This classification helps us predict how loss of spccies will affect community structure and, ultimately, ecosystem properties. While predictions are made for cases of the loss of a single species, similar predictions should hold in which the high diversity of the community forces consideration of interactions at the level of entire functional groups. First, the consequences of diversity changes under conditions in which consumers have an overall weak effect should be relatively predictable. Loss of any or all predator species will not have a major overall impact on the system. Second, for strong diffuse effects, the community structure and ecosystem-levc! processes may be largely retained provided one, or at most a few, species remain in the system. If species are fully compensatory, it should not matter which species remain to perform the role. The impact will be large if all, or perhaps most, of the species in the group are lost because the overall effect of the group is strong (Figure 14.2a). Third, the consequences of species loss in systems with strong keystone consumers are highly dependent on the identity of species lost from the community. The loss of species other than the strong interactor will have little if any effcct, whereas the loss of the keystone species will lead to major changes. If in such a system the relative importance of different species is not known, there will be a high degree of uncertainty regarding the functional consequence of species loss (Figure 14.2b).
To summarize, the loss of function will occur when (1) a keystone species is removed, or (2) all or mostly all species are removed from a strong but diffuse group. Furthermore, little change in system function will occur when (1) any number of species are removed from a group that has a weak overall effect, (2) non-keystone species are removed from a keystone system, or (3) some but not all species are removed from a group that has strong but diffuse effects.
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