The Marginal Nature of Walls

The best examples of the tenacity and opportunism of marginal nature are mosses, lichens, and plants that grow on walls. Some bird species like cliff swallows and barn swallows attach their mud nests to walls and bridges.

Urban ecologists study the wall ecology of older walls, and in Europe and Asia they have been studied as unique habitats - see Segar, Ecological Notes on Wall Vegetation (1969)