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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]

Process

Sea urchins can passively graze on drift kelp and actively graze on the stipes and other parts of the kelp "plant." If an urchin population shifts into active grazing of the kelp, they can graze through a kelp bed, leaving few to no living individuals. The shift in conditions can last years or decades. Adult urchins will then have to shift into foraging for other resources, such as "turfy" algal species.

Shift theories

An area of the subtidal where the population growth of sea urchins has gone unchecked causes destructive grazing of kelp beds or kelp forests (specifically the giant brown bladder kelp, Macrocystis). The transition from kelp forest to barren is defined by phase shifts in which one stable community state is shifted to another.^[2] The continuous phase shift is widely accepted. This describes a transition from one ecosystem state to another where the threshold for the forward shift is at the same level as the threshold for the reverse shift back to the previous state. In other words, a kelp bed can re-establish itself when urchin grazing intensity decreases to the threshold density triggering the initial shift.

Alternatively, another theory posits that both sea urchin barrens and kelp-beds represent alternative stable states, meaning that an ecosystem can exist under multiple states, each with a set of unique biotic and abiotic conditions (i.e. barren except for urchins or flourishing with kelp). Those who argue for this theory propose several criteria: that different self-replacing communities dominate the site; each state exists longer than one complete turnover of the dominant community or species; and that following a disturbance (e.g. a storm), the system returns to the previous state.

Impacted areas

In 2012, it was reported that over the preceding four decades, barrens had been reported along coastlines around the world, everywhere from Nova Scotia to Chile. These barrens range from spanning over a thousand kilometers of coastline or occurring only in small patches.^[3]