Habitat alteration by invasive European green crab (Carcinus maenas) causes eelgrass loss in British Columbia, Canada

Abstract

Dominant, habitat-forming plant species, such as seagrasses, are key components of coastal ecosystems worldwide. Multiple stressors, including invasive species that directly alter, remove, or replace the foundation plant species, threaten these ecosystems. On the Atlantic coast of North America, ecosystem engineering by invasive European green crab (Carcinus maenas) has been linked to the loss of some eelgrass (Zostera marina) beds. However, the interaction of the same co-occurring species on the Pacific coast has not been investigated. We conducted an enclosure experiment in Barkley Sound, British Columbia, to determine if the engineering impacts of green crabs on Pacific eelgrass ecosystems mirror those previously identified on the Atlantic coast. Eelgrass shoot density declined rapidly over 4 weeks, with a 73–81% greater loss in enclosures with high crab density compared to the low-density and control treatments. The low ratio of eelgrass blades to rhizomes in the high-density treatment suggests that blade shredding, rather than bioturbation of whole plants, was the main mechanism of eelgrass loss. Eelgrass was detected in green crab stomach contents, consistent with observations from the Atlantic coast. Crab density did not have a detectable effect on the biomass or community composition of benthic fauna associated with eelgrass over the duration of the experiment. The eelgrass loss we observed was consistent with losses observed on the Atlantic coast, which raises management concerns on the Pacific coast, particularly in areas where green crabs co-occur with other coastal stressors and with ecologically and economically important species such as salmon.