Date of Award


Degree Type


Degree Name

Doctor of Philosophy in Biological Sciences


Biological Sciences

First Advisor

Carol S. Thornber


Invasive species can have a variety of impacts on food web structure and interspecific interactions. They can impact recruitment rates of associated marine species, influence behavior of potential prey items, and alter predator-prey relationships. This research is designed to assess 1) the relationship between the recruitment of Lacuna vincta and two species of algal epiphytes, the native Ceramium virgatum and the invasive Neosiphonia harveyi, in the shallow subtidal zone; 2) the spatial and temporal distribution of the invasive Hemigrapsus sanguineus in the intertidal zone of cobble beaches; and 3) the top-down effects and predator-prey interactions of H. sanguineus.

Through manipulative field experiments, we found that the presence of algal epiphytes facilitated the recruitment rate of Lacuna vincta, regardless of the epiphyte species composition. We also found a positive relationship between the number of L. vincta present and epiphyte recruitment, which is disproportionately driven by higher recruitment of Neosiphonia harveyi than Ceramium virgatum.

Long-term monitoring can be used to understand population trends of invasive species. Through monthly surveys in Narragansett Bay, Rhode Island, we determined that Hemigrapsus sanguineus densities are highest in the early summer and early fall months. Juvenile H. sanguineus are most abundant in June and July and gravid females are most prevalent in August and September. H. sanguineus exhibited a density gradient with the highest densities in the northern section of Narragansett Bay and declining toward the mouth at the southern end of Narragansett Bay.

Invasive species can outcompete native and established species, thereby altering food web dynamics through changes in top-down and predator-prey interactions. Through mesocosm studies, we found that while Hemigrapsus sanguineus has an impact on Littorina littorea behavior, it does not alter the perwinkles’ grazing rates. By contrast, the combined presence of H. sanguineus and L. littorea results in a greater decrease in algal biomass than only L. littorea. In field tethering experiments, we observed that abiotic but not biotic factors were the dominant force in structuring the vertical distribution of H. sanguineus. Overall, we found that H. sanguineus does not occupy the same ecological niche as Carcinus maenas, the previously dominant crab in the intertidal zone of cobble beaches.

This research provides insight into how invasive species shape the sub- and intertidal zones by influencing the recruitment rate and behavior of native and established species. Given that marine invasions are occurring at an increasing rate due to international marine transportation, human-mediated introductions, and global climate change, fully understanding the impacts of these invasive species is critical to mitigating and adapting to changes in species composition and abundance.



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