If feedback loops determine the stability properties of dynamic systems, it is exogenous factors like temperature, rainfall, soil type, topography, etc. that set the stage on which these processes occur. Exogenous factors can affect the magnitude, or even the sign, of the functional relationships between variables, and this can alter the stability properties of the system. For example, gradual increases in temperature due to global warming could make plants more susceptible to disease, and this could reduce stability by inducing outbreaks of disease. In contrast, normal variations in climate (weather) merely disturb the system temporarily from its stable state because the average weather does not change. We should realize, however, that disturbances of metastable systems can result in major changes in the dynamics of the system.

We recognize two major kinds of exogenous processes; (1) those that cause changes in the stability structure of the ecosystem, which we call exogenous forcing functions, and (2) those that only displace the variables from their steady states at unpredictable times and do not affect the stability structure, which we call exogenous random functions.

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