Graduation Date
Fall 2025
Document Type
Thesis
Program
Master of Science degree with a major in Biology
Committee Chair Name
Dr. Paul E. Bourdeau
Committee Chair Affiliation
Cal Poly Humboldt Faculty or Staff
Second Committee Member Name
Dr. Rachael M. Wade
Second Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Third Committee Member Name
Dr. Emily Jones
Third Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Fourth Committee Member Name
Dr. Matthew S. Edwards
Fourth Committee Member Affiliation
Community Member or Outside Professional
Keywords
Ecosystem resilience, Herbivore deterrence, Physical deterrence, Applied theory, Species interactions, Northern California, Bull kelp
Subject Categories
Biology
Abstract
Kelp forests are declining globally under warming seas, altered predator–prey dynamics, and intensified herbivory, with sea urchins often driving shifts from canopy-dominated forests to persistent urchin barrens. Management commonly relies on labor-intensive urchin removals, with variable success, underscoring the need for complementary, ecosystem-based tools that directly reduce urchin grazing pressure. Focusing on far northern California kelp forests, we combined subtidal surveys, field experiments, and laboratory feeding assays to assess whether the understory alga Desmarestia cf. herbacea can provide associational defense for bull kelp (Nereocystis luetkeana) by physically deterring purple sea urchin (Strongylocentrotus purpuratus) grazing and offering low nutritional payoff for these grazers. Surveys in intact forests revealed a strong positive association between D. cf. herbacea and N. luetkeana. In urchin barrens, live D. cf. herbacea and algal-mimic barriers reduced urchins by up to 98% inside plots and by > 60% outside of the plots but within 0.25 m, and decreased kelp tissue loss at the higher-grazing site. In a six-week experiment, natural D. cf. herbacea stands sustained protection of N. luetkeana from urchin grazing relative to removals, depressing urchin densities over time and reducing encroachment from nearby barrens, yielding a net increase in N. luetkeana abundance by week 4, and extending kelp survival by ~1.7 weeks. In 24-h feeding assays, urchins consumed D. cf. herbacea and another understory alga (Pterygophora californica) at roughly half the rate of N. luetkeana. Over 30 days, urchins consumed similar amounts of D. cf. herbacea and P. californica, but urchin gonadosomatic indices were lower on the D. cf. herbacea diet, indicating reduced energetic payoff. Together, these results show that D. cf. herbacea can protect N. luetkeana by limiting urchin access in the field and by depressing urchin condition when consumed. Incorporating D. cf. herbacea, and functionally similar understory seaweed into kelp forest restoration strategies, offers an ecologically grounded complement to urchin removals for rebuilding kelp-forest resilience in northern California and beyond.
Citation Style
APA
Recommended Citation
Wiley, Brayden Tate, "Associational seaweed defense reduces sea urchin grazing on bull kelp: Potential use in restoration" (2025). Cal Poly Humboldt theses and projects. 2515.
https://digitalcommons.humboldt.edu/etd/2515