Predicting the response of a long-distance migrant to changing environmental conditions in winter

R A Stillman; E M Rivers; W Gilkerson; K A Wood; P Clausen; C Deane; D H Ward

doi:10.1002/ece3.11619

Predicting the response of a long-distance migrant to changing environmental conditions in winter

Ecol Evol. 2024 Jun 29;14(7):e11619. doi: 10.1002/ece3.11619. eCollection 2024 Jul.

Authors

R A Stillman¹, E M Rivers¹, W Gilkerson², K A Wood³, P Clausen⁴, C Deane⁵, D H Ward⁶

Affiliations

¹ Department of Life and Environmental Sciences, Faculty of Science and Technology Bournemouth University Poole UK.
² Merkel & Associates, Inc. San Diego California USA.
³ Wildfowl & Wetlands Trust Slimbridge, Gloucestershire UK.
⁴ Department of Ecoscience - Wildlife Ecology Aarhus University Aarhus C Denmark.
⁵ Department of Biology and Wildlife University of Alaska Fairbanks Fairbanks Alaska USA.
⁶ U.S. Geological Survey, Alaska Science Center 4210 University Drive Anchorage Alaska USA.

Abstract

Access to high-quality food is critical for long-distance migrants to provide energy for migration and arrival at breeding grounds in good condition. We studied effects of changing abundance and availability of a marine food, common eelgrass (Zostera marina L.), on an arctic-breeding, migratory goose, black brant (Brant bernicla nigricans Lawrence 1846), at a key non-breeding site, Bahía San Quintín, Mexico. Eelgrass, the primary food of brant, is consumed when exposed by the tide or within reach from the water's surface. Using an individual-based model, we predicted effects of observed changes (1991-2013) in parameters influencing food abundance and availability: eelgrass biomass (abundance), eelgrass shoot length (availability, as longer shoots more within reach), brant population size (availability, as competition greater with more birds), and sea level (availability, as less food within reach when sea level higher). The model predicted that the ability to gain enough energy to migrate was most strongly influenced by eelgrass biomass (threshold January biomass for migration = 60 g m^-2 dry mass). Conversely, annual variation in population size (except for 1998), was relatively low, and variation in eelgrass shoot length and sea level were not strongly related to ability to migrate. We used observed data on brant body mass at Bahía San Quintín and annual survival to test for effects of eelgrass biomass in the real system. The lowest observed values of body mass and survival were in years when biomass was below 60 g m^-2, although in some years of low biomass body mass and/or survival was higher. This suggests that the real birds may have some capacity to compensate to meet their energy demands when eelgrass biomass is low. We discuss consequences for brant population trends and conservation.

Keywords: Black Brant (Brant bernicla nigricans); eelgrass (Zostera marina); environmental change; food competition; individual‐based model.