A graph–theoretic analysis of relationships among ecosystem stressors

The development of strategies to reduce risk to ecosystems has been proposed as a centerpiece for environmental protection. An essential prerequisite to the development of such strategies is the identification and evaluation of those factors (called stressors) which cause stress to ecosystems. Stress refers to external forces which threaten an ecosystem's viability or integrity, that is, the ability to maintain its organizational structure and function. New analytical tools are needed to assist with the difficult task of ecosystem risk assessment. These tools must be sophisticated enough to enable an analyst to do more than focus on single stressor–single assessment endpoint pairs. By an assessment endpoint is meant an environmental value or ecosystem service to be protected or restored. In this paper we develop and describe analytical tools based on the mathematical subdiscipline of graph theory which can be used to gain insights into the interactive relationships which exist among the stressors themselves. The manner in which these stressors are connected to each other and interactive paths among the stressors which tend to augment or diminish the impacts of certain stressors upon the ecosystem can be delineated. When used in conjunction with analytical methods which deal more directly with the impacts of stressors upon the ecosystem itself, the results derived with the aid of these tools can be used to more clearly identify clusters of stressors which should be the focus of environmental protection activities. Applications of these analytical tools to Green Bay, Lake Michigan and the St Croix National Scenic Riverway ecosystems in the United States are discussed.