Strong Keystone Effects
Strong Diffuse Effects
Figure I4.I Overall influence of a functional group and the effects of specics loss. Solid bars represent the total effect of the group on the system, and open bars represent the relative effects of individual species within the group. Panels on the left (a,d,g) represent the system with all species within the group present, the middle panels (b,e,h) represent the system with one species removed, and the right-hand panels (c,f,i) represent the system with two species removed. The three types of systems are represented: a-c weak overall effect: d-f strong keystone cffect (sp. 3 is the keystone); g-i strong diffuse effect community patterns and processes: those with weak effects (Figure 14.1 a-c), those with strong keystone effects (Figure 14.1 d-f), and those with strong diffuse effects (Figure 14.Ig-i). These categories are not necessarily independent communities, but may gradually merge into one another along environmental gradients. Furthermore, while total species number is used here as a surrogate for the diversity within a functional group, relative abundance will also be important when evaluating changes in real systems (Estes et al. 1989). The three panels in each row represent the overall impact of all species, and the relative contribution of each species where the entire assemblage is unmanipulated (left panel), when a single species (sp. 1) is removed (center panel), and when another species (sp. 3) is also removed (right panel).
In the first class of response (Figure 14.1a-c), several species may contribute to an ecological role, but the overall effect of that role within the system is weak. Therefore, any number of species in the functional group can be removed from the system, without an effect on the system. In the second class of response (Figure 14.ld-f), the overall effect of consumers is strong but highly dependent on a single keystone species, and therefore the identity of the species lost from the system is extremely important. For example, compare loss of a non-keystone species. Figure 14.le, to loss of the keystone. Figure 14.If. in the third class (Figure I4.1g i), the overall effect of a group of species on a system is strong, but diffuse. Although each species in the group contributes significantly to the overall effect, loss of a species is compensated for by the remaining species. In a system purely of this type, only one or at most a few species of the group (or one or two of several functional groups) are necessary to perform the ecological role, and which species (or groups) those are, should not matter.
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