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90:). The transition from kelp forest to barren is defined by phase shifts in which one stable community state is shifted to another. 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.
97:, 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.
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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
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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
35:-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.
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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.
54:. 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
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David R. Schiel, and
Michael S. Foster. The Biology and Ecology of Giant Kelp Forests. Oakland, California: University of California Press, 2015. See pages 182-189.
46:), has often been cited as a cause of these barrens. When urchins are left "unchecked," their populations increase, and this has further effects on
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167:"Kelp Forests versus Urchin Barrens: Alternate Stable States and Their Effect on Sea Otter Prey Quality in the Aleutian Islands"
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decades. Adult urchins will then have to shift into foraging for other resources, such as "turfy" algal species.
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where the population growth of sea urchins has gone unchecked causes destructive grazing of
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ecosystems shift between a kelp bed and an urchin barren.
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FilbeeDexter, Karen; Scheibling, Robert E. (2014-01-09).
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