Abstract
A primary goal in the study of producer–herbivore interactions is to characterize the tradeoffs between primary producer growth and defense. Across the Aleutian Island Archipelago, the widespread decline in sea otters has resulted in reduced predation on sea urchins, which has led to increases in urchin populations, the formation of urchin barrens, and ultimately to overgrazing of much of the region’s kelp forests. The occurrence of both kelp forests and urchin barrens on islands, along with among island variation in the time period that urchin barrens have formed, presents a unique opportunity to characterize the extent to which exposure to intense herbivory and increased light may alter marine macroalgal growth and defense tradeoffs. To address this, we used a field caging experiment with Codium ritteri, a common perennial green macroalga in the Aleutian Archipelago, to test whether urchin barren macroalgae exhibit increased defenses and reduced growth relative to kelp forest individuals. Our results suggest that urchin barren C. ritteri had greater defense than growth relative to kelp forest individuals. In the laboratory, we found little evidence for urchin barren C. ritteri growth under low light or altered defenses at high light. Grazing rates on C. ritteri were correlated with urchin biomass in the field suggesting higher herbivory intensity may shift primary producer energy allocation from growth to defense. Together, our data suggest that macroalgae occurring within kelp forests grow faster but are more palatable than macroalgae occurring in urchin barrens, which may increase urchin deforestation potential.
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Acknowledgements
We thank the captains and crew of the R/V Oceanus for two life-changing, safe, and productive cruises. We thank J. Estes and the U.S. Fish and Wildlife Service National Maritime Wildlife Refuge, Jeff Williams and Lisa Spitler, for logistical support. We also thank the U.S. Geological Survey, Alaska Science Center, for facilitating logistics and support. We are grateful to A. Bland, M. Good, T. McHugh, J. Metzger, A. Ravelo, S. Small, G. Sullaway and S. Traiger for assistance with field collections. We thank S. Traiger for assistance with a laboratory-based pilot study. We would like to offer recognition and respect to the many Aleut or Unangax̂ whose land we were able to visit and learn about: Sasignan (Attu and Nizki Islands), Qax̂un (Kiska, Amchitka), Niiĝuĝis (Adak, Atka), and Akuuĝun (Yunaska). Authors would like to thank the reviewers that helped improve the quality of the manuscript.
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Partial funding was provided by the National Science Foundation (OCE1435194).
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SSG, BPW, BHK and MSE designed experiments, SSG and BPW conducted field and laboratory work, SSG analyzed data, SSG wrote the paper, all authors contributed to edits on the manuscript.
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Gabara, S.S., Weitzman, B.P., Konar, B.H. et al. Macroalgal defense phenotype correlates with herbivore abundance. Mar Biol 167, 179 (2020). https://doi.org/10.1007/s00227-020-03787-7
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DOI: https://doi.org/10.1007/s00227-020-03787-7