Seabirds have long been viewed as sentinels of large-scale change in aquatic ecosystems. They are highly visible, uniquely accessible, and due to dietary generalization, some populations can respond rapidly to environmental shifts, from species invasions to climate change. Looking back at seabirds’ response to ecosystem alteration can also demonstrate how populations adapt (or fail to adapt) to ongoing human-induced change. The Laurentian Great Lakes have experienced dramatic changes in the 20th and 21st centuries, including the introduction of dreissenid mussels and sea lamprey, loss of wetlands, and pollution. Bulk carbon and nitrogen stable isotope data (δ13Cbulk and δ15Nbulk values) suggest that throughout the Great Lakes as a whole, herring gulls (Larus argentatus) have shifted their diet to include more terrestrial and lower trophic level aquatic foods, likely in response to decreases of their original fish prey. For this study, we use molecular markers of animal diet, including mercury (Hg) concentration, bulk stable isotope analysis, and amino acid-specific stable nitrogen analysis of herring gull eggs collected from two Great Lakes colonies (Middle Island, Lake Erie and Double Island, Lake Huron) to investigate the pattern of dietary shifts over a 30 year time period (1980-2012).
Results/Conclusions
Our results from Double and Middle Island colonies show contrasting patterns in both ecosystem change and gull response. The Double Island gulls decrease in apparent trophic level over time, with variation between individuals remaining constant (decreasing δ15NGlu-Phe; no significant change in variability of δ15Nbulk and δ13Cbulk through time). The lack of change in δ15NPhe and decline in δ13Cbulk in Double Island gulls suggests that during the period of egg formation, aquatic prey still dominate their diet, but that the population has increased their reliance on benthic food webs. On Middle Island, herring gulls show limited evidence of trophic decline up to the 2000’s alongside an expansion of their isotopic niche (increased variance in both δ15Nbulk and δ13Cbulk through time). The differences between Double and Middle Island colonies likely result from variation in oligotrophy, nutrient inputs, and food web structure between Lake Huron and Lake Erie, with herring gull populations adapting to environmental change at a local scale. These data demonstrate the complexity of dietary shifts in generalist populations over broad spatial and temporal scales and the value of molecular techniques in understanding the adaptability of animal populations.