Trophic cascades on the edge: fostering seagrass resilience via a novel pathway
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Despite widespread degradation, some coastal ecosystems display remarkable resilience. For seagrasses, a century-old paradigm has implicated macroalgal blooms stimulated by anthropogenic nutrient, loading as a primary driver of seagrass decline, yet relatively little attention has been given to drivers of seagrass resilience. In Elkhorn Slough, CA, an estuarine system characterized by extreme anthropogenic nutrient loading and macroalgal (Ulva spp.) blooms, seagrass (Zostera marina) beds have recovered concurrent with colonization of the estuary by top predators, sea otters (Enhydra lutris). Here, we follow up on the results of a previous experiment at the seagrass interior, showing how sea otters can generate a trophic cascade that promotes seagrass. We conducted an experiment and constructed structural equation models to determine how sea otters, through a trophic cascade, might affect the edge of seagrass beds where expansion occurs. We found that at the edge, sea otters promoted both seagrass and ephemeral macroalgae, with the latter contributing beneficial grazers to the seagrass. The surprising results that sea otters promote two potentially competing vegetation types, and a grazer assemblage at their boundary provides a mechanism by which seagrasses can expand in eutrophic environments, and contributes to a growing body of literature demonstrating that ephemeral macroalgae are not always negatively associated with seagrass. Our results highlight the potential for top predator recovery to enhance ecosystem resilience to anthropogenic alterations through several cascading mechanisms.
KeywordsEutrophication Predator recovery Ecosystem recovery Trophic dynamics Food web
We thank the J. Trexler, two anonymous reviewers, and the following people who provided constructive comments for this manuscript: P. Raimondi, S. Williams, M. Carr, R. Kudela, J. Ruesink, M. Whalen, and J. Estes. We are indebted to the following volunteers for field and laboratory assistance: K. Beheshti, S. Abbey, and K. Davidson. This work was supported through a National Estuarine Research Reserve Graduate Research Fellowship, a Rebecca and Steve Sooy Graduate Fellowship in Marine Mammal Studies, and a grant from the University of California, Santa Cruz, Department of Ecology and Evolutionary Biology to B. B. Hughes; and a grant from the National Oceanic and Atmospheric Administration’s Estuarine Reserve Division to K. K. Hammerstrom, N. E. Grant, and K. Wasson.
Author contribution statement
BBH originally formulated the idea. BBH, RE, and KW conceived and designed the experiments. BBH, KKH, NEG, UH, and RE performed the experiments. BBH analyzed the data. BBH and KW wrote the manuscript; all authors provided editorial comments.
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