Scientists Crack the Code of When Sea Urchins Destroy Kelp Forests

We wanted to solve a puzzle that has haunted marine ecologists for decades: why do some kelp forests coexist peacefully with sea urchins while others get completely obliterated, turning into underwater deserts called urchin barrens? We suspected the answer lay in the balance between how much kelp the forest produces and how much the urchins consume.

We designed feeding experiments using red and purple sea urchins collected from California's Santa Barbara Channel, testing eight different densities of purple urchins and six densities of red urchins in laboratory mesocosms during May-August 2019. The urchin biomass in trials ranged from about 64 to 1,852 grams per square meter for purple urchins and 91 to 1,771 grams per square meter for red urchins, spanning the full range from healthy kelp forests to devastated barrens.

The experiments revealed a stark threshold. When We calculated urchin grazing capacity and compared it to kelp production using 21 years of monitoring data, they found that sea urchins caused a 50-fold reduction in giant kelp biomass once their consumption exceeded production.

We revealed how kelp detritus—the dead pieces of kelp that drift around the ocean floor—may act as a buffer against deforestation. The leading hypothesis is that urchins passively forage on kelp detritus until changes in kelp production cause urchins to actively graze the living, standing kelp that forms the forest canopy.

These results matter because when kelp forests collapse into urchin barrens, the transformation can persist for decades and results in alternative states that provide fewer services to people and nature. The research provides a tool to predict where and when these collapses might occur, potentially allowing intervention before it's too late.

The study builds a mechanistic understanding of how density-dependent foraging and primary production interact to cause consumer-driven disturbances in temperate rocky reef ecosystems.

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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