Furthermore, species differ in their individual responses to most types of stress. This complicates the interpretation and evaluation as to positive, negative, or neutral overall effects of any single stressor on the ecosystem and its attributes. Nutrient enrichment, toxic chemicals, and temperature, for example, are affected by hydrology and hydrodynamics, that is, the way tides and freshwater flow interact to determine the temporal and spatial variability of the physical environment of the estuary. Interactions among stressors and between stressors and ecosystem processes are common. The Suite of Stressors Affecting Water Quality, Habitat, and Sustainability of the San Francisco Bay Delta Is Complex and Interactive Therefore, while an immediate difficulty for some is that concern over some listed species has affected water diversions, “the problem” is harder to define biologically, and is perceived differently by various stakeholder groups, institutions, and other interests. In addition, species composition and environmental conditions in the delta have undergone large changes over the period. Some species have thrived, but others, including some listed as threatened or endangered under the federal Endangered Species Act and California’s Endangered Species Act, have declined dramatically. “The Problem” of the Delta Is Not a Single, Easily Definable ProblemĪlthough the ecosystem has been radically altered over the past 150 years, it nonetheless remains a biologically diverse and productive ecosystem. Although it would be convenient if one or only a few of these factors could be identified as the source of the “problem,” or even ranked with some certainty, it is not possible to do that, for at least three reasons: the “problem” is not easily definable, the suite of stressors is complex and interactive, and the ecosystem and its components do not react to any stressor as a single unit. Many environmental factors, including water diversions, affect the structure and functioning of biotic communities in the delta.
THE CHALLENGE: IDENTIFYING, DISTINGUISHING, AND RANKING INTERACTING ENVIRONMENTAL FACTORS AFFECTING THE BAY-DELTA ECOSYSTEM