Global regime shift dynamics of catastrophic sea urchin overgrazing

Abstract

A pronounced, widespread and persistent regime shift among marine ecosystems is observable on temperate rocky reefs as a result of sea urchin overgrazing. Here, we empirically define regime-shift dynamics for this grazing system which transitions between productive macroalgal beds and impoverished urchin barrens. Catastrophic in nature, urchin overgrazing in a well-studied Australian system demonstrates a discontinuous regime shift, which is of particular management concern as recovery of desirable macroalgal beds requires reducing grazers to well below the initial threshold of overgrazing. Generality of this regime-shift dynamic is explored across 13 rocky reef systems (spanning 11 different regions from both hemispheres) by compiling available survey data (totalling 10 901 quadrats surveyed in situ ) plus experimental regime-shift responses (observed during a total of 57 in situ manipulations). The emergent and globally coherent pattern shows urchin grazing to cause a discontinuous ‘catastrophic’ regime shift, with hysteresis effect of approximately one order of magnitude in urchin biomass between critical thresholds of overgrazing and recovery. Different life-history traits appear to create asymmetry in the pace of overgrazing versus recovery. Once shifted, strong feedback mechanisms provide resilience for each alternative state thus defining the catastrophic nature of this regime shift. Importantly, human-derived stressors can act to erode resilience of desirable macroalgal beds while strengthening resilience of urchin barrens, thus exacerbating the risk, spatial extent and irreversibility of an unwanted regime shift for marine ecosystems.

Global patterns of kelp forest change over the past half-century

“…In other instances, regional increases in kelp (e.g., west coast of Vancouver Island, Southern California Bight) can be attributed to successful local management efforts, including the recovery of previously exploited sea urchin predators (58) and reductions in local pollution levels (59). Regional variation in the trajectories of kelp change was also influenced by the underlying frequency of sea urchin-driven phase shifts that characterize many kelp forest ecosystems (28,30). For example, long-term declines in the Aleutians were associated with persistent shifts toward sea urchin barrens since the 1990s (60).…”

“…Historically, kelp forest ecosystems have demonstrated a high degree of resilience (27,28), but recent evidence suggests that the capacity of kelp forests to recover from disturbance may be eroding (29,30). Kelp forest declines have now been documented in many regions in response to a variety of stressors (18,(31)(32)(33)(34)(35), but kelp abundances have been stable or increasing in other areas (17,36,37).…”

Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp

“…This species is well known for deforesting kelp beds in cooler latitudes along eastern Australia (29). Although the impacts of urchin grazing are highly context dependent (30,31), all data available from eastern Australia indicate that the density of black urchins observed offshore (mean ± SE = 2.5 ± 0.4 individuals·m −2 ) is too low to initiate a phase shift and cause barren formation (estimated at 4-10 individuals·m −2 ) (32). Thus, although it is possible that these urchins are contributing to the maintenance of kelp-free areas, it is highly unlikely they triggered the initial shift.…”

Herbivore control in connected seascapes: habitat determines when population regulation occurs in the life history of a key herbivore

“…Nearshore macrophyte ecosystems are particularly prone to overgrazing, shifting to heavily overgrazed barrens when herbivore populations expand without control (Wolf et al 2007, Ling et al 2015. When ecological roles are disproportionately distributed across the species assemblage, the health of the ecosystem is linked intimately with the population ecology of a few key species.…”

“…There is growing evidence that herbivore outbreaks are triggering a major expansion of overgrazed areas across the world's oceans, particularly in northern temperate seas (Filbee-Dexter and Scheibling 2014, Conversi et al 2015, Ling et al 2015. However, our ability to accurately predict impending outbreaks depends on a clear understanding of the factors influencing the arrival (recruitment and immigration), growth and loss (mortality and emigration) of individuals in a population (Ripple and Larsen 2000, Ballard et al 2001, Ling et al 2009, Petraitis and Dudgeon 2015, and how these drivers vary between habitats.…”