by Whit Gibbons

April 8, 2002

I have a type of habitat in my backyard that is in everyone's neighborhood as well as in Hong Kong, Manhattan, and all national parks. The habitat is one we see daily but seldom think of in ecological terms. The habitats vary in different regions but all have one thing in common--they are vertical, because they are walls. Yes, walls. Like those around a garden, or the sides of houses and sheds.

Arnold Darlington, in his book called the Ecology of Walls (1981, Heinemann Educational Books, London) claims that walls comprise more than 10 percent of the area habitable by plants and animals in a city.

Many factors affect the extent and composition of species inhabiting walls, including the degree of inclination. Horizontal walls have shelf space and are more likely to collect dirt and debris where seeds can root. Compass direction could matter for some species. Moss is more likely to grow on the north side of a wall. The material, porosity, and composition of the wall, the climate of the region, and the history of human alteration are also major influences on what is found living on a particular wall.

One influential factor determining the vegetative character of walls is age. Algae and lichens are usually the first pioneers to become established. Vines rooted at the base produce, according to Darlington, the best "mural" vegetation on walls more than 150 years old. Our Ivy League schools come to mind as a good example. When walls get several centuries old and are left unattended, as with 2000-year-old walls built by the Romans in many parts of Europe, they become badly decomposed. Then shrubs and trees are more likely to grow from the wall ruins. Once a wall has structure in the form of vines or other plants, or as a result of crevices, animals begin to take up residence.

The ecological perspective of walls offers some new and intriguing prospects. School projects come to mind. Or just as a way to entertain yourself or children by looking at the world from a different angle.

While surveying the walls around my own yard, I once discovered an old concrete-block incinerator that has algae and lichens at the base near the ground as well as the signs that two animals had been there. A series of hollow, mud structures were made by females of the black wasps called dirt daubers. The mud nests contain spiders that have been captured and paralyzed by the dirt dauber before she lays her eggs. The first meal for the young wasps is the spider. The other animal sign was a spider nest covering a depression in one of the concrete blocks. The spider was missing. A victim of dirt daubers?

Along the brick wall of the house I found not only the obvious ivy and Virginia creeper but also a variety of small creatures. These included animals I could pin a name on--aphids, a millipede, and a caterpillar--and a few insects that I could not. Imagine the simple ecological principles and processes that could be examined as part of a science fair or classroom project. Do wood, brick, and concrete walls in an area differ in the number and kinds of plant and animal inhabitants? Does a shaded wall have more organisms than a sunny wall? How important are the wall's age, height, or position relative to ground vegetation in determining what grows on the wall?

If you want to sample a little wall ecology of your own, see how many different kinds of plants and animals you can find on walls in your neighborhood. Upon reflection, I realized some of my previous observations of lizards and snakes crawling, bats and treefrogs sleeping, and birds building nests all had one thing in common: their activities occurred on some sort of wall.

The thought came to mind: Why are so many plants and animals of flat regions already suited for living in a vertical ecosystem created by humans? Then I took a look at the trees and realized that the forests are full of walls. Plants and animals have been using them much longer than we have.

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