Many organisms build, modify or destroy physical structures in the environment.
For example, both beavers and the Army Corps of Engineers build dams. Beaver dams and many other physical structures have important ecological effects on other species because these physical structures control the amount of resources that other species can use. Rock-eating snails in the Negev Desert control the amount of soil for plants. Desert isopods control soil erosion and remove salts that decrease soil fertility for plants. Desert porcupines dig pits that trap water and seeds, making an ideal place for annual plants to grow.