Short-term effect of a toxicant on scope for change in ascendency in a microcosm community

A need in the field of toxicity testing is the development of parallel tests applicable to all levels of biological organization: single-species tests alone cannot predict effects at ecosystem levels. Scope for change in ascendency (SfCA) is a comparative index applicable to the short-term assessment of stress effects on any biological system. The SfCA of a community, defined by analogy to scope for growth of an organism, is the balance of ascendency (network size and organization) of population inputs and outputs. It takes into account the interrelated processes of growth and development, both of which can be affected by stress factors. To illustrate this approach, the effect of atrazine on SfCA of a simple epilimnetic microcosm community was investigated. Simultaneous, short-term measurements of all carbon flows between populations were made and SfCA was calculated. Compounded effects of stress on populations, measured as a relative drop in community SfCA in the presence of atrazine, were observed within 2-3 weeks. This indicates that SfCA may be useful as a comparative index for the short-term assessment of perturbation effects. A refinement of the methodology for measuring resource flows at levels other than the organism would allow full advantage of this assessment tool to be taken.