Ecological risk assessment is a process that evaluates the likelihood that adverse ecological effects may occur or are occurring as a result of exposure to one or more stressors (EPA 1992). An ecological risk assessment commonly evaluates the potential adverse effects that human activities have on the plants and animals that make up ecosystems, and has developed, in recent years, as an alternative assessment paradigm (EPA 1992; SUTER 1993a). It estimates likelihoods of specific ecological effects equivalent to risks of cancer or birth defects estimatedby human health risk assessors (SUTER 1993a). However, since problem formulation, which includes the scope and objectives of the assessment as well as a preliminary description of exposure and ecological effects, ecological risk assessments have rarely considered the often complex composition of real landscape habitat mosaics in terms of vegetation cover types and land use, and of their functional organization. The most important attributes of an ecological risk assessment are clear assessment endpoints, which represent the necessary premise for supporting further economic valuation. Risk assessment endpoints are explicit expressions of the environmental values that are to be protected, operationally defined by ecological entities and their attributes that best represent integrity for that system (EPA 1992).