developing the built environment.

Some species are particularly suited for exploiting recently disturbed sites. Vegetation with the potential for rapid growth can quickly take advantage of the lack of competition and in most cases with the abundance of sunlight. Species that are well adapted for exploiting disturbance sites are referred to as pioneers or early successional species.

Wildlife Habitat

Our prolific development of urban centers has reduced extant wildlife habitat and their opportunities for sustenance. In considering how to reintroduce wildlife habitat into our cities, one question is whether the patchwork ecology of small-scale green infrastructure interventions can provide substantial wildlife habitat. Most of these sites tend to be intimately-sized and disconnected from one another and yet wild urban plants might provide the connective link between these disparate patches. Offering food and shelter, many wild urban plants provide important value to bees, insects, birds and small mammals. For example, the gravelly substrate of vacant lots provides an excellent nesting ground for insects which feed on the nectar and pollen of the weeds. Wild urban plants offer a variety of habitats for urban wildlife providing aerial roosts in weed trees, dark, woody nesting places under the cover of vines, and protection by camouflage amidst grasses and flowering plants in an open, urban meadow.

Carbon Sequestration

Rising levels of carbon dioxide in the atmosphere are contributing to global warming and rapid climate change. Plants are the greatest natural resource we have to ward off this impending threat. Through photosynthesis, they provide us the unique service of removing CO2 from the atmosphere, storing in their tissues including stems, roots and wood and using it to produce energy for growth. With their large biomass and woody structure, trees are the most efficient carbon capturing plants. This is especially true in young, emergent forests, where there is still an increase in the biomass being produced. Metrics have been established to add value to the act of urban tree planting and to chart the ability of trees to mitigate the effect of fossil fuel emissions. Quick growing species are favored, and are represented by many species in the urban flora.

Edible

The rise of industrial scale food production has disconnected the consumer from their regional farm and food sources. Access to local food, especially in low-income urban areas, has become a major social justice and health care issue. Over the past decade, there has been a resurgence of interest in higher nutritional, fresh food which many people are finding access to through green markets or in their support of community supported agriculture. Additionally, urban foragers have helped to revive the practice of harvesting wild plants. Many nutritious and delectable fruits, shoots and greens can be found in cities requiring little or no cultivation. Although many of the city’s most common plants are edible, certain plants are rare delicacies and need to be hunted for, such as ramps (Allium tricoccum) or fiddlehead ferns (Matteucia struthiopteris). Increasingly, wild plants such as mulberry or sumac are being re-evaluated for their delicious flavor, localness and abundance.

Medicinal

The study of the medicinal qualities of plants laid the foundation for the field of pharmacology and modern medicine. The Greek philosopher Dioscorides wrote the authoritative text on medicinal plants, “De Materia Medica”, in the 1st century A.D. and the manual was actively used for the next 1500 years. Depending on the plant, roots, bark, foliage, stems, flowers and seed can be integrated into topical salves, remedies and tinctures. With a focus on urban foraging, homeopathic medicine and a distrust of large pharmaceutical companies, the collection of wild plant material for medicinal purposes has become popular once again. Recent studies have attempted to validate herbal remedies within the context and rigor of modern western medicine.

Phytoremediation

Many urban, postindustrial sites contain contaminated soil that is expensive to remove and difficult to remediate. Spontaneous urban plants are adapted to flourish on tough sites and can assist in mitigating the concentrations of pollutants in a given area. Plants can stabilize sites with their root systems, preventing soil loss and the flow of pollutants into waterways. Many vigorous plant species, called hyperaccumulators, are able to uptake heavy metals and toxins from the soil, accumulate and bind them in their cell tissue, and transubstantiate them into less harmful states. Specific types of phytoremediation include phytostabilization, rhizodegradation, metablolization and phytoextraction.

Stormwater Retention

Higher ratios of impermeable surfaces in urban areas exponentially increases the amount of water pouring into storm drains. In New York City, which has a combined stormwater and sewer system, many rainfalls throughout the year overwhelm the system’s stormwater capacity, resulting in raw sewage being released into local waterways. Additionally, runoff carries a range of surface pollutants into our waterways, including sediment, oil and heavy metals. Many wild urban plants have the ability to grow in gravel lots and abandoned parking lots which can increase the local watersheds capacity to handle significant rainfall in those areas. In particular low growing, prostrate and mat-forming plants which inhabit the street edges and sidewalks are especially effective in limiting the first flush of a rainfall, which can last only a minute or two but can hold a substantial amount of the rainfall in an event. Municipal rain gardens, or bioswales, are organized efforts which recognize the importance of bioremediation and the soils ability to retain water and assist in the removal of various pollutants. Slow it down, spread it out and let it soak in.

Ecological Disservice

Not all weeds are good neighbors. Some wild urban plants are so successful on disturbed sites that they rapidly infiltrate existing functional ecosystems and outcompete their native counterparts. These rogue invaders establish pervasive monocultures which reduces flora and fauna biodiversity and results in a destruction of native habitat. Most spontaneous urban plants provide a range of ecological services however; there are some that simply offer no benefit to humans, wildlife or the environment. These species are often toxic, parasitic or are such aggressive growers that they can’t coexist with other plants and deserve eradication. The decision about a plants inherent value should be site specific and appropriate to the surrounding context. As further research is completed, more metrics attributed to the positives and negative services each plant has to offer. Hopefully we can make more informed decisions about the direction of our urban ecology.

Erosion Prevention

Urban Heat Mitigation

Noise Regulation

Flood Prevention