Fiddler crab impacts from observational study 2020-21: Aboveground & diatom biomass, plant height, percent N, burrow & mussel density, belowground biomass, and organic matter

The fiddler crab, Minuca pugnax, expanded its range into the Gulf of Maine recently and was first observed in the Plum Island Estuary in 2014. In 2020 and 2021, we investigated the impact of this burrowing crab on benthic microalgal biomass, sediment properties and the above- and belowground biomass of the cordgrass, Spartina alterniflora. To accomplish this, we conducted a control-impact study in plots with and without fiddler crabs in three marshes in the PIE-LTER: Sawyer, Clubhead, and Metcalf. In its historical range (i.e., south of Cape Cod), M. pugnax, enhances Spartina aboveground biomass. In contrast, we found that, on average, when fiddler crabs were present, aboveground biomass was 40% lower in the PIE-LTER. We also found that belowground biomass was 30% lower and benthic microalgal biomass was 45% lower when fiddler crabs were present, which is in line with our expectations. Because fiddler crabs reduced the biomass of foundational primary producers in its expanded range, our results imply that M. pugnax can influence other saltmarsh functions such as carbon storage and accretion as they expand north. More broadly, our results suggest that as species expand or shift their range with climate change, not only can they have profound impacts in their new ranges, but that those impacts can be the inverse of what is seen in their historical ranges.