The impacts of record lake-level rise and interacting stressors on black tern (Chlidonias niger) nesting in Lake St. Clair, Michigan

Jennifer L. Fuller, Karen M. Alofs, Johannes Foufopoulos and Andrew D. Gronewold, School for Environment and Sustainability, University of Michigan, Ann Arbor, MI

Background/Question/Methods
Global climate change is expected to interact with existing environmental stressors and increasingly impact wildlife populations. Few case studies of this phenomena exist however, especially in wetland ecosystems. The black tern (Chlidonias niger) is a migratory shorebird that nests on floating vegetation in large freshwater wetlands. Great Lakes breeding colonies have experienced heavy population losses and abandonment and have fallen by almost 90% since 1991. Like many marshbird species, black terns face multiple stressors, yet the exact scope of these threats remains unclear. Record lake-level fluctuations generated by global climate change is likely both important and ill-understood. This study focuses on one of Michigan’s largest black tern colonies, located in Lake St. Clair. To understand the potential drivers of these declines and inform species management, we analyzed long-term hatching success data regarding both proximate ecological, and large-scale hydrological and geospatial habitat features. Landcover and bathymetric data were collected using remote sensing classification and were evaluated relative to nest success via stepwise binomial general linear models. Results were then applied to land cover maps to estimate change in habitat characteristics tied to nest vulnerability.
Results/Conclusions
Between 2013 and 2020, the colony’s population and nesting success collapsed in tandem with a record rise in lake-levels. Nests with significantly lower hatching success were surrounded by deeper water, more dense, monotypic vegetation, and were closer to the wave-exposed open area of the main lake. All these characteristics shifted unfavorably with rising lake levels, leading to progressively reduced nesting habitat availability, a 56% reduction in hatching success and a 77% estimated population decline. The ideal combination of interspersed native plant communities and floating nest material for successful breeding was unable to shift upland as the lake margins were either developed or invaded by Phragmites australis. Subjected to progressively deeper and unstable habitat, nests likely failed more frequently due to inclement weather and aquatic predators. Our results demonstrate that the interaction between climate change-driven lake-level rise, invasive species and coastal development are increasingly eliminating safe nesting habitat for black terns. The resilience of other coastal wetlands under these circumstances is worth investigating more broadly in Great Lakes. We conclude that management must account for multiple stressors in mitigating habitat loss and protect as much wetland reserves as possible so black terns and other marshbirds can adjust to continued hydrologic extremes.