The number of species or their identity Keystones again

Indeed, it is clear that the whole concept of a keystone species (see Section 20.2.6) is itself a recognition of the fact that the effects of a disturbance on structure or function are likely to depend very much on the precise nature of the disturbance - that is, on which species are lost. Reinforcement of this idea is provided by a simulation study carried out by Dunne et al. (2002), in which they took 16 published food webs and subjected them to the sequential removal of species according to one of four criteria: (i) removing the most connected species first; (ii) randomly removing species; (iii) removing the most connected species first excluding basal species (those having predators but no prey); and (iv) removing the least connected species first. The stability of the webs was then judged by the number of secondary extinctions that resulted from the simulated removals, such extinctions occurring when species were left with no prey (and so basal species were subject to primary but not secondary extinction). In the first place, the robustness of community composition in the face of species loss increased with connectance of the communities - further support for an increase in community stability with complexity. Overall, however, it is also clear that secondary extinctions followed most rapidly when the most connected species were removed, and least rapidly when the least connected species were removed, with random removals lying between the two (Figure 20.13). There were, moreover, some interesting exceptions when, for example, the removal of a least connected species led to a rapid cascade of secondary extinctions because it was a basal species with a single predator, which was itself preyed upon by a wide variety of species. This, finally in this section, reminds us that the idiosyncrasies of individual webs are likely always to undermine the generality of any 'rules' even if such rules can be agreed on.

between resilience and energy input per unit standing crop. This seems to depend in part on the relative importance of hetero-trophs in the system. The most resilient system, the pond, had a biomass of heterotrophs 5.4 times that of autotrophs (reflecting the short life and rapid turnover of phytoplankton, the dominant plants in this system), whilst the least resilient tundra had a hetero-troph : autotroph ratio of only 0.004. Thus, the flux of energy through the system has an important influence on resilience. The higher this flux, the more quickly will the effects of a perturbation be 'flushed' from the system. An exactly analogous conclusion has been reached by DeAngelis (1980), but for nutrient

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