Ecosystem Flips, Locks, and Feedbacks: the Lasting Effects of Fisheries on Maine's Kelp Forest Ecosystem

Abstract

Ecosystems can flip and, as a result of reinforcing feedback mechanisms, lock into alternative stable states. We studied this process in a kelp-forest ecosystem in Maine, USA, for nearly four decades and found two stable states: one dominated by green sea urchins and crustose coralline algae and the other by erect fleshy macroalgae. Herbivory by urchins drives algal deforestation, but declined after fishing for sea urchins began in 1987. As the fishery expanded northeastward, so did phase shifts to macroalgae. By 2000, macroalgae dominated nearly all of coastal Maine. Monitoring newly settled sea urchins between 1996 and 2002 revealed sites with thousands of settlers per square meter per year, but virtually none survived to become adults. Algal succession to densely branched morphologies may create nursery habitat for settling crabs that prey on settling sea urchins. Experiments intended to restore herbivory to prefishing levels, through translocation of 51,000 adult sea urchins over two consecutive years at multiple release sites (with controls), resulted in complete urchin mortality both years as a result of predation by large migratory Cancer borealis Stimpson, 1859 crabs. Population densities of this crab increased fivefold coastwide soon after the macroalgal phase shift. Persistent absence of urchins (even in no-take reserves) probably resulted from predation on newly settled and/or adult urchins. Fisheries-induced declines in herbivory may therefore have improved recruitment potential for predatory crabs that then became the region's new apex predator. Cascading sequential processes of herbivory, recruitment, and predation create reinforcing feedback, effectively locking this ecosystem into alternative stable states