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Chelation and Carbonation
Chelation occurs when organic substances (chelates) produced by living organisms eat away at rock surfaces. They do this by removing metallic ions. In turn, plants absorb these ions as nutrition. Chemical reactions occurring during absorption acidify the surrounding soil. Acidity speeds corrosion of rock. Carbonation is the action of carbonates upon minerals. Carbonates are more prevalent where living organisms are respiring, producing carbon dioxide. Mixed with rainwater, respiratory carbon dioxide produces carbonic acid. This dissolves rocks' calcium content. Limestone, marble and chalk all dissolve in acidic water.
Lichens
Lichens are the main colonizers of rock surfaces. Lichens cause weathering in four main ways. First there is physical penetration by fine, root-like hairs--hyphae and rhizines. Lichens then remove nutrients from the stone, adding to its deterioration. Then there is stress damage caused when lichens swell and contract in reaction to humidity and temperature. Creating a moist micro-climate enveloping the rock, lichens exacerbate chemical and physical weathering. In addition to organic salts and carbonic acid, many lichens produce oxalic acid. This dissolves calcium carbonate within the rock--etching the surface--and deposits calcium oxalate, altering the rocks' chemical composition.
Plants
Plants may grow in small cracks in rocks. Expanding roots extend these cracks and cause new fissures. This action of splitting rocks by expansion is caused both by growth and by imbibition--the swelling of materials as they absorb water. Seeds swelling with water prior to germination exert forces of many pounds per square inch. Roots growing beneath rock or man-made structures can cause upward pressure, creating cracks and uplift. Even small, soft plants have surprising power, pushing their way up through pavements. Shade from plants, the action of roots and the presence of dead plant material all increase water retention by the soil. Water's presence contributes to physical and chemical weathering. Respiration by roots and ion exchange during feeding acidify conditions, increasing carbonation.
Animals
Animals digging in search of food dislodge, displace and fragment rocks. Human activity--digging, climbing and walking--similarly impacts the landscape. Bioturbation is the moving and mixing of sediments by any burrowing animal--moles, tunneling bees or worms. Burrowing can move and uplift stones, exposing them to weathering by wind, rain or river water. Boring into rock, shale or mud cliffs, piddock clams are a significant cause of coastal erosion. According to the Netherlands Institute of Ecology, the entire surface of a sandy seabed passes through the intestines of lugworms every two months.