Fishes, sea urchins, and other animals that graze on algal and coral tissues have 2 effects on a reef’s ecology.

1) Selective grazing supports the diversity of species. The diversity is kept up by blocking corals from overcrowding less dominant corals. In experiments where grazers are removed from corals, the results showed that diversity does indeed decrease. At the end of the experiments, great densities of only a few dominant corals remained, rare under ordinary circumstances in nature.

2) Grazers that clear tissue off hard substrates provide areas for algae and the larvae of sessile organisms to grow. Leslie S. Kaufman from John Hopkins University has found that some species of fish systematically kill plots of coral tissue so that “farms” of algae can grow. These fishes also chase away intruders, including sometimes humans. It is unknown how much damage is done to the reef from biological space clearing as compared to slumping and storms.

On this photo of fossil of Paleopleziastraea (Poland, Miocene) you can see many holes made by grazers.