Bottom-up food-web impacts of commercial-scale resource harvest on coastal bird communities

Elliot M. Johnston, Laura A. Braun, Hannah N. Mittelstaedt and Brian J. Olsen, University of Maine, Orono, ME, Hannah Webber, Schoodic Institute at Acadia National Park, Winter Harbor, ME, Jessica Muhlin, Maine Maritime Academy, Castine, ME, Amanda J. Klemmer, School of Biological Sciences, University of Canterbury, Christchurch, New Zealand

Background/Question/Methods
Wild harvest of seaweed is increasing on a global scale as human demand grows for many seaweed-derived products. Intertidal seaweeds, such as Ascophyllum nodosum (rockweed), potentially increase the abundance and diversity of associated animal assemblages by providing structural complexity, refuge from predators, and foraging habitat for higher trophic level consumers. However, the impacts of seaweed harvesting on associated food-webs are not well understood, as most studies have focused on the impacts of harvesting on the regeneration of the resource itself. In particular, the bottom-up food-web effects of rockweed harvest on higher trophic level taxa—such as birds—are not well understood. We conducted a Before–After Control–Impact experiment at 43 sites in Maine between 2018 and 2020 to assess the impact of commercial-scale rockweed harvest on the abundance of three coastal bird guilds (shorebirds, divers, scavengers). We predicted that rockweed harvest, by decreasing primary consumers, would have a negative effect on the top predators, birds. Bird communities were surveyed for 20 minutes at each site, before-harvest, 2-4 weeks post-harvest, and 1 year post-harvest.
Results/Conclusions
Overall bird abundance was low throughout the study, with no birds present during 127 out of 215 bird surveys (59%). There was high variance in shorebird and diver abundance across sites and years, but surprisingly there was no decrease in bird abundance attributable to rockweed harvest. We suspect that, although large scale primary producer resource removal was happening in this ecosystem, these food webs were not bottom-up resource limited, and as a result there were not cascading effects on the rest of the food web. Continued study of wild macroalgae harvest along other temperate coastlines will provide insight into whether low disturbance impacts extend to other harvested macroalgae and associated coastal bird assemblages.