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


Ling, SD and Kriegisch, N and Woolley, BK and Reeves, SE, Density-dependent feedbacks, hysteresis, and demography of overgrazing sea urchins, Ecology, 100, (2) Article e02577. ISSN 0012-9658 (2019) [Refereed Article]


Copyright Statement

Copyright 2019 Ecological Society of America

DOI: doi:10.1002/ecy.2577


Sea urchin grazing can result in regime shift from productive kelp beds to sea urchin barren grounds that represent an alternative and stable reef state. Here we examine the stability of urchin barrens by defining the demographics of the Australian urchin Heliocidaris erythrogramma during regime shift to, and maintenance of, barrens. Inverse-logistic modeling of calibrated in situ annual growth increments for five urchin populations, two from kelp beds and three from barrens, demonstrate slowing of urchin growth as availability and consumption of standing and/or drift kelp declines. Population age structures were predicted from observed sizes over four years (20122015, n = 5,864 individuals), which indicated stable age distributions for populations both maintaining barrens and actively grazing among kelp beds. Younger age distributions occurred on barrens whereas more mature populations existed within kelp beds, indicating that high recruitment facilitates maintenance of barrens while overgrazing appeared more reliant on adult urchins grazing from the edges of kelp beds, as opposed to juvenile recruitment among kelp. Leslie-matrix projections indicated potential for unchecked population growth for all study populations, but which varied depending on whether local or regional recruitment rates were modeled. Ultimately, strong density dependence was observed to check population growth; with high-recruitment/high-density populations offset by reduced growth rates and decreased longevity. Increasing disease rates among older urchins in high density populations were consistent with observed density-dependent mortality, while tethering of healthy urchins revealed highest predation on small urchins within kelp beds, suggesting some remnant resilience of declining kelp habitat. Results demonstrate that the greatest opportunity for urchin population control is when reefs exist in the kelp bed state, at which point urchin populations are prone to negative feedback. Conversely, control of urchins on barrens is demonstrably difficult given positive density-dependent feedbacks that act to stabilize population size and which evidently underpin the hysteresis effect governing the persistence of this alternative stable state.

Item Details

Item Type:Refereed Article
Keywords:age, alternative stable states, catastrophic regime-shift, disease, growth, kelp beds, population dynamics, predation, sea urchin barrens, temperate reef ecology
Research Division:Biological Sciences
Research Group:Ecology
Research Field:Marine and estuarine ecology (incl. marine ichthyology)
Objective Division:Animal Production and Animal Primary Products
Objective Group:Fisheries - wild caught
Objective Field:Fisheries - wild caught not elsewhere classified
UTAS Author:Ling, SD (Dr Scott Ling)
UTAS Author:Kriegisch, N (Ms Nina Kriegisch)
UTAS Author:Woolley, BK (Miss Bailee Woolley)
UTAS Author:Reeves, SE (Mr Simon Reeves)
ID Code:130556
Year Published:2019
Funding Support:Australian Research Council (DP170104668)
Web of Science® Times Cited:30
Deposited By:Ecology and Biodiversity
Deposited On:2019-02-02
Last Modified:2020-01-06
Downloads:92 View Download Statistics

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