High Densities Of Sea Urchins Research Articles

Ocean warming undermines the recovery resilience of New England kelp forests following a fishery-induced trophic cascade.

Ecological theory predicts that kelp forests structured by trophic cascades should experience recovery and persistence of their foundation species when herbivores become rare. Yet, climate change may be altering the outcomes of top-down forcing in kelp forests, especially those located in regions that have rapidly warmed in recent decades, such as the Gulf of Maine. Here, using data collected annually from 30+ sites spanning >350 km of coastline, we explored the dynamics of Maine's kelp forests in the ~20 years after a fishery-induced elimination of sea urchin herbivores. Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20th century, we found that forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest. Forests collapsed in the southwest apparently because ocean warming has-directly and indirectly-made this area inhospitable to kelp. Indeed, when modeling drivers of change using causal techniques from econometrics, we discovered that unusually high summer seawater temperatures the year prior, unusually high spring seawater temperatures, and high sea urchin densities each negatively impacted kelp abundance. Furthermore, the relative power and absolute impact of these drivers varied geographically. Our findings reveal that ocean warming is redefining the outcomes of top-down forcing in this system, whereby herbivore removal no longer predictably leads to a sustained dominance of foundational kelps but instead has led to a waning dominance (northeast) or the rise of a novel phase state defined by "turf" algae (southwest). Such findings indicate that limiting climate change and managing for low herbivore abundances will be essential for preventing further loss of the vast forests that still exist in northeast Maine. They also more broadly highlight that climate change is "rewriting the rules" of nature, and thus that ecological theory and practice must be revised to account for shifting species and processes.

Fisheries restrictions and their cascading effects on herbivore abundance and macroalgae removal at Kenyan coral reefs

The increase of macroalgae at degraded reefs impedes several ecosystem services and calls for effective methods to facilitate a return to coral dominance. Removal of macroalgae (browsing) is typically realized by fish, but the role and identity of browsers at the heavily-fished East African coast is still largely unknown. This study investigated how browsing pressure at Kenyan reefs (−4.700, 39.396) related to fisheries management and herbivore community. From October 2018 to January 2019, consumption during 24-h buffet assays using the brown macroalgae Sargassum and Padina was determined and video recorded at six sites: two in fished zones, two in marine reserves (traditional fishing allowed) and two in no-take zones. Herbivorous fish composition, biomass and sea urchin density were also determined. Consumption of Sargassum and Padina was nearly three-fold lower in the fished zones (26% and 28% of macroalgal biomass consumed, respectively) compared to the no-take zones (62% and 82%), with intermediate consumption in the marine reserves (48% and 71%). Herbivore biomass was seven-fold higher in no-take zones and included substantially more browsers (mainly unicornfishes, Naso spp.) and scrapers (scarids), which were associated with the higher browsing pressure. Browsers and scrapers were predominantly responsible for the consumption of macroalgae as determined by video recordings, though key species differed across sites. In contrast, damselfish-dominated fished sites were associated with high sea urchin densities and low browsing pressure. These results indicate that fishing restrictions are likely to support reef resilience by increasing herbivorous fish biomass of key species and thereby promote macroalgae removal.

Interaction of marine heatwaves and grazing on two canopy-forming algae

Brown canopy-forming macroalgae species form complex assemblages known as marine forests, which are highly productive systems and provide multiple ecosystem services. In many regions worldwide, these key foundation species are being replaced by generalist species due to numerous underlying impacts acting at local, regional and global scales (i.e. overgrazing, marine heatwaves (MHWs), rising temperatures, among others). In Madeira Island (Macaronesia, NE Atlantic Ocean), rocky bottoms on the south coast are dominated by bare rocks, sediments and high densities of sea urchins and turfs. One of the most common macroalgae in these degraded systems is the cryptogenic red algae Asparagopsis taxiformis, naturally competing with other canopy-forming algae such as Cystoseira foeniculacea. In this study, we analysed 37-year high-resolution sea surface temperature data on the occurrence of MHWs in Macaronesia, finding a significant increase in the frequency of moderate and extreme events in the last 20 years. We performed a mesocosm experiment to understand how these two foundation species respond to heat stress and grazing activity by the sea urchin Paracentrotus lividus under different scenarios simulating moderate and extreme MHWs. Biomass and productivity of C. foeniculacea showed a significant reduction under the interaction of MHWs and grazing, with higher loss of biomass at the moderate event. The biomass of A. taxiformis decreased significantly by the increase in temperatures, independetly of the grazing pressure. The productivity of A. taxiformis was significantly reduced at extreme MHWs, where respiration rate was higher than C. foeniculacea, which outperformed in the dark phase. Results suggest that under control conditions and moderate MHWs, A. taxiformis is not impacted by grazing and is possibly more efficient in resource utilisation than C. foeniculacea, but losing this potential advantage under extreme MHWs. Under the context of future climate change, marine forests in Madeira are, thus, expected to be threatened by the combined effects of more common extreme climatic events (e.g. MHWs) and local changes in the biotic interactions (e.g. grazing and competition).

Assessment of Exploitation Intensity of Commercial Species and Associated Benthic Communities, in Chilean Marine Management Areas of North Patagonia

The Aysén region of Chile (North Patagonia), has had limited studies on the effectiveness of management and exploitation areas of benthic resources, and performance relative to open access areas in this region has never been evaluated. We evaluated seven management areas (MAs) and five open access areas (OAAs) between 43.9°S and 45.2°S for exploitation intensity of three commercial species (Concholepas concholepas, Loxechinus albus, and Ameghinomya antiqua) together with characterization of the benthic community. Indicators based on size, density and weight were used to evaluate exploitation intensity of commercial species. Associated benthic communities were evaluated considering density, species composition, and community structure. We found a high species richness and a community structure with low variability between MAs and OAAs. Low densities and small sizes classes of C. concholepas in most of the areas indicated high exploitation intensity in both MAs and OAAs. In this context, a permanent ban to harvest C. concholepas within OAAs may need to be reevaluated since with no enforcement and monitoring, the exploitation status of this species remains unclear in these areas. L. albus in most areas were absent in the harvestable sizes, which could be indicating high exploitation intensity in both regimes. High densities and small sizes of C. concholepas and L. albus in some MAs, indicated a potential recruitment zones which bears further investigation. A. antiqua, showed better conditions than other commercial species evaluated, with no significant differences in densities and size-based indicators when comparing OAAs and MAs. Benthic communities were dominated numerically by the Echinoidea class in both MAs and OAAs, with L. albus, Arbacia dufresnii and Pseudechinus magellanicus being the dominant species. High densities of sea urchins co-occurring with low coverage of macroalgae found in MAs-Gala could indicate that a sea urchin barren was dominant during the study period. On the other hand, high densities of Cosmasterias lurida, a predatory sea star, in conjunction with low densities of C. concholepas in most of the studied areas suggested that a shift in predator roles is occurring. No differences were estimated in terms of fisheries indicators and benthic community structure across the two management regimes, suggesting the poor performance of MAs in Aysén region. Our data also support the need to improve monitoring of MAs especially with respect to associated benthic community incorporating a broader spatial scale.

Large-Scale Sea Urchin Culling Drives the Reduction of Subtidal Barren Grounds in the Mediterranean Sea

Increasing anthropogenic pressures are causing long-lasting regime shifts from high-diversity ecosystems to low-diversity ones. In the Mediterranean Sea, large extensions of rocky subtidal habitats characterized by high diversity have been completely degraded to barren state due to high grazing pressure exerted by sea urchins, which could persist for long time. In several areas of the world, a positive effect of sea urchin removal on the recovery of overexploited subtidal rocky habitats was found. This study assessed, for the first time in the Mediterranean Sea, the effects of extensive sea urchin culling on the recovery of subtidal reefs from the state of barrens. We tested this approach within a Marine Protected Area where a combination of oligotrophic conditions, general depletion of fish stocks, dramatically high sea urchin densities, and the large extension of barren grounds caused by date mussel fishery hampered the natural recovery of shallow rocky reefs. Culling intervention (through hammering) was carried out in spring 2015, covering an area of 1.2 hectares at about 5 m depth. The effects of sea urchin removal were monitored at regular intervals, for a time span of 3 years, and were compared with two control sites adjacent to the culling area. We documented a progressive reduction of the barren grounds in the fully protected area after the intervention. Also, very low re-colonization of sea urchins was observed during the experiment, so that no additional extensive culling was necessary. Our findings suggested sea urchin culling as a promising practice, also considering the limited costs of the intervention. However, since the reduction of barren grounds was largely driven by turf forming algae, caution is needed in the interpretation of the outcomes in terms of restoration, and results are discussed considering the factors involved in the observed shift and the synergies to be carried out for a full recovery of the system.

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

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 140m depth and spanning 140km 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 1.5km of shore, in depressions and flat habitats between 40 and 85m 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.

A first approach to stock assessment of the sea urchin Paracentrotus lividus (Lamarck, 1816) in Cantabria (Bay of Biscay)

The purple sea urchin P. lividus is a marine resource greatly valued for its highly nutritious roe. The objective of this work is to analyse the spatial distribution pattern and population structure of this species in the region of Cantabria (Bay of Biscay) in order to identify the most appropriate zones for sustainable harvesting and for the establishment of adequate management strategies. Two depth ranges were studied (<5 and 5–10 m) in five coastal areas. The available stock of commercial individuals (size >55 mm) was estimated based on the density of individuals and biometric parameters. Significant differences were found among areas but not among depth ranges. Three areas showed low densities with large individuals, whereas two areas showed high densities with small individuals. Despite the average smaller size of the individuals, these high-density areas accumulated 76% of the commercial stock, making these the most suitable zones for the sustainable harvesting of P. lividus in the region. The high variability of the observed density values resulted in elevated uncertainty in the estimation of stocks. However, the high sea urchin densities together with the high percentages of commercial individuals suggest that the resource is currently underexploited. Further studies should be carried out, and precautionary measures should be adopted before opening the fishery. The suitability of many of these measures is discussed in the present work, as a necessary step in the management of the P. lividus fishery in Cantabria.

The influence of herbivores on primary producers can vary spatially and interact with disturbance

Identifying the major drivers of ecosystem change remains a central area of ecological research. Although top–down drivers of change have received particular focus, we still have little understanding of how consistently these factors control an ecosystem's shift in both directions, between different ecosystem states. Using a crossed experiment in a shallow embayment in southeastern Australia, we investigated the roles of disturbance (kelp removal) and sea urchin herbivory (via increased density) to determine their contributions to shifts away from a kelp-dominated community. In a second experiment, done in urchin barren areas at two sites, we tested whether reductions in ambient sea urchin densities allowed an algal shift in the reverse direction. In both experiments, we observed that high densities of sea urchins could negatively influence kelp and macroalgal abundance. However, in the kelp bed, a moderate or severe disturbance resulted in a comparable algal response, irrespective of urchin density. The influence of sea urchins also varied dramatically between the two urchin barren sites. Here, reducing urchin densities resulted in algal recovery at one site, but at the other site, substantial colonisation of barren areas by canopy-forming brown algae and Ulvales occurred across all (low, medium, and high) urchin density treatments. Our findings illustrate multiple pathways of urchin barren creation and algal recovery, and reveal that shifts both to and from an urchin barren state can occur irrespective of herbivore pressure. These alternate pathways can operate over short spatial distances or with different regimes of disturbance.

Fish and sea urchin grazing opens settlement space equally but urchins reduce survival of coral recruits

Grazing fishes and invertebrates are influential in marine ecosystems because they open space for benthic recruits, alter post-settlement recruit survival, and can often determine benthic community composition. On tropical reefs, grazing fishes and sea urchins can play key roles in limiting growth of fleshy macro-algae, thereby facilitating coral recruitment and maintain- ing coral-dominated communities. However, as grazer abundance increases, grazer influence on corals (or other settlers) may shift from being positive and indirect by reducing space competitors, to being negative and direct by damage or removal of coral recruits. Fishing can alter both the abundances and types of dominant grazers, with potential cascading effects on coral recruitment and subsequent community organization. In Kenya, sea urchins dominate grazing on heavily fished reefs, while herbivorous and omnivorous fishes dominate grazing within fisheries closures (marine protected areas). We used reefs under these 2 fishery management systems to investigate the effects of fish versus sea urchin grazing on the availability of settlement substrate for corals, subsequent coral settlement, and mortality of coral recruits. Fish and sea urchin grazers were equally effective at clearing benthic space for coral settlement. However, grazing associated with high densities of sea urchins on fished reefs removed many coral recruits after settlement. In con- trast, fish grazing within fisheries closures enhanced coral survival compared to non-grazed treat- ments. We conclude that the effects of reduced abundance of grazing fishes (on available space for coral settlement) may be initially offset by increased sea urchin grazing, but that higher urchin abundances may ultimately reduce coral cover by their negative influence on post-settlement survival.

Rock walls: small-scale diversity hotspots in the subtidal Gulf of Maine

The physical orientation of rocky substrate profoundly affects subtidal marine commu- nities of sessile organisms. Anecdotal descriptions of dramatic differences between communities on rock walls and adjacent horizontal rocky bottoms abound in the literature and are common knowl- edge among scuba divers, yet these differences have rarely been quantified by ecologists. We sam- pled rock walls and adjacent horizontal rock bottoms at 8 subtidal sites across the Gulf of Maine. Spe- cies richness and abundance, in percent cover, of sessile invertebrates on vertical walls averaged 4 times higher than that on horizontal platforms, whereas abundance of macroalgae on horizontal rock was about 3 times that on vertical walls. Both macroalgae and sessile invertebrates were less abundant, particularly on horizontal surfaces, at sites with high sea urchin densities. The consistency of sessile invertebrate domination of vertical walls versus macroalgal domination of horizontal rock, combined with previous experimental results, suggests that competition for space between autotro- phic and heterotrophic organisms drives this pattern. The partitioning of autotrophs (macroalgae) and heterotrophs (sessile invertebrates) between horizontal and vertical surfaces respectively implies that topographic heterogeneity plays an important role in the structure, composition and function of rocky subtidal ecosystems.

The effects of trophic interactions and spatial competition on algal community composition on Hawaiian coral reefs

AbstractMuch of coral reef ecology has focused on how human impacts change coral reefs to macroalgal reefs. However, macroalgae may not always be a good indicator of reef decline, especially on reefs with significant sea urchin populations, as found in Kenya and Hawaii. This study tests the effects of trophic interactions (i.e. herbivory by fishes and sea urchins) and spatial competition (between algae and coral) on algal community structure of reefs surrounding two Hawaiian Islands that vary in their level of human impacts. Reef‐building organisms (corals and crustose coralline algae) were less abundant and turf algae were more abundant on Maui as compared to Lanai, where human impacts are lower. In contrast to previous studies, we found no evidence that macroalgae increased with human impacts. Instead, low turf and macroalgal abundance were best explained by the interactive effects of coral cover and sea urchin abundance. Fishing sea urchin predators appeared to have cascading effects on the benthic community. The absence of sea urchin predators and high sea urchin densities correspond to a disproportionately high abundance of turf and crustose coralline algae. We propose that high turf algal abundance is a better indicator of reef decline in Hawaii than high macroalgal abundance because turf abundance was highest on reefs with low coral cover and few fish. The results of this study emphasize that understanding changes in community composition are context‐dependent and that not all degraded reefs look the same.

Density-dependent sea urchin grazing: differential removal of species, changes in community composition and alternative community states

The grazing activity of consumers causes shifts between alternative states in a variety of terrestrial and marine ecosystems. One of the best examples of a consumer-driven shift occurs on temperate marine reefs, where grazing by high densities of sea urchins results in a shift from a foliose algal- to a crustose algal-dominated state. In this study, we focussed on 2 largely untested but impor- tant issues during the transition from a foliose algal- to a crustose algal-dominated state: (1) whether sea urchins impact foliose algal community composition by differentially removing species and (2) whether any impacts of grazing vary with 2 different densities of aggregating sea urchins. We tracked the movement of a high-density front of the sea urchin Heliocidaris erythrogramma and then performed experimental manipulations of H. erythrogramma at 2 unusually high but naturally occur- ring densities. Non-metric multidimensional scaling (nMDS) followed by analysis of similarities (ANOSIM) showed differences in the foliose algal community composition, and therefore differential removal of species, between permanent plots before and during grazing (surveys), and between grazed and ungrazed plots (experiment). Of the 6 abundant foliose algae, 2 had relatively low sur- vivorship (Amphiroa anceps and Zonaria diesingiana), while 2 had relatively high survivorship (Delisea pulchra and Corallina officinalis) when grazed by high densities of sea urchins. Grazing by different densities of H. erythrogramma resulted in differences in the foliose algal community composition and for the chemically-defended D. pulchra there appeared to be a threshold sea urchin density required before its removal. Our results show that an intermediate community state composed of grazer-resistant foliose algae and crustose algae can occur, which may have important consequences for community composition.

Effects of herbivores on a Posidonia oceanica seagrass meadow: importance of epiphytes

There is increasing evidence that herbivory in seagrasses is more important than previ- ously thought. However, little experimental data evaluating the importance of seagrass grazing and the factors involved in these interactions is yet available. We carried out an experimental evaluation of the effects of macroherbivores (i.e. the sea urchin Paracentrotus lividus and the sparid fish Sarpa salpa) in a temperate seagrass (Posidonia oceanica) meadow by means of 2 macroherbivore density manipulation experiments in which several measurements of plant and epiphyte vitality, and abun- dance were carried out. The results show that P. oceanica can withstand high densities of sea urchins over long periods, as no significant effect on shoot density, shoot size, leaf growth or carbohydrate reserves was detected during the time course of both experiments. Conversely, epiphyte load was significantly higher in herbivore exclusion cages; even though this increase did not appear to reduce plant vitality. Furthermore, epiphyte biomass was greatly reduced when urchins were present even at relatively low densities (5 ind. m -2 ), suggesting that epiphytes are the most limiting food resource for sea urchins in P. oceanica meadows. Whereas epiphyte load was only reduced 30% by the pres- ence of fish alone, sea urchins at low densities (5 ind. m -2 ) decreased epiphyte load ca. 60%, reach- ing values as high as 80% in high density treatments, in which case the effect of fish was negligible. This study shows that in P. oceanica meadows, seagrass-herbivore interactions are further compli- cated by an underlying network of seagrass-epiphyte-herbivore, in which epiphytes appear to play a crucial role.

Persistence of sea urchin (Heliocidaris erythrogramma) barrens on the east coast of Tasmania: inhibition of macroalgal recovery in the absence of high densities of sea urchins

Sea urchin barrens occur commonly in temperate regions throughout the world and have significant implications for ecological processes on subtidal rocky reefs because they constitute areas of low productivity and diversity compared with habitats dominated by macroalgae. On the east coast of Tasmania, the occurrence of sea urchin (Heliocidaris erythrogramma) barrens in sheltered bays has additional implications in that they represent an important habitat of the introduced kelp Undaria pinnatifida. Identifying the factors responsible for ongoing maintenance of the barren habitat is essential in defining management options to promote recovery of native canopy-forming species. We used transplant experiments to investigate whether inhibition of recovery of native canopy-forming algae can occur in the absence of intense sea urchin grazing. High densities of native canopy-forming species successfully colonised paving blocks deployed in a dense algal bed adjacent to a sea urchin barren. Transplanting these paving blocks to plots on the barren from which sea urchins were removed resulted in >80% mortality of recruits after three months, and 100% mortality after seven months. The decline in macroalgal recruits on paving blocks transplanted to the urchin barren was associated with an increase in the cover anddepth of sediment. A persistent cover of sediment also developed on paving blocks deployed on the urchin barren, where no native canopy-forming algal recruits were observed. While sea urchins are undoubtedly important in creating urchin barrens, our results suggest that other mechanisms can influence recovery of native canopy species. In sheltered and semi-exposed bays on the east coast of Tasmania, sedimentation appears to play a critical role in inhibiting early developmental stages of native macroalgae, thereby contributing to a positive feedback that acts to maintain the barren habitat.

Algal cover and sea urchin spatial distribution at Madeira Island (NE Atlantic)

This study describes sea urchin spatial distribution in relation to environmental factors, and the relationship between Diadema antillarum density and algal abundance. Twenty-three transects around Madeira Island were surveyed by scuba divers, and sea urchin density and algal cover were determined in situ. Sampling sites along these transects were characterised in terms of distance from the tide line, water depth, substratum type, bottom declivity and water turbulence. Diadema antillarum was the dominant sea urchin species. Paracentrotus lividus and Arbacia lixula occurred at shallower depths (2-6 m), contrasting with the distribution of Sphaerechinus granularis , which occurs among D. antillarum (4-20 m). Surveys found two alternative types of communities on rocky shores: 1) a community with high algal cover and low numbers of sea urchins, along the north and south-west coasts and; 2) a community with little algal cover and high densities of sea urchins, along the south-east coast. Macroalgal cover and D. antillarum densities were inversely correlated (adjusted R 2 =75.6%; n = 429; p D. antillarum on rocky substrates. We propose a multiple non-linear regression model (using backward stepwise analysis) to explain D. antillarum abundance on the rocky shores: D. antillarum/m 2 (??)= 0.121 - 0.209 distance from shore (in m) (??) + 2.052 water depth (in m) (??) - 1.778 water turbulence level (??) - 0.007 water turbulence level4 (??); where ?? indicates data are square-root transformed (adjusted R 2 = 60.99%; n = 454; p< 0.05).

Effect of Variable Recruitment and Post-Recruitment Herbivory on Local Abundance of a Marine Alga

For marine algae, the relative importance of recruitment and post-recruitment processes in determining population abundance is not well understood. We documented patterns of recruitment and post-recruitment mortality for a common, clonally reproducing subtidal red alga from southeastern Australia, Delisea pulchra, and used a combination of empirical analyses and population simulations to assess the relative importance of the two processes to local population abundance. Using a hierarchical sampling design consisting of two depths and two spatial scales (quadrats and sites) within each depth, we followed monthly cohorts from recruitment to adulthood (6 mo of age). Recruitment of D. pulchra varied significantly in space and time and was generally decoupled from local adult abundance. Mortality was consistently high, particularly in the deep sites where only 1 of 291 plants survived >5 mo, and in the shallow sites when high densities of grazing sea urchins were present. Both early (1 mo) and late (6 mo) post-recruitment mortality of monthly cohorts were density independent at both spatial scales examined. Despite this pattern of mortality, variation in recruitment explained a significant amount of the subsequent variation in adult abundance in only 3 out of 10 cohorts at both spatial scales. This result indicates that high density-independent mortality was often important in determining adult abundance for individual cohorts. However, cumulative (across cohorts) recruitment into sites and quadrats over the entire study period explained a significant amount of the variation in the total number of plants that survived to reproduce there. Simulation models based on the measured demography of D. pulchra examined how the relative importance of recruitment and survivorship to population growth changed under two categories of sea urchin density: low (<80 urchins/m2) and high (>80 urchins/m2). These two categories represented the densities of sea urchins above and below a transition point where the mortality of D. pulchra went from being relatively low (17–33%/mo) to very high (63%/mo). When the effects of high densities of sea urchins were simulated, they caused a 95% decline in the population abundance of D. pulchra, compared to only a 31% decline when there were low densities of urchins. Significantly, the relative importance of recruitment and survivorship varied with sea urchin density: when there were high densities of sea urchins, population growth was more sensitive to increases in recruitment, whereas when there were low densities of urchins, survivorship was more important. Our findings demonstrate that the local population abundance of D. pulchra is determined by a combination of recruitment and density-independent mortality following recruitment, but it appears that the relative importance of the two processes varies in time and space.

EFFECT OF VARIABLE RECRUITMENT AND POST-RECRUITMENT HERBIVORY ON LOCAL ABUNDANCE OF A MARINE ALGA

For marine algae, the relative importance of recruitment and post-recruitment processes in determining population abundance is not well understood. We documented patterns of recruitment and post-recruitment mortality for a common, clonally reproducing subtidal red alga from southeastern Australia, Delisea pulchra, and used a combination of empirical analyses and population simulations to assess the relative importance of the two processes to local population abundance. Using a hierarchical sampling design consisting of two depths and two spatial scales (quadrats and sites) within each depth, we followed monthly cohorts from recruitment to adulthood (6 mo of age). Recruitment of D. pulchra varied significantly in space and time and was generally decoupled from local adult abun- dance. Mortality was consistently high, particularly in the deep sites where only 1 of 291 plants survived >5 mo, and in the shallow sites when high densities of grazing sea urchins were present. Both early (1 mo) and late (6 mo) post-recruitment mortality of monthly cohorts were density independent at both spatial scales examined. Despite this pattern of mortality, variation in recruitment explained a significant amount of the subsequent variation in adult abundance in only 3 out of 10 cohorts at both spatial scales. This result indicates that high density-independent mortality was often important in determining adult abundance for individual cohorts. However, cumulative (across cohorts) recruitment into sites and quadrats over the entire study period explained a significant amount of the variation in the total number of plants that survived to reproduce there. Simulation models based on the measured demography of D. pulchra examined how the relative importance of recruitment and survivorship to population growth changed under two categories of sea urchin density: low ( 80 urchins/M2). These two categories represented the densities of sea urchins above and below a transition point where the mortality of D. pulchra went from being relatively low (17-33%/mo) to very high (63%/mo). When the effects of high densities of sea urchins were simulated, they caused a 95% decline in the population abundance of D. pulchra, compared to only a 31% decline when there were low densities of urchins. Significantly, the relative importance of recruitment and survivorship varied with sea urchin density: when there were high densities of sea urchins, population growth was more sensitive to increases in recruitment, whereas when there were low densities of urchins, survivorship was more important. Our findings demonstrate that the local popu- lation abundance of D. pulchra is determined by a combination of recruitment and density- independent mortality following recruitment, but it appears that the relative importance of the two processes varies in time and space.

Effects of removing sea urchins (Strongylocentrotus droebachiensis): Stability of the barren state and succession of kelp forest recovery in the east Atlantic.

Stability properties of the barren state of a kelp forest-sea urchin system were studied in northern Norway. The ability of the sea urchin Strongylocentrotus droebachiensis to maintain high population densities and recover from perturbations, and the succession of kelp forest revegetation, were studied experimentally by reducing the sea urchin density on a barren skerry. Additional information was obtained from community changes following a natural, but patchy, sea urchin mortality that varied between sites. On the barren grounds, high sea urchin densities (30 50 per m2) is maintained by annual recruitment. Severe reductions of sea urchin densities initiated luxuriant kelp growth, while more moderate reductions allowed establishment of opportunistic algae (during spring and early summer), but no kelps. Succession of algal growth, after the severe decline in sea urchin densities, followed a predictable pattern. At first the substrate was colonized by filamentous algae, but within few weeks they were outcompeted by the fast growing kelp Laminaria saccharina. After 3-4 years of the removal experiment, the slower-growing, long-lived kelp L. hyperborea became increasingly dominant. Increased food availability after reduction in sea urchin density led to increased individual growth of the remaining sea urchins. However, the population density did not increase, neither from recruitment nor immigration from adjacent areas with high sea urchin densities. Possibly, early establishment of a dense kelp stand, may represent a breakpoint in the ability of sea urchins to reestablish a barren state. The ability of L. saccharina quickly to invade and monopolize an area may have both positive and negative effects on the succession towards the climax L. hyperborea kelp forest. Competitive interactions may slow the process, but development of a dense stand of L. saccharina will also reduce grazing risk on scattered recruits of the more slowly growing L. hyperborea.

Settlement and recruitment of sea urchins (Strongylocentrotus spp.) in a sea-urchin barren ground and a kelp bed: are populations regulated by settlement or post-settlement processes?

I sampled recruitment of very small sea urchins (Strongylocentrotus spp.) by using the anesthetic magnesium chloride to remove individuals from substrata collected in sea-urchin barren grounds (barrens) and kelp beds at Naples Reef near Santa Barbara, California, USA. Preliminary sampling found low numbers of newly settled individuals(<0.6 mm test diam) from April–July in 1984 and 1985, and in April, 1986. In early May, 1986, I found many newly settled seaurchins (0.3 to 0.6 mm, 5 to 17 d old), and I compared the densities of the cohort on several types of natural substrata in barrens and kelp-bed habitats. Newly settled individuals of both purple sea urchins (S. purpuratus) and red sea urchins (S. franciscanus) were present in similar, high densities (1 000 S. purpuratus m-2) on foliose red algal turf, a dominant substratum ofthe kelp bed, and on crustose coralline algae, the dominant substratum of an adjacent barrens. Larvae of S. purpuratus reared and tested in the laboratory showed high rates of settlement on both red algal turf and on crustose coralline algae, but significantly lower rates on rock. Larvae also settled in response to a partiallypurified extract of coralline algae. The reduced settlement on natural rock surfaces relative to either algal treatment and the significant settlement in response to the extract of coralline algae indicate that larvae discriminate between natural substrata and probably respond to a settlement cue other than, or in addition to, a simple microbial (bacterial) film. The similar densities of young recruits of S. purpuratus on dominant substrata of barrens and kelp bed show that, at least in this case, differential settlement cannot explain the high densities of sea urchins in the barrens habitat. Movement between barrens and kelp bed is unlikely given the small sizes of the newly recruited sea urchins relative to the large distances often involved. Reduced post-settlement mortality of newly settled individuals in the barrens remains the most likely mechanism leading to the higher densities of sea urchins in barrens relative to kelp-bed habitats.

Development of a subtidal epibenthic community: factors affecting species composition and the mechanisms of succession

Macroinvertebrate grazers and temporal variability were found to strongly influence species composition of communities that developed subtidally on plexiglas panels. On panels exposed to the naturally high densities of sea urchins and sea stars, only grazer-resistant algal crusts, a diatom/blue-green algal film and short-lived filamentous algae became abundant. On those panels protected from grazers, however, other algae and sessile invertebrates were also common. Both the effects of grazing and the abundance of individual taxa differed on panels immersed at different times of the year.Resident species also affected subsequent recruitment. Some colonists were found more frequently on panels with established communities than on recently immersed plates. Others became more abundant on younger than on older panels. Considerable small-scale spatial variation in the abundance of species was also found among panels within treatments and appeared to persist throughout the 13 months of the study.I suggest that since the interactions that determine which mechanisms are important in succession occur between individuals (generalized here to species), not between successional stages, factors such as those examined that can determine which species will interact, indirectly determine the mechnaisms that are important in the development of a community. Models that deal with interactions between successional stages may lack the detail neccessary to predict or explain changes in species composition in diverse communities.