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Density-dependent feedbacks, hysteresis, and demography of overgrazing sea urchins


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Ling, SD ORCID: 0000-0002-5544-8174, Kriegisch, N, Woolley, BK and Reeves, SE 2019 , 'Density-dependent feedbacks, hysteresis, and demography of overgrazing sea urchins' , Ecology, vol. 100, no. 2 , pp. 1-19 , doi: 10.1002/ecy.2577.

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Sea urchin grazing can result in regime shift from productive kelp beds to seaurchin barren grounds that represent an alternative and stable reef state. Here we examine thestability of urchin barrens by defining the demographics of the Australian urchin Heliocidariserythrogramma during regime shift to, and maintenance of, barrens. Inverse-logistic modelingof calibrated in situ annual growth increments for five urchin populations, two from kelp bedsand three from barrens, demonstrate slowing of urchin growth as availability and consumptionof standing and/or drift kelp declines. Population age structures were predicted from observedsizes over four years (2012–2015, n = 5,864 individuals), which indicated stable age distributionsfor populations both maintaining barrens and actively grazing among kelp beds. Youngerage distributions occurred on barrens whereas more mature populations existed within kelpbeds, indicating that high recruitment facilitates maintenance of barrens while overgrazingappeared more reliant on adult urchins grazing from the edges of kelp beds, as opposed tojuvenile recruitment among kelp. Leslie-matrix projections indicated potential for uncheckedpopulation growth for all study populations, but which varied depending on whether local orregional recruitment rates were modeled. Ultimately, strong density dependence was observedto check population growth; with high-recruitment/high-density populations offset by reducedgrowth rates and decreased longevity. Increasing disease rates among older urchins in high densitypopulations were consistent with observed density-dependent mortality, while tetheringof healthy urchins revealed highest predation on small urchins within kelp beds, suggestingsome remnant resilience of declining kelp habitat. Results demonstrate that the greatest opportunityfor urchin population control is when reefs exist in the kelp bed state, at which pointurchin populations are prone to negative feedback. Conversely, control of urchins on barrensis demonstrably difficult given positive density-dependent feedbacks that act to stabilize populationsize and which evidently underpin the hysteresis effect governing the persistence of thisalternative stable state.

Item Type: Article
Authors/Creators:Ling, SD and Kriegisch, N and Woolley, BK and Reeves, SE
Keywords: age, alternative stable states, catastrophic regime-shift, disease, growth, kelp beds, population dynamics, predation, sea urchin barrens, temperate reef ecology
Journal or Publication Title: Ecology
Publisher: Ecological Soc Amer
ISSN: 0012-9658
DOI / ID Number: 10.1002/ecy.2577
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Copyright 2019 Ecological Society of America

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