Lichens are the first colonist on exposed rock. They secrete acids that gradually break down the rock surface on which they grow, contributing to soil development. Lichens grow extremely slowly, adding only 0.1 mm to 10 mm of growth per year. Much of the mineral nutrition needed by lichens is obtained from the surrounding air and precipitation to which they are exposed. For this reason, lichens are extremely sensitive to poor air quality, especially sulfur dioxide pollution.