Graduation Date

Fall 2024

Document Type

Thesis

Program

Master of Science degree with a major in Biology

Committee Chair Name

Paul E. Bourdeau

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Erik S. Jules

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Bret C. Harvey

Third Committee Member Affiliation

Community Member or Outside Professional

Fourth Committee Member Name

Gina M. Contolini

Keywords

Kelp Forest, Kelp Forest restoration, Rock crabs, Purple urchins, Bull kelp, Cancer productus, Romaleon antennarium, Strongylocentrotus purpuratus, Kelp forest ecology, Experimental Ecology, Applied Ecology, Kelp Forest Conservation, Northern California, Marine Ecology, Marine Biology

Subject Categories

Biology

Abstract

Kelp forests are threatened by regional abiotic and biotic stressors, including overgrazing by sea urchins. On the north coast of California, purple sea urchins (Strongylocentrotus purpuratus) are prolific grazers of kelp and with the regional extinction of their main predator (sunflower stars, Pycnopodia helianthoides), it is unknown whether any remaining mesopredators could exert top-down control of purple urchin populations. Other known predators of purple urchins in northern California include rock crabs, which have been shown to control sea urchin populations in Atlantic urchin barrens and indirectly cause phase shifts back to kelp forests. Yet, little is known about the importance of these predators in mediating purple urchin distribution, abundance, and grazing behavior in the eastern north Pacific. To better understand the ecological effects of rock crabs on purple urchins in northern California, I conducted laboratory mesocosm and field experiments to determine: (1) the feeding rates of two common rock crab species (Romaleon antennarium and Cancer productus) on urchin barren and kelp forest urchins; and (2) the influence of crab risk cues and damaged conspecific urchins on urchin grazing rates. In mesocosms, actively feeding rock crabs of both species and all adult sizes consumed approximately two urchins per day, regardless of urchin condition. The effect of the presence of non-feeding rock crabs on urchin grazing depended on urchin condition, with crab risk cues strongly suppressing grazing by well-fed urchins compared to the presence of crushed conspecific urchins and no-cue controls, but only weakly suppressing the grazing of starved urchins. In a natural urchin barren, risk cues from caged rock crabs reduced urchin grazing by 64% when compared to no-crab plots. As we search for novel restoration efforts, our findings provide insight into potential strategies for suppressing urchin grazing, through rock crab consumptive and non-consumptive effects. Together, my findings suggest that rock crab enhancement could be incorporated with other methods as a tool for kelp forest restoration until sunflower stars can be re-introduced to these vital ecosystems.

Citation Style

APA

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