The present is the key to the past: linking regime shifts in kelp beds to the distribution of deep‐living sea urchins

Abstract

Understanding processes that drive sudden shifts in ecosystem structure and function has become an important research focus for coastal management. In kelp bed ecosystems, regime shifts occur when high densities of sea urchins destructively graze kelp and create coralline algal barrens. While the importance of predation and disease in mediating shifts between kelp beds and barrens on shallow rocky reefs has been well documented, little is known about the role of deep-living urchins in these alternative stable-state dynamics. In this study, we test the hypothesis that deep-living urchins along the central Atlantic coast of Nova Scotia move onshore and trigger shifts from kelp beds to barrens on shallow rocky reefs. We documented urchin distribution and abundance using tow-camera surveys down to 140 m depth and spanning 140 km of coast and created a predictive species-distribution model using these observations and spatial data on environmental factors that likely delineate suitable habitat for urchins. We used a random forest model to generate our predictions, which correctly classified 91% of observations into a positive or negative occurrence of urchins. Sea urchins predominantly occurred within 1.5 km of shore, in depressions and flat habitats between 40 and 85 m depth. We found that shallow regions where destructive grazing fronts have been documented over the past four decades were closer to deep-living sea urchin habitats compared to regions that remained in a kelp bed state during the same period. This supports our prediction that deep-living urchins play an important role in driving shallow regime shift dynamics, and indicates that their distribution can help identify areas of coast that are most vulnerable to a collapse to barrens.

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

“…As such, H. erythrogramma is behaviorally similar to Strongylocentrotids not only by its mode of barrens formation (Feehan et al. , Filbee‐Dexter and Scheibling ), but also in its propensity to switch from a passive/cryptic grazer of detached kelps to an active over‐grazer of standing kelp beds under conditions of low drift kelp supply (Harrold and Reed , Ling et al. , Kriegisch ).…”

“…Thus, the combined increase in predation mortality of young urchins within kelp beds (also see Rowley 1990) and potential for greater migration of older individuals to kelp beds indicates an overgrazing mode of H. erythrogramma contingent on adult urchins grazing from the edges of kelp beds (outside in), as opposed to significant recruitment of juveniles among kelp. As such, H. erythrogramma is behaviorally similar to Strongylocentrotids not only by its mode of barrens formation (Feehan et al 2012, Filbee-Dexter andScheibling 2017), but also in its propensity to switch from a passive/cryptic grazer of detached kelps to an active over-grazer of standing kelp beds under conditions of low drift kelp supply (Harrold and Reed 1985, Ling et al 2010, Kriegisch 2016. Notably, grazing by H. erythrogramma therefore contrasts with the primary overgrazing mode of the sympatric Australian sea urchin Centrostephanus rodgersii, which typically forms urchin barrens from the inside out by recruiting within kelp beds and creating incipient barrens patches, which coalesce into large-scale barrens features (Flukes et al 2012).…”

Leveraging the blue economy to transform marine forest restoration

“… e Coarse estimate of area based on the average kelp forest margin of 0.23 km from shore (Filbee‐Dexter and Scheibling 2017) multiplied by the km of coastline loss. …”

Interaction among sea urchins in response to food cues

“…Density is an important characteristic of a group 19 , 20 . Sea urchin groups of high densities are closely associated with the overgrazing and consequent low-productive barrens 5 , 21 , 22 . Field experiment showed that sea urchins aggregated more in a density of 20 ind/m 2 than of 4 ind/m 2 23 .…”