The transition from evaluating the potential risk from chemical exposure via single species toxicity tests to multispecies tests is still evolving and the subject of much discussion, particularly in regard to regulatory decisions. The underlying premise of all the chapters presented in this section is that for regulatory agencies to receive maximum information from complex surrogate ecosystems requires that they be evaluated in a more integrated, holistic fashion, rather than as a profusion of single-species tests. The chapter discusses the ecosystem stability can be evaluated through the compartmentalization and connectance of food webs. In order to provide risk assessments of pesticides on natural ecosystems using data collected from surrogate manmade ponds, we need to understand how the structural and functional components of the test systems relate to natural systems. Modeling approaches, like those discussed in this chapter, may ultimately prove to be useful in extrapolating from mesocosms to natural ecosystems.