Sea Urchin Grazing Research Articles

Effects of macroalgae and sea urchin grazing pressure on zoantharians growth under laboratory conditions

In the context of ocean warming, thermophilic organisms such as zoantharians are expanding and altering shallow benthic habitats. Here, a four-month laboratory experiment was performed to examine the influence of three types of macroalgae morphotypes common in the Canary Islands (turf algae, Lobophora spp., and crustose coralline algae) on the growth of two zoantharian species, Palythoa caribaeorum and Zoanthus pulchellus. Additionally, the grazing effects of echinoids Diadema africanum and Paracentrotus lividus were assessed as facilitators of substrate colonization by means of controlling macroalgae cover. Colony and algal coverages were measured at the beginning, middle and end of the experiment, and increments were calculated. Results indicated a general decrease in zoantharian colony sizes in contact with different algal types in the absence of sea urchins. However, P. caribaeorum colonies showed significant growth in the presence of D. africanum, highlighting the ecological importance of sea urchins in zoantharian population proliferation and subsequent community modification. This study represents the first investigation into zoantharian-macroalgae interactions under controlled conditions.

Consequences of kelp forest ecosystem shifts and predictors of persistence through multiple stressors.

Ecological communities can be stable over multiple generations, or rapidly shift into structurally and functionally different configurations. In kelp forest ecosystems, overgrazing by sea urchins can abruptly shift forests into alternative states that are void of macroalgae and primarily dominated by actively grazing sea urchins. Beginning in 2014, a sea urchin outbreak along the central coast of California resulted in a patchy mosaic of remnant forests interspersed with sea urchin barrens. In this study, we used a 14-year subtidal monitoring dataset of invertebrates, algae, and fishes to explore changes in community structure associated with the loss of forests. We found that the spatial mosaic of barrens and forests resulted in a region-wide shift in community structure. However, the magnitude of kelp forest loss and taxonomic-level consequences were spatially heterogeneous. Taxonomic diversity declined across the region, but there were no declines in richness for any group, suggesting compositional redistribution. Baseline ecological and environmental conditions, and sea urchin behaviour, explained the persistence of forests through multiple stressors. These results indicate that spatial heterogeneity in preexisting ecological and environmental conditions can explain patterns of community change.

Regime shift in the northernmost coral ecosystem: decline of the threatened coral Acropora pruinosa due to grazing by Diadema setosum

Once thriving in Suruga Bay, Japan, the threatened coral species, Acropora pruinosa, faced drastic declines due to climate change and sea urchin grazing. Fourteen-years of observation (2009–2023) showed near extinction, highlighting the urgent need for conservation of corals, even in temperate zones, to preserve biodiversity.

Giant Kelp Forest Resilience to Ocean Warming; Historical and Modern Systems

Giant kelp forests of the North Pacific are iconic among cool temperate marine communities. They are also amongst the most productive marine ecosystems, harboring significant biodiversity and supporting coastal economies. The fate of these systems over the next eight decades is uncertain, given the variance of outcomes for global warming scenarios and the complexities of ecological forecasting. There is a growing consensus that giant kelp will be susceptible to warming, leading to a decline of the communities. Kelp forest dynamics, however, are also controlled by biotic interactions. In the North Pacific, the main biotic factors today are kelp herbivory, especially by grazing sea urchins, and predatory control of the urchins, particularly by sea otters and sunflower sea stars. A recent study demonstrated that as late as the 18th century, the now extinct mega herbivore Steller’s sea cow, an obligate kelp browser, had a significant impact on North Pacific kelp forest dynamics. Mathematical models comparing the historical community to modern ones indicated that the sea cow would have increased forest resilience against reductions of predation of urchins caused by multiple types of perturbation. In addition to epidemics and temperature anomalies, it is expected that global temperatures will increase 2-4 degrees Celsius by the year 2100. Here we use the mathematical models to explore forest dynamics under various warming scenarios, including the changing impacts of epidemics and the frequency and intensity of warming anomalies. Preliminary results suggest that in contrast to a stable temperature regime, warming to 4 degrees increases chaotic dynamics, extinction of both sea urchins and sunflower sea stars, and abundance of understory algae. Those results also suggest that chaos and algal increase would be lessened in the presence of the extinct herbivore. These findings bear implications for future kelp forest management, conservation, and human economic exploitation.

Macroalgal habitats support a sustained flux of floating biomass but limited carbon export beyond a Greenland fjord

Despite growing attention on the contribution of macroalgae to carbon cycling and sequestration (blue carbon), more observational data is needed to constrain current estimates. In this study, we estimate the floating macroalgal carbon flux within and beyond a large sub-Arctic fjord system, Nuup Kangerlua, Greenland, which could potentially reach carbon sinks. Our study estimates 1) the fjord-scale area with macroalgal coverage and barrens caused by sea urchin grazing, 2) the floating macroalgal biomass in the fjord, and 3) the annual export flux of floating macroalgae out of the fjord system. ROV surveys documented that macroalgal habitats cover 32 % of the seafloor within the photic zone (0-30 m) with an average coverage of 39.6, 22, and 7.2 % in the depth intervals 0–10, 10–20, and 20-30 m, respectively. 15 % of the area suitable for macroalgae was denuded by sea urchin grazing. Floating macroalgae were common with an average biomass of 55 kg wet weight km−2. Densities and species composition varied seasonally with the highest levels after storms. The floating biomass was composed of intertidal macroalgal species (58 %) (Fucus vesiculosus, Fucus distichus, and Ascophyllum nodosum) and kelps (42 %) (Saccharina longicruris, S. latissima, and Alaria esculenta). We deployed surface GPS drifters to simulate floating macroalgal trajectories and velocity. Data indicated that 80 % of the floating biomass is retained in the fjord where its fate in relation to long-term sequestration is unknown. Export beyond the fjord was limited and indicated an annual floating macroalgal export beyond the fjord of only 6.92 t C yr−1, which is equal to ~0.02 % of the annual net primary production. Our findings suggest that floating macroalgae support a limited blue carbon potential beyond this fjord and that future research should focus on the fate of retained floating macroalgae and subsurface export to resolve the connectivity between macroalgal habitats and long-term carbon sinks.

The differences in the trophic structure of semi‐enclosed and open coastal communities under the influence of an alien top predator (red king crab in the Barents Sea)

AbstractThe impact of the introduced red king crab (RKC), Paralithodes camtschaticus, in the Barents Sea was mostly studied through its direct predation on native species. This study uses stable isotope analysis of carbon and nitrogen to characterize major trophic groups (including grazing sea urchins) and specifically looks at RKC's trophic position and niche partitioning with the native hermit and spider crab species. To achieve this, we sampled organisms from the food web of a small fjord‐like Zelenaya Inlet on the Murmansk coast in the southwestern Barents Sea and the open sea coast, just outside the inlet for comparison. There, macrophyte‐derived carbon is the major source of organic matter along with the phytoplankton. In general, the fucoid source of primary production was more prominent within the inlet food web compared to the nearby open coastal zone. Isotopic trophic niches calculated using SIBER analysis showed partition between the native hermit and spider crabs suggesting, some degree of trophic segregation between coevolved species. RKCs are clustered together in the center of native decapod species' niches. There were some distinct differences between the feeding habits of RKCs within the inlet and the open sea coastal waters. The subadult RKCs that are present in the inlet throughout the year have a narrower (in the area and specifically in δ15N dimension) niche than the adults from the open sea. The latter have a wider variety of food items due to their migration and may prey on food items from different trophic levels. Sea urchins are an important trophic link transferring the macrophyte carbon to RKC. However, P. camtschaticus may also directly consume plant residues. Despite generally similar calculated trophic levels of decapod species, our isotopic data and literature data on the food composition of Paralithodes camtschaticus in the Barents Sea do not indicate significant RKC's competition with, and predation on, native species of crabs.

Compounded effects of sea urchin grazing and physical disturbance on macroalgal canopies in the lagoon of Moorea, French Polynesia

Release from herbivory is a factor underpinning coral replacement by macroalgae. Once macroalgae have achieved dominance, shifts back to the coral-dominated state can be hindered by stabilizing feedbacks. Thus, restoring herbivore assemblages alone can be insufficient to trigger coral recovery. However, herbivores could control macroalgal recovery in the aftermath of physical disturbances removing macroalgae. Diadematid urchins at Moorea (French Polynesia) have collapsed in the last decade. By means of a manipulative field experiment, we tested the interactive effects of physical disturbance and increased diadematid densities on macroalgae inside the lagoon. Massive Porites colonies, referred to as ‘bommies’, were assigned to 3 different macroalgal removal treatments (removal of stipes and fronds of the canopy-forming macroalgae Turbinaria ornata and Sargassum pacificum, total removal of erect macroalgae or untouched) and exposed to 3 different urchin densities (absent, low [~0.5 ind. m-2], and intermediate [~1 ind. m-2]). After 1 yr, sea urchins had no effect on the covers of S. pacificum and T. ornata when macroalgal canopies were left untouched. Urchins could control the recovery of S. pacificum on total macroalgal removal bommies, but not that of T. ornata. However, urchins, even when at intermediate densities, did not generate major changes in the structure of benthic assemblages on experimental bommies. Our study indicates that a moderate increase in diadematid densities is unlikely to reduce the extent of macroalgal stands in Moorea back reefs unless associated with the recovery of other herbivore guilds able to remove adult macroalgae (i.e. browsers).

Effects of Natural and Anthropogenic Stressors on Fucalean Brown Seaweeds Across Different Spatial Scales in the Mediterranean Sea

Algal habitat-forming forests composed of fucalean brown seaweeds (Cystoseira, Ericaria, and Gongolaria) have severely declined along the Mediterranean coasts, endangering the maintenance of essential ecosystem services. Numerous factors determine the loss of these assemblages and operate at different spatial scales, which must be identified to plan conservation and restoration actions. To explore the critical stressors (natural and anthropogenic) that may cause habitat degradation, we investigated (a) the patterns of variability of fucalean forests in percentage cover (abundance) at three spatial scales (location, forest, transect) by visual estimates and or photographic sampling to identify relevant spatial scales of variation, (b) the correlation between semi-quantitative anthropogenic stressors, individually or cumulatively (MA-LUSI index), including natural stressors (confinement, sea urchin grazing), and percentage cover of functional groups (perennial, semi-perennial) at forest spatial scale. The results showed that impacts from mariculture and urbanization seem to be the main stressors affecting habitat-forming species. In particular, while mariculture, urbanization, and cumulative anthropogenic stress negatively correlated with the percentage cover of perennial fucalean species, the same stressors were positively correlated with the percentage cover of the semi-perennial Cystoseira compressa and C. compressa subsp. pustulata. Our results indicate that human impacts can determine spatial patterns in these fragmented and heterogeneous marine habitats, thus stressing the need of carefully considering scale-dependent ecological processes to support conservation and restoration.

Disease Outbreak in a Keystone Grazer Population Brings Hope to the Recovery of Macroalgal Forests in a Barren Dominated Island

Macroalgal forests play a key role in shallow temperate rocky reefs worldwide, supporting communities with high productivity and providing several ecosystem services. Sea urchin grazing has been increasingly influencing spatial and temporal variation in algae distributions and it has become the main cause for the loss of these habitats in many coastal areas, causing a phase shift from macroalgae habitats to barren grounds. The low productive barrens often establish as alternative stable states and only a major reduction in sea urchin density can trigger the recovery of macroalgal forests. The present study aims to assess if the 2018 disease outbreak, responsible for a strong reduction in the sea urchinDiadema africanumdensities in Madeira Island, was able to trigger a reverse shift from barren grounds into macroalgae-dominated state. By assessing the diversity and abundance of benthic sessile organisms, macroinvertebrates and fishes before, during and after that particular mass mortality event, we evaluate changes in benthic assemblages and relate them to variations in grazer and herbivore densities. Our results revealed a clear shift from barren state to a macroalgae habitat, with barrens characterized by bare substrate, sessile invertebrate and Crustose Coralline Algae (CCA) disappearing after the mortality event. Overall variations in benthic assemblages was best explained by four taxa (among grazers and herbivores species). However, it was the 2018 demise ofD. africanumand its density reduction that most contributed to the reverse shift from a long stable barren state to a richer benthic assemblage with higher abundance of macroalgae. Despite this recent increase in macroalgae dominated habitats, their stability and persistence in Madeira Island is fragile, since it was triggered by an unpredictable disease outbreak and depends on howD. africanumpopulations will recover. With no control mechanisms, local urchin populations can easily reach the tipping point needed to promote a new shift into barren states. New conservation measures and active restoration are likely required to maintain and promote the local stability of macroalgal forests.

Can portunid crabs protect massive coral against the attack by long-spined sea urchins?

Sea urchin grazing can damage corals, but little is known how such damage can be remediated. We conducted three caging experiments in the field to determine the relationships among the portunid crab Thalamita prymna, the long-spined sea urchin Diadema setosum and the massive coral Platygyra carnosa. Experiment 1 showed that urchins caused coral tissue loss and reduced coral weight, and such effects were stronger at higher urchin densities. Experiment 2 showed that crabs could be effective urchin predators. Experiment 3 revealed that, when a rock with fouling organisms (an alternative food source) was available, some urchins managed to survive, but they did not cause appreciable coral damage. Together, our results indicate that, crab predation, an overlooked relationship in coral reefs, can be exploited to help control urchin corallivory and bioerosion. Prohibiting fish trapping in reef areas could reduce the by-catch of these crabs and protect reefs against urchin attack.

Interactive effects between sinking polyethylene terephthalate (PET) microplastics deriving from water bottles and a benthic grazer

The information concerning the toxicity of sinking microplastics (MPs) on benthic marine animals, particularly benthic grazers, is still scant. No study focused on biological weathering of sinked MPs operated by benthic organisms. This study aims at investigating the ingestion and the effects induced by 7-days dietary exposure to environmentally relevant amount (8, 80 and 800 particles/g of food) of irregular shaped and sized (diameter 12.6−1,065 μm; mean diameter 316 ± 12 μm) polyethylene terephthalate microplastics (PET-MPs) on a common marine benthic grazer, the sea urchin Paracentrotus lividus. Adverse effects were investigated on digestive tract at biochemical (oxidative stress biomarkers) and tissue level (histopathological analyses). Potential alteration of MP structure/surface and PET macromolecules due to the ingestion of PET-MPs within the sea urchin digestive tract were investigated. Results showed that PET-MPs were efficiently egested by sea urchins without producing histological alterations on digestive tract tissues, only inducing a slight modulation of oxidative status. Sea urchin grazing activity and the related transit of PET-MPs within animal digestive tract slightly affected MP structure and PET composition. These findings suggest that PET-MPs might represent an hazard for benthic grazer organisms, which can partially contribute to the degradation of PET in marine ecosystems.

Injured conspecifics as an alarm cue for the sea urchin Evechinus chloroticus

Predator mediation of sea urchin grazing pressure may occur via lethal removal of individual sea urchins or non-lethal modification of sea urchin behaviour. Several studies have shown that predation-related cues can affect sea urchin movement and grazing rates, but generalisations about the types of cues that prompt responses and the magnitude of those responses will require further research on a wider variety of species. We examined the effects of potential alarm cues on behaviour of the habitat-forming sea urchin Evechinus chloroticus (Echinometridae) on fished rocky reefs in northeastern New Zealand, where predators are uncommon and the sea urchins form barrens. Exposed E. chloroticus (i.e. those not in crevices) rapidly fled from injured conspecifics within a 1 m radius of the cue, but showed no apparent reaction to injured sea urchins belonging to another family (Centrostephanus rodgersii, Diadematidae), diced pilchards or the disturbance caused by fish attracted to the cues. Densities of exposed sea urchins in an area containing injured conspecifics did not return to control values for at least 20 h, while cryptic individuals remained crevice-bound when injured conspecifics were nearby. Injured conspecifics thus provide a strong, albeit localised, cue for E. chloroticus. By restricting sea urchins to crevices where they have a reduced impact on living kelp, this non-consumptive effect may complement the lethal effects of predation in marine reserves where populations of predators such as rock lobsters and large fish are allowed to recover from overharvesting by humans, thereby reinforcing the trophic cascade initiated by those predators.

Sea urchin grazing preferences on native and non-native macroalgae

Abstract Herbivory plays a major role in shaping community dynamics across freshwater, marine and terrestrial habitats, by controlling patterns of abundance and distribution of primary producers, including seaweeds. In the context of biological invasions, the proliferation of non-native seaweeds has been often attributed to limited grazing by native herbivores on introduced species (“Enemy Release Hypothesis”, ERH). In our study, we aimed to explore the potential of an abundant generalist herbivore (the sea urchin Paracentrotus lividus) to graze on non-native in comparison to native macroalgae species. For this purpose, we used manipulative experiments to assess sea urchin preference on native and non-native seaweed species present in the northwest coast of Portugal. Specifically, we determined the preferences of P. lividus on brown seaweeds i.e. Laminaria ochroleuca, Saccorhiza polyschides and the non-native Undaria pinnatifida, and red seaweeds i.e. Chondrus crispus, Mastocarpus stellatus and the non-native Grateloupia turuturu. For each group of seaweed species, sea urchin preference and biomass consumption were examined. The nutritional (organic carbon and nitrogen) and chemical (phenolic content) features were also analysed, in order to assess their effect on sea urchin feeding. According to the results, P. lividus did not show a specific preference for any of the different seaweeds of each phylum. These results suggest that P. lividus is a generalist herbivore, not exerting a differential grazing pressure on non-native seaweeds when compared to native ones.

Sea urchin control of macroalgal communities across a productivity gradient

Opposing bottom-up ‘resource-driven’ and top-down ‘consumer-driven’ forces interact to shape the structure of ecosystems. While these counteracting forces are well recognised, debate remains regarding which is more influential across space and time. Here we explore bottom-up versus top-down control of macroalgal communities for temperate rocky reef communities in highly urbanised Port Phillip Bay (PPB), southeast Australia. Field surveys show macroalgal cover to paradoxically decline with increasing ‘bottom-up’ nutrient inputs while the abundance of grazing sea urchins (Heliocidaris erythrogramma) increased. The mechanisms underpinning this pattern were examined by constructing urchin-exclusion plots using octocoral (Erythropodium hicksoni) colonies that grow on urchin barren grounds and form natural barriers to grazing sea urchins. Octocoral plots were constructed by cutting 200 mm by 200 mm squares to expose bare reef substratum within the centre of octocoral colonies, which enabled efficient replication of urchin-exclusions on barren grounds across three distinct zones of anthropogenic nutrient input in PPB. Octocoral plots successfully excluded urchins across zones and, in the absence of grazing, macroalgal production increased with increasing nutrient concentration as expected. This novel opportunity to efficiently replicate urchin-exclusions on high-density barren grounds across different zones of ‘bottom-up’ forcing demonstrates that urchin overgrazing can keep pace with and overwhelm increasing macroalgal productivity. Our findings also highlight that impacts of grazing can be greatest where bottom-up forces enable large abundances of herbivores to accumulate, which is counter to perceptions that impacts of herbivores will be greatest where macroalgal productivity is low.

Octocoral barrier to grazing sea urchins allows macroalgal recovery on barrens ground

Experimental exclusion of marine herbivores is a powerful approach to reveal their effects on the structure and function of ecosystems. However, such manipulations can be logistically challenging, particularly for subtidal reef systems. Here we report novel observations of a natural barrier to grazing sea urchins formed by the octocoral Erythropodium hicksoni (Family: Anthothelidae, Utinomi, 1971), which allowed localised growth of standing macroalgae on barrens ground despite sea urchin (Heliocidaris erythrogramma) densities locally exceeding 40 individuals m−2. Effectiveness of the octocoral barrier was experimentally tested by cutting 200 mm by 200 mm plots within the centre of octocoral colonies (providing clean reef surfaces completely enclosed by octocoral), which were otherwise surrounded by sea urchin barrens. Within 1 month, macroalgae completely colonised reef surfaces inside octocoral plots, which continued to proliferate over 12 months relative to background barren ground plots. Furthermore, a procedurally-controlled experiment (whereby barren control plots were also scraped clean of all biota to the same dimensions as octocoral plots), revealed octocoral-barriers to again exclude urchins and enable greater recovery of macroalgae relative to the procedural-control. In all octocoral plots, algae grew to the octocoral edge, indicating a lack of allelopathic/ shading effects by octocoral, which forms a thin encrusting veneer over reef surfaces <20 mm height. However, octocoral regrew to infill the excised portion of the colony at a mean rate of 0.23 mm ± 0.01SE day−1, which was routinely trimmed to maintain original plot dimensions. Competition for space between octocoral and macroalgae was also evident when recovered macroalgae overhung octocoral barriers and scoured surrounding octocoral. Results demonstrate the existence of natural biological barriers to grazing urchins, which can be utilized to efficiently replicate experimental exclusions, taking only ~10 min to construct per plot. While bio-chemical/physical mechanism(s) of the barrier were not determined, the potential for such biological barriers to form important local refugia for macroalgae and applications over larger scales remains largely unexplored.

Assessing a macroalgal foundation species: community variation with shifting algal assemblages

Foundation species have strong, positive effects on local community structure; increasing biodiversity and species abundances by providing food and habitat. On coastal temperate and subpolar rocky reefs, canopy-forming kelps form three-dimensional habitats that support numerous fish, invertebrate, and algal species. Throughout the Aleutian Archipelago, unregulated sea urchin grazing has largely removed the foundation canopy-forming kelp, Eualaria fistulosa, and most subcanopy algae. Consequently, most nearshore rocky reefs have shifted from kelp to sea urchin (Strongylocentrotus spp.) dominated habitats. These latter habitats are either urchin barrens devoid of all fleshy macroalgae, or transition forests devoid of all fleshy macroalgae algae but E. fistulosa. These three distinct communities (kelp and transition forests, and urchin barrens) were used to test the influence of E. fistulosa and sea urchins on the associated communities. Contrary to initial expectations, in transition forest habitats where E. fistulosa is the lone macroalga, no differences in community structure or in the size structure of benthic invertebrates were seen relative to urchin barrens. In kelp forests, where E. fistulosa coexisted with subcanopy macroalgae and urchins were less abundant, faunal communities were more species rich with higher abundance, biomass, and percent cover of numerous filter feeders. These findings stress not only the strong negative impact which urchins can exert on the kelp forest communities, but also the context-dependent nature of foundation species.

Top-down sea urchin overgrazing overwhelms bottom-up stimulation of kelp beds despite sediment enhancement

Biogenic marine habitats are threatened by a multitude of human stressors, and urbanised temperate coasts in particular are exposed to an intense and accumulating range of impacts. Here we use a 3-way factorial design to test the individual and combined effects of an approximate doubling of nutrients, sedimentation and sea urchin herbivory on rocky reef macroalgal bed structure over a 14-month field experiment within the urbanised setting of Australia's largest embayment (Port Phillip Bay, Victoria). Our aim was to examine whether these stressors would result in altered macroalgal bed structure (i.e. canopy and understorey components) when applied singly and in combination. The experiment was designed to evaluate the opposing influences of “top-down” (urchin overgrazing) and “bottom-up” (elevated nutrients) control of macroalgal bed structure in the presence and absence of elevated sedimentation. We show that kelp cover and abundance respond positively to nutrient enrichment (approximately twice ambient nutrient levels overall; with NH3, NOx and FRP being 2.6, 1.5 and 1.4 times higher respectively), but when combined with enhanced abundance of grazing sea urchins (at double ambient abundance), the local positive effect of nutrient enrichment was overwhelmed by the negative effect of increased herbivory. Fucoid cover showed no response to experimental treatments indicating greater resilience to urban stressors, however the trends in abundance of fucoids indicated the potential for positive nutrient and negative urchin effects manifesting beyond 14-months. Turf cover showed a very different trend, with no detectable response to nutrient enrichment, yet showed a positive response to urchin enhancement mediated by competitive release from kelp as a result of preferential urchin grazing of kelp. Enhanced sedimentation (twice ambient load), had no detectable effects on kelp, fucoids or other algal guilds, but appeared to negate the effect of urchin grazing when applied in combination with enhanced urchin abundance. Cover of non-turf understorey showed no response across all experimental treatments, indicating relatively high resilience of this guild. Collectively, our study demonstrates that ‘top-down’ effects of urchin grazing can overwhelm the positive ‘bottom-up’ effect of nutrient enhancement on kelp, and providing urchin abundance does not exceed the overgrazing threshold, that macroalgal community structure appears to demonstrate resilience to an approximate doubling in nutrient levels and/or sedimentation.

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

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 insitu 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.

Can sea urchin grazing of kelp forests in the Arctic make rocky shore systems more vulnerable to oil spills?

In Arctic Norway, the risk of major marine oil spills associated with increasing offshore drilling, land terminal, and maritime transport activities is a cause for concern. Intertidal and subtidal kelp and seaweed communities found in this region are highly productive ecosystems, although sea urchin (Strongylocentrotus droebachiensis)-overgrazed areas (urchin barrens) have been and still are a widespread problem. In the present study, we developed a kelp (Laminaria hyperborea) distribution model and studied fauna distribution, dispersal and recolonisation potentials in seaweed communities, kelp forests and sea urchin-grazed areas to assess sensitivity and vulnerability of these rocky shore communities to additional stress from oil spills, especially if the oil is released close to the coast. Our results suggest that sheltered and moderately exposed rocky shore seaweed communities close to urchin barrens are the most sensitive systems when a combined ecological effect from grazing and oil spill pollution is considered. Our study may have implications for which oil spill countermeasures that represent the best environmental practice (BEP) in the northern coastal areas that already are ecologically stressed because of sea urchin overgrazing.

Reduced resistance to sediment-trapping turfs with decline of native kelp and establishment of an exotic kelp.

Understanding the strength and type of interactions among species is vital to anticipate how ecosystems will respond to ongoing anthropogenic stressors. Here, we examine the ecological function of native (Ecklonia radiata) and invasive (Undaria pinnatifida) kelps in resisting shifts to sediment-trapping turf on reefs within the highly urbanized temperate Port Phillip Bay (PPB), Australia. Short-term (30days) and long-term (232days) manipulations demonstrated that kelp laminae can clear and maintain the substratum free of turfs, while conversely, removal of kelp leads to a proliferation of turfs. Analyses looking at the relationship between total length of E. radiata and U. pinnatifida and the area cleared of turf algae showed that the clearing effect of E. radiata over a year was greater than that of U. pinnatifida due to the annual die-back of the invasive. A natural experiment (608days) identified that ongoing sea urchin (Heliocidaris erythrogramma) grazing led to native kelp bed decline, facilitating turf dominance. Even though U. pinnatifida establishes once native beds are disturbed, its ecological function in clearing turf is weaker than E. radiata, given its annual habit. In PPB, turfs represent the more persistent and problematic algal group and are likely changing the structure, function, and energy flows of shallow temperate reefs in this urbanised embayment.