Response of fish biota to dams in the Lower Colorado River Basin: empirical findings and utility for predicting responses to climate and water use change
Report for Ecogeomorphology field class, UC Davis. Spring 2014
Over the past century, development of the Colorado River Basin for water resources changed a dynamic river into a managed system. Seasonality in flow, temperature, and turbidity is reduced, and formerly riverine habitats are now reservoirs. These changes have played out in the fish fauna. Native species have declined and non-natives have increased. The key alteration, relative to the past, is increased competition and predation, especially at juvenile stages, which is likely mediated by more stable flow and resource regimes. A dataset (SONFISHES) assembled by W. L. Minckley and covering 150 years of fish occurrence in the Lower Colorado River (below Glen Canyon Dam) has provided insight into patterns in extirpation among native fishes and range expansion among non-native fishes. Several studies have combined these data with measures of extinction risk to explain how range fragmentation and species’ traits correlate with observed extinctions or threats in native species. Another study analyzed range shifts within a strategy space of potential fish life histories (originally introduced by K. O. Winemiller and K. A. Rose) to understand how human activity has created and removed ecological niches. Together, these studies support a causal link between environmental and faunal change. This strongly suggests that continuing the status quo will result in further extirpation and extinction of native fishes. I argue, however, that current knowledge is not yet sufficient to quantitatively forecast the magnitude and timescale of the response in fish biodiversity to any projected future environment. Such forecasting requires a model that quantitatively relates the environment to persistence in the strategy-space of fish life histories. Unfortunately, we do not need such a model to make the qualitative prediction: threatened “big-river” fishes will eventually go extinct without intervention.
Note: a version of this paper prior to peer review was published at the course website (link below).