The Pacific marine heatwave did not impact all marine bird species equally

Predator species had different responses during the marine heatwave. Those that relied on species that experienced population declines were impacted more than those that had relatively stable food sources. Seaducks were minimally impacted because their benthic invertebrate prey base remained stable.

Photo courtesy of Nicole LaRoche

The Pacific marine heatwave (PMH) was an intense environmental disturbance that negatively affected many organisms across the North Pacific. Marine mammals and marine birds that rely on pelagic prey species, such as forage fish, experienced large-scale declines that, in many cases, have persisted through the years following the PMH. Declines in the availability of forage fish to marine predators was the likely culprit in the wide-spread loss of marine bird and mammals. However, not all marine species fared the same. Some marine birds, such as sea ducks and some shorebirds rely on benthic invertebrates as prey. These benthivores marine birds remained relatively stable throughout and following the PMH. In contrast to forage fish, benthic invertebrates were not significantly impacted by the heatwave and in some cases, even increased in abundance throughout our study areas. This finding suggests that across marine food webs, predator responses to the heatwave were driven by differing trophic pathways. Because sea ducks primarily feed on benthic invertebrates, and those populations remained stable during the heatwave, the sea duck population were not impacted. Measuring varying responses to inform which species may or may not respond to certain types of perturbations is important as marine heatwaves and other anthropogenic stressors are expected to increase in frequency.

Lack of strong responses to the Pacific marine heatwave by benthivorous marine birds indicates importance of trophic drivers

Abstract

The Pacific marine heatwave (PMH) of 2014-2016 was an intense, long-lasting environmental disturbance expressed throughout the north Pacific. While dramatic consequences of the PMH on pelagic food webs have been well documented, effects on nearshore food webs, i.e. those based on macroalgal primary productivity, benthic invertebrate intermediate consumers, and specialized benthivorous top predators including some marine birds, are not well understood. We conducted summer and winter coastline marine bird surveys in 2 National Parks in the northern Gulf of Alaska from 2006 to 2022. We evaluated changes in abundance of benthivorous marine birds in relation to the PMH, after accounting for effects of season and region. We also evaluated changes in abundance of nearshore benthic invertebrate prey to allow specific consideration of a prey-based mechanism for effects of the PMH across food webs. We found that benthivorous marine birds, consisting of sea ducks and shorebirds, did not show a strong response to the PMH, unlike significant effects demonstrated by piscivorous birds in pelagic biomes. In contrast to extreme reductions in quantity and quality of forage fish documented elsewhere, we found that common benthic invertebrate prey abundance remained relatively stable. Our results support the hypothesis that, across food webs, top predator responses to the PMH were driven primarily by how and whether the PMH affected their prey availability. These findings show how a large-scale environmental perturbation affects biological communities differently through various trophic pathways, which provides insight into ecosystem resiliency and can inform management strategies in the face of persistent climate change.

Robinson, B., H. A. Coletti, B. Ballachey, J. L. Bodkin, K. Kloecker, S. B. Traiger, and D. Esler. 2023. Lack of strong responses to the Pacific marine heatwave by benthivorous marine birds indicates importance of trophic drivers. Marine Ecology Progress Series.