Tj

few many

Number of species lost yes

Figure 14.2 Relationship between the fulfillment of a functional role and progressive loss of species when the relative importance of species within the group is unknown, (a) For systems with strong diffuse effects species within the functional group compensate for loss of other species in the group, and therefore the function is fulfilled until most or all species are removed, (b) For systems with strong keystone effects, bccause the function is primarily fulfilled by a single species, the functional consequences of the loss of a species is highly uncertain few many

Number of species lost all

Figure 14.2 Relationship between the fulfillment of a functional role and progressive loss of species when the relative importance of species within the group is unknown, (a) For systems with strong diffuse effects species within the functional group compensate for loss of other species in the group, and therefore the function is fulfilled until most or all species are removed, (b) For systems with strong keystone effects, bccause the function is primarily fulfilled by a single species, the functional consequences of the loss of a species is highly uncertain

14.4.3 Caveats

Three caveats should be highlighted in interpreting the above predictions that reflect our ignorance about critical features of most ecosystems. First, a conservative approach should be followed for systems in which we have not established the relative importance of functional groups and of individual species within the groups. The system should be treated as one with keystone dynamics because of the chance that loss of a randomly selected species could be dramatic (Figure 14.2b). Presuming either a high degree of compensation or an overall weak effect without understanding the dynamics of species interactions within the system invites surprises. A conservative course is further justified by noting that keystone-type systems are not rare, at least among well-studied communities in marine systems, and that the characteristics of a keystone species are sufficiently ambiguous to preclude their classification without experimental manipulations in situ (Menge et al. 1994; Power et al. 1996).

Second, these predictions apply to a single ecological role. Because all species perform multiple roles, it is conceivable that a single species may play a minor role for one particular function, yet be a strong interactor for another. To understand the total community impact of the loss of a species, all functional roles that a species plays should be considered.

Finally, the above predictions assume a constant environment. It is likely that human-caused alterations such as climate change will have differential effects on the species within functional groups. The consequences of such changes are impossible to predict, but it is likely that some species would be lost from the system. Systems characterized by diffuse effects, and therefore ones in which compensation occurs among species within a particular functional group, should be more resilient to change, i.e. to loss of critical ecosystem functions. In other words, diffuse effects imply compensation, which in turn confers insurance against loss of function (Chapin et al. 1995).

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