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| Crabs are a favorite prey item for sea otters Enhydra lutris in Elkhorn Slough.
Photo by Ron Eby
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| Fig. 2. (A) Interaction web of top-down and bottom-up effects in the eelgrass study system. The top predator is the sea otter (E. lutris), the mesopredators are crabs (Cancer spp. and Pugettia producta), the epiphyte mesograzers are primarily an isopod (I. resecata) and a sea slug (P. taylori), and algal epiphyte competitors of eelgrass primarily consist of chain-forming diatoms, and the red alga Smithora naiadum. |
Abstract
A fundamental goal of the study of ecology is to determine the drivers of habitat-forming vegetation, with much emphasis given to the relative importance to vegetation of “bottom-up” forces such as the role of nutrients and “top-down” forces such as the influence of herbivores and their predators. For coastal vegetation (e.g., kelp, seagrass, marsh, and mangroves) it has been well demonstrated that alterations to bottom-up forcing can cause major disturbances leading to loss of dominant vegetation. One such process is anthropogenic nutrient loading, which can lead to major changes in the abundance and species composition of primary producers, ultimately affecting important ecosystem services. In contrast, much less is known about the relative importance of apex predators on coastal vegetated ecosystems because most top predator populations have been depleted or lost completely. Here we provide evidence that an unusual four-level trophic cascade applies in one such system, whereby a top predator mitigates the bottom-up influences of nutrient loading. In a study of seagrass beds in an estuarine ecosystem exposed to extreme nutrient loading, we use a combination of a 50-y time series analysis, spatial comparisons, and mesocosm and field experiments to demonstrate that sea otters (Enhydra lutris) promote the growth and expansion of eelgrass (Zostera marina) through a trophic cascade, counteracting the negative effects of agriculturally induced nutrient loading. Our results add to a small but growing body of literature illustrating that significant interactions between bottom-up and top-down forces occur, in this case with consequences for the conservation of valued ecosystem services provided by seagrass.
Keywords: eutrophication, food web, estuary, resilience
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| Eelgrass beds in Elkhorn Slough benefit from the presence of sea otters.
Photo by Ron Eby
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| Both sea slugs and Idotea (the crustacean between the two sea slugs above) feed on algae and increase in numbers when the crab population is controlled by sea otters.
Photo by Brent Hughes
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Brent B. Hughes, Ron Eby, Eric Van Dyke, M. Tim Tinker, Corina I. Marks, Kenneth S. Johnsone and Kerstin Wasson. 2013. Recovery of a Top Predator mediates negative Eutrophic Effects on Seagrass. PNAS. http://pnas.org/cgi/doi/10.1073/pnas.1302805110
Sea Otters Promote Recovery of Seagrass Beds
— Scientists studying the decline and recovery of seagrass beds in one of California's largest estuaries have found that recolonization of the estuary by sea otters was a crucial factor in the seagrass comeback. Led by researchers at the University of California, Santa Cruz.
