All those “weeds” add up to a lot of urban biodiversity

Featured Article: Bonthoux, S., L. Voisin, S. Bouché-Pillon and S. Chollet. 2019. More than weeds: Spontaneous vegetation in streets as a neglected element of urban biodiversity. Landscape and Urban Planning 185: 163–172. DOI: https://doi.org/10.1016/j.landurbplan.2019.02.009

Urban residents might regard the scruffy-looking volunteer plants in their neighborhood as “weeds”, if they notice them at all. But a study by Bonthoux and colleagues suggests that even plants along pedestrian sidewalks may represent an important chunk of urban biodiversity, and can make up hundreds of species found in the “natural” ecosystems of  cities.

Tough enough for the city

In the urbanizing 21st century, more attention is being paid to recognizing and fostering “nature” in cities, in growing recognition of the benefits exposure to “nature” can bring to city dwellers. But the word “nature” itself, applied to cities, probably makes most people think of managed open areas like parks, or perhaps some of the more industrial-appearing green spaces like New York’s beloved High Line.

Even a lifelong urbanite might be surprised to learn that many of the living things that call the city home can be found intimately mingled into our own neighborhoods. True, some are more obvious, like the ubiquitous sparrows and pigeons, and some may only merit notice when caught on nocturnal raids of the garbage bins (looking at you, racoons and rats). But consider the living things with whom you might share your street hiding in plain sight: Plants, especially the “Weeds” – a somewhat derogatory word often applied to any vegetation that establishes itself spontaneously without human help. More pointedly, some of these volunteers have a reputation of prevailing even against humanity’s best efforts, as anyone who ever mowed a lawn full of dandelions or tended a backyard garden can attest.

“Weeds” are often plants adapted to difficult conditions. (Photo: Pxfuel)

But there’s another way to look at them: “Weedy” plants that sprout in human-dominated areas often represent “pioneer species” that evolved specifically to thrive in places that have been recently disturbed. This preference for wild frontiers makes them experts at handling the rough and variable conditions, like poor soil, variable moisture, and short windows of opportunity, that predominate in places like road verges, between pavement cracks, or even the walls of buildings. When humans came along and made cities, we also inadvertently created habitat just to their liking.

The jungle underfoot

So-called “grey spaces”, and the “weeds” that call them home, may be an important part of the urban ecosystem and an important reservoir for plants relied on by pollinators. But these areas receive less interest than the greener parks and open spaces. The study authors wanted to shed light on this neglected aspect of the urban ecosystem, and better understand how these more heavily used zones might be better managed for the good of the occupants of the city, human and otherwise.

The study by Bonthoux and colleagues looked into just how abundant and diverse were the spontaneous plant communities found in the small-sized city of Blois, in central France. This city provided an ideal test location because it recently phased out the use of some herbicides to manage unwanted vegetation, putting less pressure on where urban plants could grow. The scientists carefully surveyed the plant communities growing in sections of sidewalk all around the city, recording in each stretch the species of plants observed and their coverage, the types of pavement and what ground cover was nearby, and many other variables about the landscape nearby. They then analyzed their findings statistically to examine the effect of these factors on the ecological plant communities they documented.

The medieval city of Blois in central France (Photo: Pxfuel)

The researchers found a surprising amount of plant biodiversity growing out of the sidewalks – more than 300 species in total. Far from representing an ecological dead zone, about 20% of the plant species known to exist in the region could be found right beneath people’s feet in the city. Many were familiar grass species, or common low-growing plants tolerant of some human foot traffic. A few were invasive species brought in from elsewhere, but most were native to the region, some even found commonly in marginal habitats like the edges of agricultural fields.

The authors found that the main determinant of plant cover and biodiversity in these communities was the kind of growing environment offered by the sidewalk. Older sidewalks made of pavers with some cracks between had a moderate amount of plant cover and diversity, while newer pavers with fewer gaps had very little. Perhaps unsurprisingly, sidewalks made of compacted sand, with a bit more permeability than pavement, hosted the most plant cover and biodiversity. More plant cover was also found where sidewalks were not adjacent to built structures and with proximity to parks and street trees. Species richness was also higher at the periphery of the city where presumably there were more opportunities for seeds to establish from outside the city. And the plant communities appeared to respond to human behavior – industrial and commercial areas with less human traffic (and presumably less volunteer weeding by residents) had more plant cover, while in more maintained areas towards the city center species richness declined.

Making room

The authors offer some key recommendations to urban planners keen to optimize urban ecology and the needs of residents. In Blois, they suggest that since many sidewalks are already wide enough, perhaps allowing some marginal soil to remain unpaved could provide additional space for these spontaneous plant communities. They also suggested that given the relatively small effect of intentional weeding on eliminating plants, a better strategy might be to focus maintenance efforts on more highly trafficked areas like the historic city center, while allowing diversity to flourish with less intensive management in quieter areas.

Only by first noticing a thing can we come to value it, or understand it. Studies like the one by Bonthoux and colleagues invite us to notice the less beloved green co-habitants of our city, and perhaps to reimagine our relationship to the “natural” environment of the cities we share. The scraggly volunteers in our neighborhoods may sometimes thwart our higher ambitions for perfect flower beds or the fatter tomatoes, but they are admirable survivors in their own right – and we are still working to understand the part they play in making the natural urban world tick.

Reviewed By: Kara Cromwell

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Andrew Trlica

Andrew Trlica

Recent PhD out of Boston U's Dept. of Earth & Environment. Interested in landscapes defined by the human presence, and how humanity’s choices connect our landscapes to the causes and consequences of climate change. My own research has dealt with the urban carbon cycle and urban forest, the urban heat island, agricultural practices and soil quality, and disturbed land reclamation. Twitter/Insta: @places_we_made

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