Urchin barren

An urchin barren is commonly defined as an urchin-dominated area with little or no kelp. Urchin grazing pressure on kelp is a direct and observable cause of a "barren" area. However, determining which factors contribute to shifting a kelp bed to an urchin barren is a complex problem and remains a matter of debate among scientists.

Loss of "top" predators, particularly the historic hunting of sea otters (Enhydra lutris), has often been cited as a cause of these barrens. When urchins are left "unchecked," their populations increase, and this has further effects on primary production in the ecosystem. This type of shift is called a trophic cascade. Such theories have emphasized the "top-down" pressures by predators, including other urchin predators, exerting pressure at different life stages (including at the planktonic larval stage). Others theories have emphasized "bottom-up" factors, including abiotic environmental variables affecting urchin recruitment and the abundance and resiliency of kelp (including water temperature, nutrients, pollution, and other factors). Today, many scientists acknowledge that there is a mix of top-down and bottom-up factors that affect when, how, and where these ecosystems shift between a kelp bed and an urchin barren.^[1]