Date of Award


Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)


Biology/Environmental Biology

First Advisor

Ron Etter

Second Advisor

Alan Christian

Third Advisor

Robyn Hannigan


Shallow subtidal epibenthic communities worldwide are under threat from exploitation, pollution, eutrophication, acidification, climate change, and invasive species, with implications for ecosystem diversity, productivity, function, and services. Subtidal ecosystems in the Gulf of Maine are particularly impacted, making it crucial to understand these habitats so that our impacts can be predicted and mitigated. I investigated the basic ecological forces that structure shallow subtidal epibenthic communities in this region, and how invasive species integrate themselves into these communities. I used community phylogenetic and functional trait analyses to investigate if invertebrate communities in the rocky subtidal are assembled via deterministic or random forces, experimental manipulations to quantify how macroalgae might influence sessile invertebrates on subtidal surfaces, and measurements of life history traits of Botrylloides violaceus, an invasive colonial ascidian, to estimate whether growth of this species differs among man-made versus natural habitats. Based on community phylogenetic analyses, rocky subtidal invertebrate communities appear to be structured by deterministic forces, with evidence for both competitive exclusion and environmental filtering operating at different spatial scales. These findings support existing studies that show that competition structures communities at local scales, and also expand our knowledge of the processes that act regionally, i.e. environmental filtering. On shallow sunlit experimental surfaces suspended from floating docks, macroalgae had little effect on invertebrate abundance or diversity, contrary to findings from experiments in the rocky subtidal. Macroalgae did influence composition as well as enhance invertebrate colonization in the early stages of community assembly. Different factors appear to influence the balance between heterotrophs and autotrophs in floating dock and rocky subtidal systems with implications for community structure, function and productivity. In different habitats, colonies of the invasive ascidian B. violaceus exhibited differences in life history traits. It grew faster and attained larger sizes in man-made floating dock versus natural rocky subtidal and eelgrass bed habitats. Again, differences among habitats appear to influence invasion success. In conclusion, competitive exclusion, facilitation, and environmental filtering play key roles in controlling the structure, composition, and function of shallow subtidal communities. Invasive species have the potential to disrupt these forces as they integrate themselves into man-made and subsequently natural habitats.

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