The dilemma that confronts biologists who are called on to give evidence in court is generally how to predict the impact of a development on a ecosystem. Biologists are expected to give definite responses, whereas they are trained to assess impacts on the basis of probability under certain conditions. Using an estuarine ecosystem as an example of a system that changes naturally over time, this paper develops a hypothetical scenario in which a proposed marina development would necessitate extensive reclamation of wetlands. The paper comments on various developments which have been proposed for estuarine ecosystems in New South Wales and offers some suggestions about how to resolve the dilemma facing biologists in court.