Lexikon

living organisms contribute to the weathering process in many ways.

Trees put down roots through joints or cracks in the rock in order to find moisture. As the tree grows, the roots gradually prize the rock apart.

Even the tiniest bacteria, algae and lichens produce chemicals that help break down the rock on which they live, so they can get the nutrients they need.

Many animals, such as these Piddock shells, bore into rocks for protection either by scraping away the grains or secreting acid to dissolve the rock.
Source: http://www.geolsoc.org.uk/gsl/education/resources/rockcycle/page3568.html