Keystone Species

Some keystone species are important because they play a crucial role in nutrient cycling, particularly if that function cannot be carried out by other species. Good examples of these sorts of keystone species are those that play unique roles in the nitrogen cycle, in particular in the ecologically important process known as nitrification.

Nitrification is a process during which highly specialized bacteria oxidize the positively charged ion ammonium (NH₄₊) to the negatively charged ion nitrite (NO₂-) and then to nitrate (NO₃-). Nitrification is a very important component of the larger nitrogen cycle because most plants prefer to utilize nitrate as the chemical form by which nitrogen, an essential nutrient, is absorbed from soil or water. Because this preference for nitrate is true of most agricultural species of plants, nitrification is also an ecological function that is important for human welfare.

Nitrification occurs in two discrete steps. The first stage is the oxidation of ammonium to nitrite, a process that is only carried out by specialized bacteria in the genus Nitrosomonas. The nitrite formed is then quickly oxidized to nitrate by other specialized bacteria in the genus Nitrobacter. Neither Nitrosomonas or Nitrobacter are abundant in soils or water. These microorganisms can, however, be viewed as keystone organisms because nitrification is such an important aspect of the nitrogen cycle in ecosystems, and it is only carried out by these bacteria.