Editing Ecology (section)

=== Keystone species ===
{{main|Keystone species}}
[[File:Sea otters holding hands, cropped.jpg|thumb|[[Sea otter]]s, an example of a keystone species]]
A keystone species is a species that is connected to a disproportionately large number of other species in the [[food-web]]. Keystone species have lower levels of biomass in the trophic pyramid relative to the importance of their role. The many connections that a keystone species holds means that it maintains the organization and structure of entire communities. The loss of a keystone species results in a range of dramatic cascading effects (termed ''trophic cascades'') that alters trophic dynamics, other food web connections, and can cause the extinction of other species.<ref name="Fisher06"/><ref name="Libralato06"/> The term keystone species was coined by Robert Paine in 1969 and is a reference to the [[Keystone (architecture)|keystone]] architectural feature as the removal of a keystone species can result in a community collapse just as the removal of the keystone in an arch can result in the arch's loss of stability.<ref>{{Cite journal |last=Paine |first=R. T. |date=January 1969 |title=A Note on Trophic Complexity and Community Stability |journal=The American Naturalist |volume=103 |issue=929 |pages=91–93 |doi=10.1086/282586 |bibcode=1969ANat..103...91P |s2cid=83780992 |issn=0003-0147}}</ref>

[[Sea otter]]s (''Enhydra lutris'') are commonly cited as an example of a keystone species because they limit the density of [[sea urchins]] that feed on [[kelp]]. If sea otters are removed from the system, the urchins graze until the kelp beds disappear, and this has a dramatic effect on community structure.<ref name="Mills93"/> Hunting of sea otters, for example, is thought to have led indirectly to the extinction of the [[Steller's sea cow]] (''Hydrodamalis gigas'').<ref name="Anderson95"/> While the keystone species concept has been used extensively as a [[Conservation biology|conservation]] tool, it has been criticized for being poorly defined from an operational stance. It is difficult to experimentally determine what species may hold a keystone role in each ecosystem. Furthermore, food web theory suggests that keystone species may not be common, so it is unclear how generally the keystone species model can be applied.<ref name="Mills93"/><ref name="Polis00"/>