Life on the Vertical: Exploring Nature on City Walls

Human civilization has evolved rapidly and so too has its most fundamental architectural unit, the wall. From humble walls that gave our ancestors shelter, to ones that reach for the sky in modern cities, we’ve built walls visible from space, and dams that break the flow of mighty rivers. With over six billion people estimated to live in urban spaces by 2050, the dominance of walls in our city landscapes will only grow. Is there any non-human life in these seemingly mundane landscapes? Delightfully, yes! Let us enter the fascinating world of wall ecology.

A vast diversity of life has adapted to urban landscapes including walls. Four broad factors seem to play a key role in shaping the combinations and permutations of life forms that live comfortably on walls: the material used to make the wall; the microclimate and moisture around it; the nutrients available; and the surrounding ecology. Each combination of factors creates its own unique assemblage of life on walls. Here’s a glimpse of some of the incredible and quirky biodiversity of life on walled surfaces in Bangalore city.

Growing through the cracks

In India today, concrete is the most popular construction material. It is used both as filler and choice of material for building blocks. It is relatively cheap, versatile, and available in vast quantities. That concrete is an ecologically disastrous material, is a story for another day. In developing countries like India, concrete walls are often constructed with higher volumes of sand to cut costs. These poorly constructed walls are quick to develop a network of cracks. Various plants including ferns and figs make the best of these gaps and often grow from these dusty fissures. In the wild, strangler fig species have adapted to grow from seeds pooped by birds and bats onto the branches of other trees, giving them access to abundant sunlight closer to the top of the canopy. A nifty adaptation that seems to have also helped them conquer our concrete-walled canopies.

These small outcrops of green on walls may not seem interesting to those of us that busily stroll past them. They are, however, of great importance to some of the tiny lives that share space with these plants. The fig plant growing out of a second-storey wall in residential neighbourhoods is a source of fibre for paper wasps building their nests. On a busy street, we found leaves of ferns growing out of a wall providing an ideal retreat for a spider.

High-rise homes

After gathering fibre from the stems of plants or dead wood, paper wasps build their nests in the nooks and crannies of urban environments. If they have not been bothered too much, they rather boldly build their nests on the surfaces of walls. Another winged inhabitant of aging brick walls is the affable stingless bee. These tiny bees normally make their homes in the hollows of tree branches or in small rock crevices. In urban ecosystems, they have made their homes in the cavities of walls. Like most bees, these inconspicuous wall dwellers play a vital role in pollinating the flowers in their immediate environment.

Another group of individuals that love to hang around walls are spiders. They breathe life into walls with their silken graffiti. From jumping spider retreats that look like cosy sleeping bags to elaborate three-dimensional art installations by tent-web spiders, they decorate walls with quaint patterns that work as effective insect traps. The ubiquitous Uloboridae, an orb-weaving spider can often be seen waiting patiently on bathroom and kitchen walls in urban homes. Even two-dimensional orb-weavers like the signature spider (Argiope) are sometimes seen building their webs inches away from walls — adapting to the only structural support that is now available.

Among the many spiders that live on walls, there is a specialist that would make one think that the invention of the wall was this spider’s idea. The two-tailed spider (Family: Hersiliidae) lies flat on walls, just as it would on the bark of a tree. Its colours and patterns often merge seamlessly with its background. Often the only tell-tale signs that give the spider away are the silken layers on the surface it is sitting on. Other delightful groups that seem to find walls safe havens are the trash carrying case-worm moth caterpillars, lacewing larvae, snails, slugs, lizards, and a diversity of wasps. Among our eight-legged friends making homes along grooves and crevices of walls are the very tiny but sneakily abundant disc-web weavers (also known as wall spiders) belonging to the genus Oecobius and the cobweb building spiders belonging to the Pholcidae family. Our checklist continues to grow as we lovingly scan the surfaces of different types of walls across the city for life.

Greening urban walls

As urban spaces continue to get more built-up and cramped, it behoves us to think of and empathise with our cohabitants in these spaces as we grow towards the sky. Walls present excellent spaces for us to plan and implement scientifically backed urban greening. While it may not be ideal to let a fig sapling grow and climb out of your home wall, there are a wide variety of native climbers like the Bengal clock vine or Mysore trumpet vine that can add colour and life to home walls, and provide nectar to a wide diversity of life. Many urban homes are making good use of the walls around their steads by propping potted plants on and around them. Sadly, community commons like the pillars under our flyovers seem to get plastic-filled “aesthetic” upliftment instead of the more functional, simpler alternatives like native ecologically important shrubs, herbs, and creepers. If our walled spaces are used to grow kitchen variety herbs, they also become ecological green spaces that benefit their human hosts as well as their non-human visitors and residents. Recognising, engaging with, and understanding the wondrous nature that lives on our concrete walls can be an enriching experience and fuel the empathy needed to coexist with them peacefully.