Assessing Ecological Impact Assessment: Lessons from Mono Lake, California

Because of its high salinity and alkalinity, Mono Lake, in eastern California (USA), is a relatively simple ecosystem. It has become the focus of an environmental controversy over the effects of 50 yr of diversions of water from tributary streams to supply water to Los Angeles. Diversions lowered the lake level, increased the salinity, changed the availability of aquatic habitats, and altered the configuration of the shoreline and of islands that support breeding colonies of gulls. We consider (1) how two independent panels of experts synthesized scientific information on the lake ecosystem to assess the environmental consequences of these changes, and (2) how the findings of these groups influenced policy decisions and how well subsequent changes in the lake matched expectations. Despite differences in composition and approach, the two panels reached generally similar conclusions. These conclusions have been a major component of legal activities and the development of management plans for the lake and basin ecosystem. Both panels concluded that, because of the simplicity of the lake ecosystem, ecological consequences of changes in lake level and salinity associated with continuing diversions were likely to be unusually clear‐cut. At certain lake levels these changes would be expected to alter algal and invertebrate populations and the populations of aquatic birds that feed upon them or to disrupt breeding activities in gull colonies. Projections about when critical lake levels might be reached, however, have not been met. This is largely because stream flows into the lake have been altered from recent historic patterns by the cessation of water diversions due to governmental and legal actions (prompted in part by the panels' findings) and by a prolonged drought. These events illustrate the difficulty of projecting a timetable for environmental changes, even in simple and well‐studied ecosystems.