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.

Restoration of eastern oyster (Crassostrea virginica) reefs in the Chesapeake Bay and its tributaries is important, as oyster reefs provide habitat for temperate estuarine communities and shoreline protection. Oysters that settle in crevices, such as those found on natural shell substrates, suffer low mortality, but natural shell is becoming a limited resource in the Chesapeake Bay. Finding an alternative settlement substrate that is complex like natural shell and mimics the benefits of shell substrates with less expense could be the best way to encourage oyster recruitment and survival. The two main goals of this experiment were to (1) understand which artificial substrate type (granite stone, oyster castle, diamond, c-dome, and x-reef) promotes the highest oyster recruitment and survival compared to natural shell and (2) determine the effects of reef presence on macrofaunal community structure and productivity. It was hypothesized that a new settlement substrate, oyster diamonds, given the sloping surfaces with large surface area, will be best for oyster recruitment. Additionally, it was hypothesized that oyster reef presence will substantially enhance macrofaunal community abundance and increase macrofaunal productivity compared to unstructured sediment. To address these goals, infaunal macrofauna and sediment samples were taken at three experimental sites and two control sites along the York River in June 2021. Then, two replicates of each of six reef types were deployed in a randomized block design at each of three experimental sites. The reef types included loose oyster shell, granite, oyster castles, oyster diamonds, c-domes, and x-reefs. The fall after deployment, the structures were sampled for oyster density and shell height. One year after deployment, the structures were physically sampled, removing oysters to determine oyster density and biomass, and the macrofaunal community associated with the reefs. Control sites were sampled for benthic infauna and sediment analyses. Oyster densities and biomass were extremely high and were highest on the loose shell reef (9,852 oysters/m2 and 743 g AFDW/m2 based on model estimates), and the x-reefs had the second-highest recruitment and biomass (3,816 oysters/m2 and 531 g AFDW/m2 based on model estimates). Prior to deployment of reefs, control and impact sites had similar density, biomass, and diversity of macrofauna, but one year after reef deployment, the reef impact sites had higher densities, richness, diversity, biomass, and secondary productivity of macrofaunal organisms than the control sites with the impact sites having 145 times greater secondary productivity than the controls. The diversity among reef types did not vary but the granite reefs had the highest secondary productivity overall. All reef structures showed successful oyster and macrofaunal community recruitment based on Chesapeake Bay Fisheries Goal Implementation Team metrics and densities were among the highest for alternative substrates. Based on the results of this study, researchers and managers could choose from a variety of successful alternative reef types. Based on restoration goals, the use of different reef types could lead to differences in the achievement of goals.

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