One objective of the International Geosphere±Biosphere Programme is to provide a scienti®c basis for sustainable development policies. Land use change and ecohydrology are important components of this scienti®c basis, but predicting change is dicult because of the scale and complexity of the interactions between non-linear ecohydrological and socio-economic processes at dierent spatial and temporal scales. A systems framework, the Ecosystem Approach, has been developed to conceptualize these interactions for the purpose of providing information for sustainable development policy. The Ecosystem Approach combines the dynamics of the Holling ®gure-eight model Ð a conceptual model of dynamics that stresses discontinuous change and destruction as an internal property of the system Ð and the properties of self-organizing systems with the socio political aspects of decision making.
The Ecosystem Approach highlights the problems of managing change in complex systems when that change may involve unpredictable shifts to a dierent attractor. Although there are methods available to detect the occurrence of such shifts, both detection and modelling are complicated by the presence of semi-stable attractors. When a model or an ecosystem is on a semi-stable attractor, it may appear to remain stable for an extended period prior to changing as a consequence of inherent instabilities. When the shift to a new attractor occurs, it is quite sudden and unpredictable. A technical discussion on prediction under conditions of semistability and chaos is included because it enhances our understanding of the role of surprise in ecosystems, as well as the utility of simulation models.
The principles of the Ecosystem Approach are derived from the theoretical discussion and an example of a land use policy in the Huron Natural Area in south-western Ontario. These principles provide a clear role for scienti®c research, and particularly simulation modelling, within the larger context of policy and land use management.