Scientists discovered that sea urchins cause severe kelp forest collapse when their grazing overwhelms kelp production, but kelp detritus (dead kelp pieces) can prevent this collapse by feeding urchins and reducing their appetite for living kelp. Researchers conducted laboratory feeding experiments with different densities of sea urchins, then combined those results with 21 years of kelp forest monitoring data from the Santa Barbara Channel to predict when urchin grazing would exceed kelp production. When urchin grazing capacity exceeded kelp production, sea urchins caused a 50-fold reduction in giant kelp biomass. The balance between herbivory and production determines when kelp forests collapse into urchin barrens. Detrital kelp supply suppresses deforestation by providing alternative food for urchins. Urchin foraging rates vary predictably with urchin biomass, allowing scientists to forecast ecosystem collapse. The research provides a mechanistic understanding of kelp forest collapse, helping scientists predict when and where these productive ecosystems might shift to low-diversity urchin barrens that persist for decades and provide fewer services to people and nature.

Citation

Rennick, Mae; DiFiore, Bartholomew P.; Curtis, Joseph; Reed, Daniel C.; Stier, Adrian C. (2022). Detrital supply suppresses deforestation to maintain healthy kelp forest ecosystems. Ecology.

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Can the urchins out-eat the drift?

Urchins mostly eat kelp that has already broken off and drifted to the seafloor. This study found that kelp forests flip to barrens when urchins can eat more than that drift supplies. Set the urchin biomass and the drift supply, and see which side wins.

Drift supply 20 Urchin grazing 14
700 g/m²
Marks show urchin biomass thresholds from earlier studies (668 g/m², Ling et al. 2015; 1246 g/m², Dean et al. 1984). This study found kelp often stayed healthy above both.
0 g/m²
Red urchins ate less than half as much kelp per gram as purple urchins.
20 g/m²/day

Model and numbers from Rennick, DiFiore, Curtis, Reed & Stier (2022), Ecology: grazing slopes from the mesocosm experiments (Fig. 2); the 50-fold, 91% and 42% figures from 21 years of SBC LTER data (Figs. 4–5). The kelp drawing is illustrative. The drift slider's range is set for display; the paper estimated drift at each site from monthly kelp-loss data.

Cite this article

Rennick et al. (2022). Kelp Forest Collapse Depends on Urchin Grazing and Kelp Supply. Ocean Recoveries Lab. https://doi.org/10.1002/ecy.3673