A CRACK IN THE SIDEWALK

Concrete or asphalt covers a large percentage of urban and industrial areas. These are very hostile environments which are not favorable for most living things. There is no soil, and extreme fluctuations of temperature and moisture are the norm. Such places are essentially artificial deserts. However, things do live in deserts and things do live on sidewalks, but not many things. This is an artificial ecosystem where the only living things are those that are adapted to harsh conditions. It is an ecosystem that very few people are even aware of, but one that we see every day. It is also a fascinating one, which beckons from our doorsteps. It is an ecology you have to study on your hands and knees, preferably with a magnifying glass.

The concrete itself may appear totally devoid of life, but that is not the case. It is covered by micro-organisms. Even in the middle of a well used sidewalk there are thousands of species of bacteria. At the edges of the sidewalk where few people walk, you begin to see the first visible evidence of living organisms. There is often a green layer of algae. This alga coating is only one cell layer thick, but is easily seen because it forms a continuous coating over the concrete surface. It consists of single cells, but these cells do the same thing that leaves of higher plants do. They contain chlorophyll, which combines water from the surface of the concrete, and carbon dioxide from the air to produce glucose and oxygen. So even the sidewalk is doing its little bit to combat global warming. The most common alga here is probably Pleurococcus, the same one that forms green coatings on tree trunks and fences. Old fences are also combatting global warming.

On the edges of sidewalks you will often see pale circular patches. Some of them are chewing gum, but others are crust lichens. You will need to look rather closely to tell which are which. If a patch has tiny round saucers on it, it is a lichen. A lichen is a composite organism composed of an alga and a fungus. The alga part carries out photosynthesis and the sugars produced are also used by the fungus component. Lichens produce quite a few unique acids, and these acids slowly dissolve the sidewalk. This, however, is a very slow process, so the concrete is not going to disappear any time soon. The most common crust lichen on road and sidewalk edges is a member of the genus Lecanora.

If you have a magnifying glass you may see springtails and mites wandering about. These animals are just barely visible without a lens, but you need the lens to see anything about them. Most of them are feeding on algae, micro-fungi or bacteria. You will undoubtedly see several species of ants, foraging for whatever they can find. Some of them also prey on the springtails and mites. 

Which brings us to the cracks. Sidewalks and roads have cracks, grooves and depressions. Small amounts of soil form, or get washed into these cracks, and it is here that annual flowering plants and mosses can attain a foothold. Look along the green line between one piece of concrete and another piece. The plant diversity here is not tremendously great, but there are a number of different plants in several families. They share a number of factors in common, and these factors do not relate to how closely related the plants are. They are characteristics concerned with their ability to adapt to this harsh environment. The limiting factor here is not competition with other plants, which is often the most important factor in other environments. It is the extremes of the physical environment - heat, drought, and lack of nutrients.

If the crack is very shallow, there is not enough soil for even a small flowering plant. Such a crack, however, favors very small mosses that are adapted to extreme habitats. These mosses are often ones that grow on limestone, as concrete is alkaline, and functions as artificial limestone. There are three common mosses in such sites - Bryum argenteum, Bryum bicolor, and Ceratodon purpureus. The most distinctive one is the silvery bryum (Bryum argenteum). It is shaped like a tiny worm, with the leaves pressed closely together, giving the plant a cylindrical shape. It has a silvery sheen because the leaf tips do not have any chlorophyll. 

                                                                               Silvery Bryum - Rosemary Taylor.

If the crack is a little bigger it favors two small weeds that are not much bigger than mosses. They both have tiny narrow grasslike leaves, but they look quite different when in flower. You need to look very close, though, to see the flowers. They are very tiny. The bird’s-eye pearlwort (Sagina procumbens) has four green petal-like sepals. The other species, red sand-spurry (Spergularia rubra) has little pink flowers with five petals.

                                                                          Red Sand-spurry - Rosemary Taylor.

On southern Vancouver Island a recently introduced weed from farther south in North America is spreading on sidewalk areas. This is the spotted spurge (Chamaesyce maculata). Maculata refers to a single dark spot on each of the plant’s lopsided leaves.

                                                                        Spotted Spurge - Rosemary Taylor.

If the crack is larger still, bigger plants such as white clover (Trifolium repens) and Kentucky bluegrass (Poa pratensis) can get a foothold.

Next time you walk down the street have a closer look at your sidewalk. Even here you will find some of the wonders of nature, and see some of the processes of ecological adaptation.