Long-term monitoring and remote sensing reveal waterbird colony collapses with rising lake levels and multiple stressors.

Global climate change is expected to exacerbate existing environmental stressors and increasingly impact biodiversity. While likely widespread, changes in wildlife populations resulting from the interaction of climate change and other environmental stressors remain poorly understood. We investigated climate-mediated lake level fluctuations in coastal wetland ecosystems undergoing shoreline development and species invasions in the Laurentian Great Lakes. We evaluated the potential of multiple stressors to reduce ecological resilience and generate alternative stable states, by examining effects on a declining wetland habitat indicator species, the Black Tern (Chlidonias niger). We monitored one of the largest breeding Black Tern colonies in the Great Lakes over 8 years (2013-2020), recording population size, nest location and hatch success, clutch initiation date, and nest habitat characteristics (relative initial nest water depth, mean NDVI and % dense vegetation within a 7 and 12-m radius, and distance to open water and developed land). We applied a generalized linear mixed-effects model (GLMM) to determine hatch success response to biological and habitat predictors. We mapped the GLMM's predictors to quantify the spatial extent of hatch success likelihood and used a second GLMM to relate sub-colony breeding pair abundance to habitat variables derived from the spatial model. From 2013 to 2020, the mean breeding season lake level (May-July) increased by 1.03 m, coinciding with a 55% decline in hatch success rate and 77% decline in breeding pairs. Hatching success was negatively related to clutch initiation date, relative initial nest water depth, and % dense vegetation, and positively related to distance from the main lake and mean NDVI. The GLMM indicated breeding pair abundance was positively related to the area of sub-colony habitat with >50% hatch success likelihood. Our findings demonstrate rapid hydrological change, coastline development, and common reed (Phragmites australis) invasion reduced habitat migration potential and available nesting habitat, likely contributing to near abandonment of the colony. While future hydrological projections are uncertain, we expect Black Tern colony populations to fluctuate in response to lake levels in the region. Our study exemplifies how multiple stressors can undermine wetland resilience to climate change and present a conservation challenge for vulnerable wetland taxa.