Erosion

The wearing away and transport of rock, soil or reef material by wind, water, ice or living organisms. Erosion is usually pictured as a purely physical process, but living organisms drive a significant share of it in both directions. Plant root systems bind soil and slow water's ability to carry it away, so the loss of forest or grassland cover is one of the most reliable predictors of accelerated soil erosion in a landscape; on a coral reef, by contrast, living organisms are themselves the agents of erosion, a process reef scientists call bioerosion, as parrotfish graze algae directly off the coral skeleton, taking mouthfuls of limestone with it and excreting it as sand, and boring sponges and worms tunnel into the reef structure from within. A healthy reef's rate of new coral growth normally outpaces this bioerosion; where it does not, the reef structure itself shrinks even without a single storm.