A range expanding ecosystem engineer influences historical and expanded habitats via the same causal pathways

Species are shifting their ranges in response to climate change. There remain many unknowns about relative impacts of range-expanding ecosystem engineers between historical and expanded habitats, however. The mud fiddler crab Minuca pugnax (= Uca pugnax ) is shifting its range northward likely due to increased warming in the Gulf of Maine. A burrowing crab, M. pugnax affects ecosystem functioning in salt marshes south of Cape Cod, Massachusetts with unknown effects in expanded marsh habitats over 150km to the north. We therefore studied the M. pugnax range expansion to determine the extent that range expanding ecosystem engineers are influencing ecosystem functioning expanded ranges relative to historical habitats. We installed in 2017 and 2018 a series of crab-inclusion cages at both the UMass Boston Nantucket Field Station (historical range) and the Plum Island Estuary Long Term Ecological Research site (PIE-LTER, expanded range). For each site, year, and block, we measured in the beginning and end of the three-month experiment metrics of sediment strength, primary production, and decomposition. We developed and tested causal models using structural equation modeling (SEM) to determine direct and indirect effects of fiddler crabs on ecosystem functions. Despite site, year, and block variability, local environment influenced burrow density, which directly affected sediment strength and indirectly affected primary production in both ranges. Overall, understanding range-expanding ecosystem engineers in historical ranges was predictive for how they influence expanded habitats, despite inter-site heterogeneity. Therefore, it is critical to study relative impacts of range-expanding ecosystem engineers to understand total impacts of global range shifts. ### Competing Interest Statement The authors have declared no competing interest.