Abstract

  This paper attempts to fit the history of pollution and recovery of Lake Orta, Italy, to the ecological theory of resilience, or the ability to restore structure following perturbation. The evolution of this lake can be represented by a model of a folded surface. In the case of Lake Orta, a disturbance, represented by the onset of pollution in 1926, determined the transition to conditions of an extreme environment. If an opposite change is imposed on the environment (in this case, the liming of the whole lake), the model predicts that the ecosystem will not simply revert to the pathway that it has followed up to this point but will evolve in a different way (hysteresis). Ultimately, the system could arrive either at similar (but never completely identical) conditions to those a/sting prior to the disturbance, or at some completely different state.
  The variations of phytoplankton, zooplankton, benthos, and fish in Lake Orta have been summarized by applying a stochastic exponential population growth model This model succeeds in showing how resilience and resistance combine together, offering a fairly good description of both the pollution and the recovery of the lake. It can be concluded that at least nothing contradicts the hypothesis that past changes can be explained by the ecological theory of resilience and resistance.

Keywords: resistance, liming, ecosystem recovery, stochastic exponential population growth mode