Changes in aquatic bird communities of terminal lakes in the western Great Basin in relation to water availability

Background/Question/Methods

Terminal lakes of the Great Basin provide a critical migration stop-over resource for a large variety of aquatic birds. Over past decades, droughts, water diversions, climate change, and associated changes in salinity have affected the resource value of some terminal lakes for migrating aquatic birds in the western Great Basin. In particular, the water level of Walker Lake, near Hawthorne NV, has fallen over 100 feet over the last century. This has resulted in a suite of inter-related changes in lake’s chemistry and biology, including declines in the lake’s historical fishery. Studies suggest that Walker Lake is nearing a “tipping point” when these changes cause a major reorganization of the biological communities.  In this study, we ask (1) how different species of piscivorous birds have been affected by the changes at Walker Lake and (2) how different annual water-availability scenarios have affected migrating waterfowl over five decades. To address these questions, we use the results of our waterbird surveys (2012 – 2014) in four western terminal lakes including Walker Lake, data made available to us by Nevada Department of Wildlife from aerial waterfowl surveys over the past 50 years, and locally collected climate data for the same time period.

Results/Conclusions

Our results indicate that the tipping-point conditions at Walker Lake have caused a major shift and simplification of migrant species composition, resulting in losses in piscivorous species populations, particularly those specializing on small fish sizes, such as Common Loon (Gavia immer), and an increase in saline-lake-tolerant species, such as Eared Grebe (Podiceps nigricollis). The avian species composition now approaches a similar scenario as recorded at the hypersaline Mono Lake, California. In the waterfowl trends, we observe a species-dependent migrant population response to various water availability scenarios. Here, we highlight a representative set of waterfowl species and their responses to locally obtained snowpack and climate condition metrics, as well as differences in population trends in different terminal lakes. The long-term trends in migrating waterfowl populations vary significantly among different terminal lakes and different migrant species, and using water availability data, we determine to what degree different species are influenced by water deliveries to the lakes, and how water deliveries may be optimized by resource management to maintain migrant populations of the species assemblage in the region.