Marine Community Assembly in a Dynamic Ecotone

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Species distributions are shifting with climate change. By altering the presence and distribution of biogenic foundation species, climate change effectively modifies habitat. Where biogenic habitats meet, a patchy ecotone landscape forms. The impacts that range shifts and habitat modification have on broader ecological communities will depend in part on how communities assemble in frontier landscapes of patchy habitat. Here, as a case study, I investigate marine fauna community formation and habitat associations along a wetland ecotone in which tropical mangroves invade temperate saltmarsh. When foundation species shift ranges, resulting changes in geographic context and local conditions will affect the contributions of dispersal limitation and species sorting to assembly. By evaluating the presence of community structure – grouping of species – in larval supply and settlers in each pure landscape and into the ecotone, I determine that ecotone marine communities are shaped by habitat-based sorting but not dispersal limitation. Where inhabitant species can access the ecotone, the attributes that inform habitat use and the scale(s) at which inhabitants distinguish between habitat types within an ecotone should determine the apparency of emerging patches along the range edge, affecting the precision with which inhabitants occupy them. I monitored marine fauna within an experimental array that isolated physical structure from broader habitat patch attributes, revealing that nested scales of habitat sensitivity should result in increasing community divergence as habitat patches expand along the range edge. Finally, habitat associations at settlement may be driven by preference or survival. I determine habitat-specific recruitment patterns of Callinectes spp. (Decapoda: Portunidae) crabs in the ecotone and use lab trials to determine that associations are driven by preference for and superior survival in vegetation with branched architecture. Together, these results demonstrate that marine fauna are sensitive to changes in structural attributes and fine-scale emergence of mangrove habitat within marshes, which do not provide equivalent habitat. This work also contributes to our understanding of community formation in a transitional landscape, illuminating the influence of patchy foundation species expansion on community-structuring ecological processes.