Line | While the most abundant and dominant species |
| within a particular ecosystem is often crucial in |
| perpetuating the ecosystem, a “keystone” species, |
| here defined as one whose effects are much larger |
(5) | than would be predicted from its abundance, can |
| also play a vital role. But because complex species |
| interactions may be involved, identifying a keystone |
| species by removing the species and observing |
| changes in the ecosystem is problematic. It might |
(10) | seem that certain traits would clearly define a species |
| as a keystone species; for example, |
| Pisaster ochraceus is often a keystone predator |
| because it consumes and suppresses mussel |
| populations, which in the absence of this starfish |
(15) | can be a dominant species. But such predation on a |
| dominant or potentially dominant species occurs in |
| systems that do as well as in systems that do not |
| have species that play keystone roles. Moreover, |
| whereas P. ochraceus occupies an unambiguous |
(20) | keystone role on wave-exposed rocky headlands, |
| in more wave-sheltered habitats the impact of |
| P. ochraceus predation is weak or nonexistent, |
| and at certain sites sand burial is responsible for |
| eliminating mussels. Keystone status appears to |
(25) | depend on context, whether of particular |
| geography or of such factors as community |
| diversity (for example, a reduction in species |
| diversity may thrust more of the remaining species |
| into keystone roles) and length of species |
(30) | interaction (since newly arrived species in particular |
| may dramatically affect ecosystems). |