Cooperative Fish and Wildlife Research Units Program: all
Education, Research and Technical Assistance for Managing Our Natural Resources

Identifying relationships between population dynamics and system drivers such as weather, climate, habitat, and resource availability is critical in understanding how populations respond to changing conditions. In coastal areas, the transfer of nutrients across the marine and terrestrial interface leads to further complexity. Nesting populations of bald eagles along the Pacific coast, although terrestrial, are largely dependent on marine resources during the breeding season and therefore represent a good focal species for understanding linkages of nutrients between terrestrial and marine systems. However, coastal areas are susceptible to a variety of perturbations, from both land and sea, which can be further exacerbated by climate change. For example, the northeast Pacific Marine Heatwave (PMH) of 2014-2016 had wide-ranging impacts on the marine ecosystem, providing a convenient natural experiment for further exploring how marine conditions can impact terrestrial systems. We used a spatially-explicit multi-state occupancy modeling framework to analyze >30yrs of bald eagle nest occupancy data collected in 4 large national parks along a coastal-interior gradient in Alaska, USA. We assessed occupancy state in relation to weather conditions, salmon abundance, access to alternate prey resources, and the PMH event to help elucidate the factors affecting bald eagle occupancy dynamics over time. We found that occupancy probability was higher in areas where prey resources were concentrated (e.g., near seabird colonies, where bears facilitate access to salmon carcasses), and the probability of success was higher during warmer, drier springs with higher-than-average salmon abundance. After the onset of the marine heatwave, success declined in the areas most dependent on non-salmon marine resources. These findings confirm the importance of spring weather conditions and access to salmon resources during the critical chick-rearing period, but also reveal that marine heatwaves may have important secondary effects through a reduction in the overall quantity or quality of prey available to bald eagles. Given ongoing warming at high latitudes and the expectation that marine heatwaves will become more common, our findings are useful for understanding ongoing and future changes in the transfer of nutrients from marine to terrestrial ecosystems and how such changes may impact terrestrial species such as bald eagles.