Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


David S Johnson

Committee Member

Mark Brush

Committee Member

Donna Bilkovic

Committee Member

Jeff Shields


Animals across Earth are shifting or expanding their ranges because of climate change. These are climate migrants. Although climate migrants are well-documented, their impacts on recipient ecosystems are not. Climate migrants that are also ecosystem engineers (species that modify or create habitats) will likely have profound effects on ecosystem functioning. The Atlantic marsh fiddler crab, Minuca pugnax, is a burrowing crab that recently expanded its range into the northeast United States. The historical range of M. pugnax was between southern Cape Cod, Massachusetts and Daytona Beach, Florida (Grimes et al., 1989). In 2014, Johnson (2014) found that M. pugnax was found to have extended its northern limit to New Hampshire, moving north from Cape Cod into the Gulf of Maine. We tracked the M. pugnax population within the Great Marsh, in northeast Massachusetts, since it was first detected in 2014 using burrow counts. Thus, the objective of my first chapter was to quantify the densities of M. pugnax within the Great Marsh in northeast Massachusetts using camera traps. The second objective was to measure the relationship between burrow counts and fiddler crab densities. Because burrow counts can overestimate fiddler-crab density, we used camera traps to determine the relationship between burrow densities and fiddler-crab densities in 2019. The burrow count surveys show a 6-fold increase in M. pugnax density in the Great Marsh from 2014–2019. This result indicates that the fiddler-crab population in the expanded range is established and growing. Based on burrow counts, however, the density of M. pugnax in the expanded range (6 burrows m–2) is much lower than those found in the historical range (between 75 - 300 burrows m–2). Based on camera traps, we determined that burrow counts overestimated fiddler-crab densities by 47% in 2019. Stated another way, there was, on average, one crab detected for every two burrows observed. However, this is strictly for crabs that were observed and does not account for ovigerous females who are known to incubate deep in their burrows. Therefore, further work will be needed to adjust our estimates. Minuca pugnax is an ecosystem engineer that can influence saltmarsh functioning and the magnitude of that influence is related to its density. Following, the objective of my second chapter was to examine its ecological impact on salt marshes. In a control-impact study, we found that when crabs were present, Spartina alterniflora plant height was 9% lower, aboveground biomass was 40% lower, belowground biomass was 30% lower, benthic microalgae biomass was 44% lower, bulk density was unaffected, and sediments became more reduced. Our results show that M. pugnax can have strong ecological impacts in its expanded range, which has implications for other salt marsh functions such as carbon storage and accretion. In its historical range, M. pugnax promotes the aboveground biomass of Spartina as a result of burrowing (it does not eat live Spartina) suggesting a plant-crab commensalism. We found the opposite pattern in M. pugnax’s expanded range. Thus, what was a commensalism appears to be an amensalism in the expanded range. Our results imply that not only can engineering climate migrants have strong ecological impacts but also that those impacts may be the opposite of what has been historically seen.




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