Date of Award


Document Type

Honors Thesis (Open Access)


Colby College. Environmental Studies Program


Allison Barner

Second Advisor

Christopher Moore

Third Advisor

Caitlin Cleaver


Brown algae, including the canopy-forming macroalgae Fucus distichus, are declining in abundance around the world due to stressors associated with climate change. Local extinction of Fucus impacts the intertidal community through cascading effects as the ecosystem loses a dominant primary producer at the base of the food web. However, Fucus is not only a resource species; it is also an ecosystem engineer. This thesis investigates the effect of Fucus extinction on the abundance of two herbivorous marine snails, L. saxatilis and L. obtusata, with a focus on the role of Fucus as a habitat- modifier. Specifically, I explored: 1) how the removal of Fucus impacts snail abundance, 2) how Fucus affects the temperature and humidity of snail microhabitats, and 3) how the temperature and humidity of snail microhabitats influence snail abundance. Using species count data from the Barner Lab and microhabitat temperature and humidity data that I collected on Colby’s Island Campus in the Gulf of Maine, I found that: 1) the removal of Fucus canopy decreases L. obstusata abundance but increases L. saxatilis abundance, 2) the presence of Fucus increases microhabitat air temperature, decreases substrate temperature, and increases humidity, and 3) the temperature and humidity of snail microhabitats have a weak effect on snail abundance compared to the presence of canopy itself. Overall, the biotic effects of Fucus are stronger drivers of snail abundance in this system than the abiotic effects of Fucus.


Littorina obtusata, Littorina saxatilis, Fucus distichus, Extinction, Ecosystem Engineer, Rocky intertidal

Available for download on Tuesday, May 20, 2025