of ecological (also biological) variables b which interact in their dynamic t evolution: det dbt dt = f (et, bt)' dt = 9 (et, bt). Among the solution paths to this interaction between economic and ecologi cal variables, we look for those which are sustainable. Sustainable paths are typically those along which the values of certain key stocks are always pos itive, these key stocks being important environmental resources. The types of paths on which certain variables can be positive forever include station ary solutions with appropriate positivity conditions, or limit cycles or chaotic...