Biodiversity can improve life in cities by offering ecosystem services like shade, pleasant sensory experiences and pollution treatment in soil and water. But what do cities offer biodiversity?
Biodiversity is the result of evolution: whichever species persist at any given site can be assumed to have developed helpful adaptations that have allowed them to survive the environment, climate and neighboring species inhabiting the same site*. What counts as a helpful adaptation is always related to both random events (or stochasticity in ecologist-speak) like earthquakes and forest fires, and more long-lasting or cyclic conditions like water and food availability or genetic potential for sufficient immunity systems. Some forms of life have found a specialized niche in situations where most things do not thrive, and consequently have become completely dependent on very specialized conditions. A good example of this would be Ranunculus glacialis, THE flower of the arctic, which can only be found on nutrient-poor, well-drained northern slopes of snowy mountains that are occasionally disturbed by ground frost. Anywhere else it will be outcompeted. Other species, like common dandelion** (Taraxacum officinale), brown rat (Rattus norvegicus), or human (Homo sapiens) can seemingly survive on a wide variety of sites - the trade-off being that they need to spend their energy on competition for food and habitat instead. The former group is usually called specialists, and the latter are generalists. As generalists, we humans tend to avoid extreme conditions, a tendency which is reflected in our cities.
Species that live in cities tend to adapt to the "human habitat" and the presence of humans. Many non-human city-dwellers have arrived at their own accord, like the aforementioned brown rats, and found their niches in the city. After all, many of them have had hundreds or even thousands of years of city life to get used to! Other species have been intentionally brought in by humans. These are often completely dependent on human care, especially if they've been transported far away from their origins like ornamental tropical houseplants or pet desert rodents. Cultivated crop plants like wheat and cabbage also belong to this category. As breeding for food has made them less viable as competitors, their populations would quickly dwindle without human intervention. This makes human activity one of the forces that maintain biodiversity in cities.
But as we all know, human activity is also a force that diminishes biodiversity, as we have replaced whole ecosystems by transforming forests, swamps and mountaintops into cities which we ameliorate through constructing roads, buildings and parks. Any populations of species strictly bound to forested, swampy or mountainous conditions will be weakened or shut out, since mainstream infrastructure engineering demands open space, dry subsoils and relatively flat topography. On one hand this is completely understandable: Ease of access for both machines and people with compromised movement improve safety and speed, and stable foundations for buildings and roads lower risks to human life and property. Earlier in history these difficult to build areas could be avoided in favor of better sites, but as urbanization continues we have wanted and needed to expand even on sites that were previously considered uninhabitable. The remaining natural objects and subjects are not safe even after the infrastructure is in place, as they suddenly need to accommodate human wishes and needs for cleanliness, orderliness and safety. Off with hazardous old trees, untrimmed hedges, and all of the creatures living on them! And so we are left with only the toughest plants and animals that can duke it out among each other, and either escape human scrutiny or adapt to it. How do we make sure that urban biodiversity isn't even further diminished? Or would there be ways to make cities more inhabitable to a wider variety of species again?
Soil and water are the basis for all plant life; plant life is the basis for most non-plant organisms. This means that plant biodiversity is bound to variation in soil types (oxygen and nutrient availability, pH, water retention and so on) and water availability, which in turn vary based on site history, topography and climate. In urban situations much of this variation is lost, as planting soils tend to be standardized in structure and sometimes isolated from natural subsoils and groundwater. Traditionally, the standard planting soils have been highly productive (good water and nutrient availability) and favoring a smallish set of strong generalist competitors that take over plantings in a flash. But anybody who works with urban plantings knows that stressful situations abound. Urban plant beds are often small and become compacted with time, which hinders the movement of air, water and nutrients in the soil. While water availability in urban plant beds may be improved by an increased tendency to lead stormwater into plantings, it may also lead to an increased risk of drowning the plants if drainage is insufficient. Stormwater can also transport oil, salt and other pollutants into plantings, with adverse effects. Additionally, plants are sensitive to the availability of light, which becomes scarcer as streets become narrower and buildings become higher. To top it all off, we humans can't be bothered to appreciate many of the species that actually would manage in these situations, further homogenizing plant selection in public urban spaces.
Based on the above, urban areas could on one hand be characterized as extreme sites for plants due to their high levels of drought-, shade-, pollutant-, and oxygen deprivation stress. Shouldn't this allow us to then utilize species that are specialized to withstand these kinds of conditions? Well, yes and no. Many plants adapted to highly stressful situations are small and nondescript, and tend not to satisfy human wishes for obvious beauty and space-making. Lichens and mosses, for example, can be found on similarly extreme sites in heavily shaded forests and bare rock. But if we start by looking for drought-tolerant plants, we find plenty of attractive alternatives. Out of these, a very limited amount will endure extended flooding. This list we filter further by demands for salt- and shade tolerance. Finally we make demands on the height and spread of these plants so that they fit into our narrow streets patrolled by trucks that cannot drive through tree canopies. I calculated loosely that in central Sweden the result amounts to something like 4 tree alternatives, 6 shrubs, and about 15 species of perennials and grasses. This is workable if you only need to plant one street or a few blocks. But when this palette is the only one available to you across zoning plans, city districts and cities, you start to understand that even extreme sites encourage homogeneity when the types and combinations of extremes repeat themselves from site to site. Using a small palette of specialists is no better for biodiversity than using a small palette of generalists.
So what do we do to secure diverse combinations of site conditions in urban areas? Here are some practical ideas:
Secure the existing variety of green areas and create or enhance their connectivity through coherent corridors of water and vegetation.
Respect the topography and soils of the site, as these provide the basis for varying degrees of available water, light and oxygen.
Think about variation in building heights, volumes and orientations as well as street widths to create mosaics of light and shade.
Retain existing vegetation (especially old trees that often act as keystone species for invertebrates, lichen, fungi, birds, small mammals and more) to uphold habitat continuity. See also the value in thickets of spontaneous vegetation that house insects and birds.
Let plant debris like dead wood, fallen leaves and branches lie on the ground to allow carbon recycling on site. Plant debris also provides habitat and shelter to many invertebrates, working as an all-you-can-eat restaurant for birds!
Include small water bodies like streams and ponds which can serve triple duty in recreation, stormwater management and habitat.
Prioritize habitat-based planting strategies for plantings that utilize site diversity and are likely to succeed in the long run.
Utilize a broad planting palette that includes flowering and fruiting species for every occasion, from annual bedding in city centres to home gardens to suburban shelterbelts of trees and shrubs. This ensures food provision for pollinators and fruit-eating birds, especially generalist species.
Use native species with a network-perspective in mind to provide stepping stones between native habitats and to provide native specialist microbiota, insects and other animals with hosts, habitat and food sources.
Utilize green roofs and -facades as a complement to vegetation on the ground level, not replacement! Vegetation on ground level is more accessible, carries more biodiversity, and has better chances of becoming self-sufficient.***.
Biodiversity can only persist with site diversity, and site diversity requires space and time. Successful integration of biodiversity goals and actions is only possible when they are considered on a variety of scales simultaneously, from region-scale to district-scale to individual project. Thus biodiversity can also be seen as a political question, an objective that could be prioritized in strategic urban development. So every time you ask in your work: how can I make a positive impact on biodiversity in this project? Make also sure to ask: How can I make a positive impact on the biodiversity of the city as a whole?
(*Let's take a moment of silence for all the maladapted species and individuals that did not survive. They tried so hard, and got so far- and in the end, it does too matter! Somebody probably got a good meal out of them.)
(**yes, I know that dandelion isn't strictly speaking a species.)
(*** I confess, I'm not a fan of green facades at all, at least not in my climate. They require artificial irrigation and are prone to freezing damage. I've seen better and worse examples of them in Sweden (only bad examples in Finland), and consider them mostly a novelty feature here since they do not appear to have potential for becoming self-sustaining. But hey, maybe I'll be convinced otherwise later when related technologies have been developed even further. Until then, I'm more favorable towards tropical green facades based on epiphytic species. In tropics, where they belong.)
(**** You may note that I do not mention insect hotels or birdhouses. The latter have proven their worth, and can be encouraged, but since in my understanding they are not decisive for urban bird biodiversity I've left them out of this list. Insect hotels, on the other hand, become obsolete if we follow points 4,5 and 9 and leave clay and sandy soils unbuilt. It's much easier and gives better results. Really. How many amateur entomologists do you know that could engineer suitable habitat alternatives for the approximately 1500 species of invertebrates found in hedgerows?)
This text is a translated and extended version of a previously published opinion piece.
Comments