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

Abstract

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 (2012–2015, 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.

Re-examination of the effects of food abundance on jaw plasticity in purple sea urchins

“…Stunted jaw growth is especially evident when the high and high/low treatment animals are compared; there was no signiicant diference in jaw growth between the high treatment (new growth from the site of calcein stain = 0.54 ± 0.12) and the high/low treatment (0.60 ± 0.18), indicating that growth ceased when urchins were switched to the low food diet. While this result counters previous claims that material is reallocated to the jaws when food is scarce (Fansler 1983;Edwards and Ebert 1991;Ebert 1996), it corresponds to observations in natural urchin populations where jaws from urchins in barrens grow at highly reduced rates compared to those in urchins from macroalgal beds (Pederson and Johnson 2008;Ling and Johnson 2009;Ling et al 2019). It remains unclear whether these ield observations result from reallocating material to the jaws to maintain even minimal growth or from simply a slowing of urchin growth rate overall.…”

“…The possible efects of early life food stress on jaw plasticity suggest that, in addition to habitat, urchin density, and species diferences (Constable 1993;Fernandez and Boudouresque 1997;Epherra et al 2015;Haag et al 2016), resource limitation, which is closely linked to habitat and density diferences, may be an important factor underlying the presence or absence of cyclical reversibility to seasonal changes in food abundance (Ebert 2014). It may also help to explain why diferences in relative jaw lengths between urchins in barrens and macroalgal beds persist and appear to be reinforced over time (Ling et al 2019). If urchins were afected by food limitation as juveniles, then even if food conditions shift in the future, they will maintain the relative jaw lengths acquired as juveniles.…”

“…of the test that changed the jaw length to test diameter ratio, and not changes in the jaw, contrary to what has been purported previously (Ebert 1980b;Fansler 1983;Levitan 1991;Ebert 1996). Given that the test appears to grow at high rates in macroalgal beds (Pederson and Johnson 2008;Ling and Johnson 2009;Ling et al 2019), we hypothesize that test diameter is also important for deining relative jaw length in the ield as well.…”

“…For example, urchins from barrens tend to be smaller, less dense, and have thinner tests (Ling and Johnson 2009). These diferences are thought to reduce individual performance but ultimately increase population size in barrens by augmenting turnover of the population (Ling and Johnson 2009;Ling et al 2019). Conversely, the relatively longer jaws observed in urchins from barrens are thought to aid their success by increasing feeding eiciency on encrusting and calcareous algae, because longer jaws may facilitate scraping or grabbing algae from rocks (Ebert 1980b(Ebert , 2014Edwards and Ebert 1991;Fernandez and Boudouresque 1997).…”

From marine deserts to algal beds: Treptacantha elegans revegetation to reverse stable degraded ecosystems inside and outside a No‐Take marine reserve

“…Despite these efforts, attempts to recover macroalgal beds from sea urchin barren grounds remain challenging due to the high hysteresis of stable barren state and the difficulty of reestablishing populations of natural predators and consequent trophic cascades (i.e. in old and well‐enforced NTZs) that help reduce pervasive sea urchin overgrazing (Ling et al , ).…”