Resilience of an ecological system relates more to the system's functioning than to the stability features of its constituent populations. Moreover, resilience cannot be easily observed in natural systems, and there seems at present to be no direct relationship, for example, between resilience and ecosystem diversity. As an example, tropical terrestrial ecosystems are characterized by stable and diverse populations having low resilience, whereas temperate ecosystems in areas with low diversity can show greater resilience. Some scientists argue that coastal and estuarine ecosystems, typically of low species diversity since they experience periodic physical changes and have a high degree of organism mobility, are highly resilient because the low species diversity is compensated by high functional group diversity. The importance of response diversity in maintaining lake resilience has been acknowledged by experiments in which lakes are under stress by toxic chemicals and acidification. Ecosystem processes persist in lakes with low levels of stress because of the variable response of plankton and bacteria species. Populations of stress-sensitive species decline, but compensatory growth of less sensitive species results in minimal changes in ecological processes. At higher stress levels, response diversity deteriorates and the smaller populations cannot sufficiently maintain ecosystem processes. Some modeling studies have shown that biodiversity provides cross-scale resilience. In this manner, species combinations form an integrated set of reinforcing influences that spread risks and benefits to the system so that overall consistency in performance is maintained independent of variability in individual species.
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