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Proc Biol Sci. 2018 Jul 25;285(1883). pii: 20180553. doi: 10.1098/rspb.2018.0553.

Sudden collapse of a mesopredator reveals its complementary role in mediating rocky reef regime shifts.

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School of Resource and Environmental Management, Simon Fraser University, Burnaby, BC, Canada V5A 1S6
Hakai Institute, Heriot Bay, British Columbia, Canada V0P 1H0.
Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, CA 95060, USA.
School of Resource and Environmental Management, Simon Fraser University, Burnaby, BC, Canada V5A 1S6.
Department of Biological Science, Florida State University, Tallahassee, FL 32306, USA.
Department of Zoology, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4.


While changes in the abundance of keystone predators can have cascading effects resulting in regime shifts, the role of mesopredators in these processes remains underexplored. We conducted annual surveys of rocky reef communities that varied in the recovery of a keystone predator (sea otter, Enhydra lutris) and the mass mortality of a mesopredator (sunflower sea star, Pycnopodia helianthoides) due to an infectious wasting disease. By fitting a population model to empirical data, we show that sea otters had the greatest impact on the mortality of large sea urchins, but that Pycnopodia decline corresponded to a 311% increase in medium urchins and a 30% decline in kelp densities. Our results reveal that predator complementarity in size-selective prey consumption strengthens top-down control on urchins, affecting the resilience of alternative reef states by reinforcing the resilience of kelp forests and eroding the resilience of urchin barrens. We reveal previously underappreciated species interactions within a 'classic' trophic cascade and regime shift, highlighting the critical role of middle-level predators in mediating rocky reef state transitions.


kelp forests; predator diversity; regime shift; sea otter; sea star wasting disease; trophic cascade

[Available on 2019-07-25]

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