Recovery of a top predator mediates negative eutrophic effects on seagrass

• Published in: Proceedings of the National Academy of Sciences - PNAS 2013-09, Vol.110 (38), p.15313-15318
• Main Authors: Hughes, Brent B., Eby, Ron, Van Dyke, Eric, Tinker, M. Tim, Marks, Corina I., Johnson, Kenneth S., Wasson, Kerstin
• Format: Article
• Language: English
• Subjects: Analysis of Variance; Animal and plant ecology; Animal, plant and microbial ecology; Animals; Applied ecology; Aquatic plants; Biological and medical sciences; Biological Sciences; Brachyura - physiology; Brackish; Brackish water ecosystems; California; Conservation; Conservation, protection and management of environment and wildlife; Crabs; Enhydra lutris; Environmental degradation: ecosystems survey and restoration; Epiphytes; Estuaries; Eutrophication - physiology; Food Chain; Fundamental and applied biological sciences. Psychology; Habitats; Herbivores; History, 20th Century; History, 21st Century; Marine; Marine ecology; Marine ecosystems; Nitrates; Nitrates - analysis; Otters; Otters - physiology; Pollution load; Population Dynamics - history; Predation; Predators; Seas; Synecology; Vegetation; Zostera marina; Zosteraceae - growth & development
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1302805110

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.