Dynamic and context-dependent keystone species effects in kelp forests
Edited by Mark Hay, Georgia Institute of Technology, Altanta, GA; received July 12, 2024; accepted November 11, 2024
Significance
Keystone predators can reshape ecological communities and are essential to the functioning of many ecosystems. However, when and why these species exert such large effects is often unclear. We compare two 30-y community-wide datasets documenting kelp forest dynamics during recolonization of an iconic keystone species, the sea otter. By modeling interactions as dynamic state-dependent rates that change as communities change, we show that indirect interaction chains in southern Californian kelp forests sharply weakened the trophic cascades that sea otters triggered, relative to cascades off Vancouver Island, BC. We also show that dynamics of the species interactions themselves, with waxing and waning strengths as otters recolonize, are essential to understanding the mechanisms underlying the effects of sea otters as keystone predators.
Abstract
Sea otters are an iconic keystone predator that can maintain kelp forests by preying on grazing invertebrates such as sea urchins. However, the effects of sea otters on kelp forests vary over their geographic range. Here, we analyze two 30-y datasets on kelp forest communities during the reintroduction of sea otters along the west coast of Vancouver Island, BC, Canada, and around San Nicolas Island, CA. We developed a community model to estimate species interactions as dynamic rates, varying with community state. We find evidence of a classic trophic cascade off Vancouver Island; the arrival of otters quickly led to depletion of urchins and recovery of kelp. However, this cascade was muted around San Nicolas Island, with otters, urchins, and kelp all coexisting at intermediate densities for multiple years. Our models show that this difference came from a pulse of strong otter impacts on urchins following recolonization off Vancouver Island, but not off San Nicolas Island. The mean effects of otters on urchins and urchins on kelp were not stronger in the north, indicating that interaction dynamics and not average interaction strength are key to explaining differences in community trajectories. We also find stronger multistep interaction chains in the south, arising from competitive interactions that indirectly buffered otter effects. These findings shed light on long-standing hypotheses about how interspecific interactions can alter the function of keystone species across community contexts. More broadly, we show how community change can be more accurately predicted by considering dynamic interaction strengths.
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Data, Materials, and Software Availability
R code and preprocessed data are available on GitHub (https://github.com/langendorfr/Dynamic-and-context-dependent-keystone-species-effects-in-kelp-forests). Data for the SNI region are also available in a U.S. Geological Survey data release, as an entire ecosystem dataset (54) and another containing data only on sea otters (55).
Acknowledgments
Off west Vancouver Island we gratefully acknowledge the Ka:’yu:’k’t’h’/Che:k:tles7et’h’, Mowachaht/Muchalaht and Huu-ay-aht First Nations within whose territories this research was conducted. Financial and logistical support for this work was provided by the Friends of the Ecological Reserves, Vancouver Island University, Fisheries and Oceans Canada, and Bamfield Marine Sciences Centre. Numerous colleagues and volunteers assisted over the years, in particular we thank Leah Saville, Gina Lemieux and the community of Kyuquot. Sea otter and kelp forest data from San Nicolas Island were collected by the U.S. Geological Survey. We thank the U.S. Navy for providing access and logistical support for work at SNI and for funding the sea otter and kelp forest work there during the last decade. James Estes was an Emeritus Scientist at the United States Geological Survey during this work. Comments and suggestions from the editor and two anonymous reviewers greatly improved the manuscript. We are also grateful for support and feedback from Matt Burgess. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Author contributions
R.E.L., J.A.E., J.C.W., M.C.K., B.B.H., M.T.T., E.W., M.L.D., and D.F.D. designed research; R.E.L., J.C.W., M.C.K., B.B.H., M.T.T., E.W., M.L.D., and D.F.D. performed research; R.E.L. and D.F.D. contributed new reagents/analytic tools; R.E.L., E.W., M.L.D., and D.F.D. analyzed data; and R.E.L., J.C.W., M.C.K., B.B.H., M.T.T., E.W., M.L.D., and D.F.D. wrote the paper.
Competing interests
The authors declare no competing interest.
Supporting Information
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Copyright © 2025 the Author(s). Published by PNAS. This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
Data, Materials, and Software Availability
R code and preprocessed data are available on GitHub (https://github.com/langendorfr/Dynamic-and-context-dependent-keystone-species-effects-in-kelp-forests). Data for the SNI region are also available in a U.S. Geological Survey data release, as an entire ecosystem dataset (54) and another containing data only on sea otters (55).
Submission history
Received: July 12, 2024
Accepted: November 11, 2024
Published online: March 3, 2025
Published in issue: March 11, 2025
Keywords
Acknowledgments
Off west Vancouver Island we gratefully acknowledge the Ka:’yu:’k’t’h’/Che:k:tles7et’h’, Mowachaht/Muchalaht and Huu-ay-aht First Nations within whose territories this research was conducted. Financial and logistical support for this work was provided by the Friends of the Ecological Reserves, Vancouver Island University, Fisheries and Oceans Canada, and Bamfield Marine Sciences Centre. Numerous colleagues and volunteers assisted over the years, in particular we thank Leah Saville, Gina Lemieux and the community of Kyuquot. Sea otter and kelp forest data from San Nicolas Island were collected by the U.S. Geological Survey. We thank the U.S. Navy for providing access and logistical support for work at SNI and for funding the sea otter and kelp forest work there during the last decade. James Estes was an Emeritus Scientist at the United States Geological Survey during this work. Comments and suggestions from the editor and two anonymous reviewers greatly improved the manuscript. We are also grateful for support and feedback from Matt Burgess. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Author contributions
R.E.L., J.A.E., J.C.W., M.C.K., B.B.H., M.T.T., E.W., M.L.D., and D.F.D. designed research; R.E.L., J.C.W., M.C.K., B.B.H., M.T.T., E.W., M.L.D., and D.F.D. performed research; R.E.L. and D.F.D. contributed new reagents/analytic tools; R.E.L., E.W., M.L.D., and D.F.D. analyzed data; and R.E.L., J.C.W., M.C.K., B.B.H., M.T.T., E.W., M.L.D., and D.F.D. wrote the paper.
Competing interests
The authors declare no competing interest.
Notes
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Dynamic and context-dependent keystone species effects in kelp forests, Proc. Natl. Acad. Sci. U.S.A.
122 (10) e2413360122,
https://doi.org/10.1073/pnas.2413360122
(2025).
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