Abstract
Margaref asserted that Ashby's law of requisite variety-the more complexity and intensity of interaction among elements in the system increase, the more stable the system becomes-would be valid in the ecological systems based on his practical studies of ecological systems.
This paper proposes an index for stability in the sense that the ecological system is more stable when less influenced by the envirmental fluctuations. A concrete example is given to support Margaref's assertion.
Our model is a modified Volterra predatorprey model including the hysterisis effect and two inputs which are the environmental input and the human control.
To prepare for studying the stability we also discuss the statistical relations between the environment and the population, the population response for the control input, and the control of macro growth rate.
