Dynamically Fragile versus Dynamically Robust Communities

Communities that are stable only under a limited range of environmental and ecological conditions are considered dynamically fragile, whereas communities that are stable over a wide range of conditions are dynamically robust. Figure 5.2 is a two-dimensional representation of the difference between a dynamically fragile and a dynamically robust system. Of course, a hypervolume with numerous axes would more adequately represent natural environmental conditions. A fragile community is one in which only a limited set of variables would keep the system stable (figure 5.2A), whereas a robust community can stay stable under a greater range of conditions (figure 5.2B). Again, these are subjective and scale-dependent terms, but they have significance when comparing communities and the effects of disturbances on communities under different environmental regimes.

In examining communities in this way, it has been hypothesized that predictable and stable environments will allow for the persistence of dynamically fragile communities, whereas environments with highly variable conditions would favor the presence of dynamically robust associations. It has also been hypothesized that stable environments would favor ^-selected organisms, which are resistant to perturbations but may have low resilience. On the other hand, highly variable environmental conditions could lead to r-selected populations, which are predicted to have low levels of resistance and higher resilience.

Although formal definitions for generalists and specialists have not been well developed, one could apply the definition of dynamically robust and dynamically fragile not only to communities but also to species. Generalist taxa can be considered as those that are dynamically robust, whereas specialist taxa can be considered as those that are dynamically fragile. This concept may be related to the idea that generalists are more long-lived than specialists, as discussed subsequently.

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