by Whit Gibbons

May 29, 2011

Aside from the mountains, any place within 300 miles of where I live reached temperatures approaching 100 degrees last week. During such hot weather, nature-watching can be disappointing at midday. Birds are less active. Turtles stop basking on logs. Lizards retreat to shady out-of-sight spots. Amphibians have gone underground. While contemplating that truth, I remembered a long-ago column about a habitat that will always yield some life to observe.

The habitat is in my backyard and everyone's neighborhood. It is a habitat we see daily but seldom think of in ecological terms. I am referring to walls. Yes, walls. Like the sides of houses and sheds or a fence around a garden. Walls make up a significant portion of the world's terrestrial habitats. Arnold Darlington, in his 1981 book titled "Ecology of Walls," 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 shadiest 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 is the age of the wall itself. Algae and lichens are usually the first pioneers to become established. According to Darlington, vines rooted at the base produce the best "mural" vegetation on walls that are more than 150 years old, such as at the Ivy League schools. 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. I once suggested that wall ecology would make ideal science fair projects. The hypothesis would relate to biodiversity and be stated something like: plants and animals will live on any available space if given enough time, even on a vertical wall. Included would be fences, concrete incinerators, and even the sides of trees, which are natural walls. Questions can be posed and answered. 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?

One feature of a science fair project involving the ecology of walls that will appeal to some students is that there will be plenty of time to procrastinate. A wall ecology project could be completed one or two weeks before it is due, maybe in a day under desperate conditions. But imagine the data set a student who starts now could accumulate through summer and into fall to make the point that walls are important to the biodiversity of an area.

Examining walls around your home can even be a way to entertain yourself or children by observing the world from a different perspective. See how many different kinds of plants and animals you can find on walls in your neighborhood. It was too hot during my wall search last week to expect to find animals, but upon reflection, I realized some of my previous observations of lizards and snakes crawling, bats and treefrogs sleeping, and birds building nests had been activities that occurred on some sort of wall. Walls are much more interesting ecologically than most people would think.

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