Altering eelgrass: The impact of invasive European green crabs on critical habitat

Habitat-forming marine plants like eelgrass (Zostera marina), are critical to the functioning of coastal ecosystems. However, these near-shore ecosystems are frequently invaded and altered by non-native species. For example, it is pretty well established that invasive European green crabs (Carcinus maenas) can contribute to the loss of eelgrass beds on the Atlantic coast of North America by tearing and digging up individual plants. However, there was no literature on whether we could expect the same impacts of invasive green crabs on eelgrass on the Pacific coast. We therefore spent a month working out of the Bamfield Marine Sciences Centre conducting a field experiment in Barkley Sound, British Columbia, to answer this question.

To do this we constructed 18 large cages and, with the help of many volunteers including researchers, undergraduate students, and BMSC staff, deployed them throughout an eelgrass bed. These volunteers also put in some very early mornings to help us collect data over the course of four weeks!

The data showed that eelgrass density declined at an average daily rate of 4.1 to 7.5 shoots per meter when green crabs were absent or present at low densities. However, when exposed to high densities of green crabs (> 5 crabs per m2), eelgrass declined 2.4 to 4 times faster, averaging 17.6 shoots lost per meter every day. We were also able to determine that the mechanism of eelgrass loss was mostly from green crabs pulling apart plants, and in some cases eating them, rather than uprooting them when digging.

Green crabs are a successful invader on much of the outer coast of Vancouver Island and are hyper-abundant in some places, meaning our experimental densities reflect real green crab populations in some parts of Barkley Sound. Ultimately, this study demonstrates that these invasive species require consideration if we want to protect one of the most ecologically and economically important ecosystems on our coast.

Howard, B. R., Francis, F. T., Côté, I. M., & Therriault, T. W. (2019). Habitat alteration by invasive European green crab (Carcinus maenas) causes eelgrass loss in British Columbia, Canada. Biological Invasions, 0123456789, 3607–3618.