A COMPARISON OF DRAGON KELP,EUALARIA FISTULOSA, (PHAEOPHYCEAE) FECUNDITY IN URCHIN BARRENS AND NEARBY KELP BEDS THROUGHOUT THE ALEUTIAN ARCHIPELAGO1

The Aleutian Archipelago coastal ecosystem has undergone a dramatic change in community composition during the past two decades. Following the removal of ～99% of the sea otters, Enhydra lutris, from the ecosystem, changes to the benthic communities resulted in widespread losses to most of the region’s kelp beds and corresponding increases in the prevalence of urchin barrens. Within the urchin barrens, the few kelps that have remained are exposed to elevated light, nutrients and currents, all of which may enhance their physiological condition and thus result in greater fecundity. To explore this further, we examined patterns of sporophyte fecundity in the dominant canopy‐forming kelp, Eualaria fistulosa, in both urchin barrens and in nearby kelp beds at seven Aleutian Islands spanning a range of 800 km. We found that the average weight of E. fistulosa sporophyll bundles was significantly greater on sporophytes occurring in the urchin barrens than in the nearby kelp beds. Furthermore, the average number of zoospores released per cm² of sporophyll area was also significantly greater in individuals from the urchin barrens than the nearby kelp beds. When these two metrics were combined, our results suggest that individual E. fistulosa sporophytes occurring in the urchin barrens may produce as many as three times more zoospores than individual E. fistulosa sporophytes occurring in the nearby kelp beds, and thus they may contribute disproportionately to the following year’s sporophyte recruitment in both urchin barrens and the adjacent kelp beds.     

Experimental effects of kelp canopies on subtidal coralline algae

Plants are often grouped as canopy species or understorey species because it is thought that that these sets of taxa interact in predictable ways. Mensurative experiments in southern Australia demonstrated that the percentage cover of encrusting coralline algae was greater, and articulated (branching) coralline algae less, on boulders under a canopy of dense kelp (>7 plants per m²), Ecklonia radiata, than on boulders without kelp. Experimental clearances of kelp and reciprocal transplants of boulders between patches of E. radiata and patches without kelp showed that canopies maintained and facilitated the growth of encrusting coralline algae and reduced the cover of articulated coralline algae. Potential artefacts associated with clearing kelp and transplanting boulders were not detected when tested with a series of translocation controls. These results reject the model that the co‐occurrence of E. radiata and encrusting corallines is just an assemblage of plants caused by spatial and temporal coincidence. Instead, they support the model that kelp facilitates the growth and survival of understorey algae.     

Effects of Local Deforestation on the Diversity and Structure of Southern California Giant Kelp Forest Food Webs

It has been hypothesized that the high diversity of giant kelp forests is due primarily to the provision of energy and habitat by the giant kelp (Macrocystis pyrifera). In this article, I use a 19-year-long kelp forest-monitoring data set from the Channel Islands National Park (a) to identify associations between subtidal species and forested or deforested habitats, (b) to generate an idealized food web for Southern California giant kelp forests in order to identify the primary conduits of energy flow through the system, and (c) to determine changes in the diversity and complexity of this food web due to localized giant kelp deforestation. A total of 275 common species were observed in the park between 1982 and 2000, of which 36% occurred significantly more often in kelp-forested areas than in deforested areas (that is, sea urchin barrens); 25 species were found exclusively in forested areas. Most of these associations were clearly identified as trophic and/or structural associations with giant kelp itself. The producer level of the food web was diverse, although giant kelp apparently represents the greatest single source of fixed carbon through either direct grazing or the production of phytodetritus. Primary, secondary and tertiary consumer levels were also represented by numerous species, and generalist consumers were common. With deforestation, the source of primary production shifts from primarily kelps to ephemeral microalgae, macroalgae, and phytoplankton. These results support the reliance of giant kelp forest food-web structure and diversity on the presence of the forest itself.     

Coralline algal maerl frameworks-Islands within the phaeophytic kelp belt

Summary In the subtropical belt highly productive ecosystems are formed by coral reefs in oligotrophic seas. Towards more eutrophic conditions, coral reefs diminish and are subsequently replaced by highly productive kelp forests. In high latitudes framework constructing carbonate production is enhanced by the growth of branching coralline algae which predominantly generate maerl-type deposits. On a global view, these coralline algal ecosystems show an island-like distribution pattern within the phaeophytic kelp belt. Compared to kelp ecosystems, coralline-algaldominated ecosystems have low rates of productivity. Therefore, it is reasonable to seek the pronounced competitive value of the extremely slow-growing corallines. Due to their low annual growth increment, the coralline algae studied are very endangered by abiotic physical disturbances and by overgrowth of rapidly growing filamentous algae or sessile invertebrates. To overcome fouling pressure and storm-triggered physical disturbances, coralline algae thrive well in wave-sheltered headlands or skerry areas and generate characteristic ‘denuded areas’ by intense herbivory. This general distributional pattern is also true for high-boreal to subarctic coralline algal bioherms in northern Norway. Such a complex biological feedback maintains a high potential of self-regulation or self-organization in the algal reef bioherms. The different proponents involved in feedback processes include bacterial colonization, diatom microfouling and selective induction of larval metamorphosis. The negative impact of diatom microfouling and the important role of herbivores are relevant activities in the feedback system on a microscopic scale. Macroscopically, intense herbivory on coralline algae create denuded conditions, which are a widespread phenomenon in coralline algal ecosystems.     

Cobble community DNA as a tool to monitor patterns of biodiversity within kelp forest ecosystems

Kelp forest ecosystems dominate 150,000 km of global temperate coastline, rivalling the coastal occurrence of coral reefs. Despite the astounding biological diversity and productive ecological communities associated with kelp forests, patterns of species richness and composition are difficult to monitor and compare. Crustose coralline algae are a critically important substrate for propagule settlement for a range of kelp forest species. Coralline‐covered cobbles are home to hundreds of species of benthic animals and algae and form a replicable unit for ecological assays. Here, we use DNA metabarcoding of bulk DNA extracts sampled from cobbles to explore patterns of species diversity in kelp forests of the central California coast. The data from 97 cobbles within kelp forest ecosystems at three sites in Central California show the presence of 752 molecular operational taxonomic units (MOTUs) and 53 MOTUs assigned up to the species level with >95% similarity to current databases. We are able to detect spatial patterns of important management targets such as abalone recruits, and localized abundance of sea stars in 2012. Comparison of classic ecological surveys of these sites reveals large differences in species targets for these two approaches. In order to make such comparisons more quantitative, we use Presence/Absence Metabarcoding, using the fraction of replicate cobbles showing a species as a measure of its local abundance. This approach provides a fast and repeatable survey method that can be applied for biodiversity assessments across systems to shed light on the impact of different ecological disturbances and the role played by marine protected areas.     

Survival of kelp adjacent to areas grazed by sea urchins in New South Wales,Australia

Abstract The sea urchin Centrostephanus rodgersii grazes areas of reef free of large brown algae (the barrens habitat). Survival of the kelp Ecklonia radiata adjacent to patches of barrens habitat was compared to that in the centre of kelp forests (centre) and edges of forests not adjacent to patches of barrens habitat (ungrazed). Estimates of rates of instantaneous mortality for tagged plants, as described by the slope of a negative exponential model (± 95% CI) were: barrens, 0.078 ± 0.004; centre, 0.051 ± 0.004 and ungrazed edge, 0.065 ± 0.007. Survival of plants was greatest in the middle of forests and least on the margins of patches of barrens habitat. A significant proportion of mortality in the barrens and centre positions was caused by herbivorous fish. When these plants were excluded from analysis there were no significant differences in survival between the barrens and ungrazed positions. It is concluded that C. rodgersii has little impact on the abundance of Ecklonia outside sharply defined boundaries.     

Disturbance and organization of macroalgal assemblages in the Northwest Atlantic

Large seaweeds are often structurally dominant in subtidal and intertidal rocky shore benthic communities of the N.W. Atlantic. The mechanisms by which these algal assemblages are maintained are surprisingly different in the two habitats. In the subtidal community, kelps are dominant space competitors in the absence of strong grazing interactions. In contrast, the large perennial seaweeds of intertidal zones (fucoids and Chondrus crispus) are competitively inferior to both sessile filter feeders and ephemeral, pioneer algal species. Intertidal seaweed beds are maintained by carnivory of whelks, which reduces filter feeder populations, and by herbivorous periwinkles which reduce ephemeral algal populations. Through most of the intertidal zone, disturbance, both biological and physical, dictates which species shall compete and equilibrium conditions obtain subsequently.The roles of subtidal consumers are quite different. Sea urchins are the major algal herbivores and these voracious animals maintain an equilibrium state in which large tracts of subtidal coralline pavement are kept free of kelp forests. Urchins do not seem to play a successional facilitative role for kelps in the way that periwinkles do for fucoids in the intertidal. Control of herbivore populations is thus a key to the maintenance of subtidal foliose algal beds. It is clear that parasitic amoebas can decimate sea urchin populations so that kelp forest dominance is assured. However, the importance of carnivory in limiting urchins in the subtidal community is unclear in the absence of appropriate manipulation experiments. It is possible that carnivorous decapods and fin fish control sea urchin populations and hence foliose algal abundance, but this must remain speculative. The seaweed-dominated state of the subtidal system is an alternative equilibrium condition to the urchin/coralline alga configuration. The structure of the kelp beds is relatively uniform in responding to frequent small-scale, infrequent large-scale, or no, disturbance.     

Can multitrophic interactions and ocean warming influence large‐scale kelp recovery?

Ongoing changes along the northeastern Atlantic coastline provide an opportunity to explore the influence of climate change and multitrophic interactions on the recovery of kelp. Here, vast areas of sea urchin‐dominated barren grounds have shifted back to kelp forests, in parallel with changes in sea temperature and predator abundances. We have compiled data from studies covering more than 1,500‐km coastline in northern Norway. The dataset has been used to identify regional patterns in kelp recovery and sea urchin recruitment, and to relate these to abiotic and biotic factors, including structurally complex substrates functioning as refuge for sea urchins. The study area covers a latitudinal gradient of temperature and different levels of predator pressure from the edible crab (Cancer pagurus) and the red king crab (Paralithodes camtschaticus). The population development of these two sea urchin predators and a possible predator on crabs, the coastal cod (Gadus morhua), were analyzed. In the southernmost and warmest region, kelp forests recovery and sea urchin recruitment are mainly low, although sea urchins might also be locally abundant. Further north, sea urchin barrens still dominate, and juvenile sea urchin densities are high. In the northernmost and cold region, kelp forests are recovering, despite high recruitment and densities of sea urchins. Here, sea urchins were found only in refuge habitats, whereas kelp recovery occurred mainly on open bedrock. The ocean warming, the increase in the abundance of edible crab in the south, and the increase in invasive red king crab in the north may explain the observed changes in kelp recovery and sea urchin distribution. The expansion of both crab species coincided with a population decline in the top‐predator coastal cod. The role of key species (sea urchins, kelp, cod, and crabs) and processes involved in structuring the community are hypothesized in a conceptual model, and the knowledge behind the suggested links and interactions is explored.     

Climate‐driven disparities among ecological interactions threaten kelp forest persistence

The combination of ocean warming and acidification brings an uncertain future to kelp forests that occupy the warmest parts of their range. These forests are not only subject to the direct negative effects of ocean climate change, but also to a combination of unknown indirect effects associated with changing ecological landscapes. Here, we used mesocosm experiments to test the direct effects of ocean warming and acidification on kelp biomass and photosynthetic health, as well as climate‐driven disparities in indirect effects involving key consumers (urchins and rock lobsters) and competitors (algal turf). Elevated water temperature directly reduced kelp biomass, while their turf‐forming competitors expanded in response to ocean acidification and declining kelp canopy. Elevated temperatures also increased growth of urchins and, concurrently, the rate at which they thinned kelp canopy. Rock lobsters, which are renowned for keeping urchin populations in check, indirectly intensified negative pressures on kelp by reducing their consumption of urchins in response to elevated temperature. Overall, these results suggest that kelp forests situated towards the low‐latitude margins of their distribution will need to adapt to ocean warming in order to persist in the future. What is less certain is how such adaptation in kelps can occur in the face of intensifying consumptive (via ocean warming) and competitive (via ocean acidification) pressures that affect key ecological interactions associated with their persistence. If such indirect effects counter adaptation to changing climate, they may erode the stability of kelp forests and increase the probability of regime shifts from complex habitat‐forming species to more simple habitats dominated by algal turfs.     

Dispersion and phagokinesis in the echinoid Evechinus chloroticus (Val.)

Movement in the echinometrid sea urchin Evechinus chloroticus (Val.) was investigated in relation to the availability of a preferred food species, the laminarian alga Ecklonia radiata (C. Ag.). Patterns of microhabitat occupancy, dispersion, and movement were described in two habitats: Coralline Flats, dominated by encrusting coralline algae; and Sediment Flats, occupied by turfing corallines, Ecklonia and the mussel Modiolus areolatus (Gould). In both habitats, sea urchins were positively associated with encrusting coralline algae, despite the fact that encrusting corallines occupied a minor proportion of the substratum in the Sediment Flat habitat. Sea urchins were significantly clumped in both habitat types, and more so in sites within the Sediment Flats habitat. Movement was not directional in either habitat, but the magnitude of movement was almost twice as high in sites from the Coralline Flats habitat, where kelp was absent, as in the Sediment Flats sites, where kelp was present. Behaviour in response to the presence of Ecklonia in both habitats was investigated experimentally. In both habitats, urchins exposed to a “drift” kelp plant at a distance of 1–2 m exhibited net movement approximately twice as great as that shown by control urchins, not exposed to a kelp plant. Despite the relatively small distances the urchins were from the kelp, there was no evidence of directionality in movement. We conclude from the present study that further understanding of the feeding behaviour of urchins requires that a distinction be made between attached and drift algae, and between directionality and magnitude of movement, and that the interaction of these and other factors are best investigated under field conditions.     

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.     

Inhibition of kelp recruitment by turfing algae and consequences for an Australian kelp community

In the sublittoral kelp forests of New South Wales, dictyotalean turfing algae dominate the substratum in areas where kelp canopies have been removed by storms. The effects of the presence of these algae on the recruitment of kelp plants and the subsequent structure of the benthic community were determined. Areas of turfing algae in clearings were experimentally cleared of turf at a time of dense kelp settlement, and areas of the kelp canopy were also removed. These treatments were compared with unmanipulated areas of turf in clearings and areas under the kelp canopy. The number of kelp recruits in each replicate plot was recorded throughout time as were the abundances of most of the macroscopic and microscopic species living on the substratum. Kelp was found to recruit very quickly to areas of substratum which had the overlying kelp canopy removed, but few recruits appeared under the natural canopy, or in areas with turfing algae, or in clearings where turfing algae had been removed. The results showed that whilst the presence of a natural kelp canopy and/or turfing algae inhibited the recruitment of kelps, removal of turfing species from clearings did not facilitate kelp recruitment. This indicated that the presence of turf did not solely inhibit kelp recruitment, but some lingering influence of turf seems to affect the substratum in clearings such that kelp cannot recruit to such areas. Turfing species returned and dominated the substratum in clearings where turf had been removed. After an initial decline, encrusting species increased in abundances in clearings where kelp recruited as these recruits developed a canopy. Turf and encrusting species remained great in cover in the respective control treatments. The results are discussed in terms of alternate stable states within this community. Such concepts are concluded to be dependent upon the spatial and temporal scales of one's investigation.     

Articulated coralline algae of the genus Amphiroa are highly effective natural inducers of settlement in the tropical abalone Haliotis asinina

The initiation of metamorphosis in marine invertebrates is strongly linked to the environment. Planktonic larvae typically are induced to settle and metamorphose by external cues such as coralline algae (Corallinaceae, Rhodophyta). Although coralline algae are globally abundant, invertebrate larvae of many taxa settle in response to a very limited suite of species. This specificity impacts population structure, as only locations with the appropriate coralline species can attract new recruits. Abalone (Gastropoda, Haliotidae) are among those taxa in which closely related species are known to respond to different coralline algae. Here we identify highly inductive natural cues of the tropical abalone Haliotis asinina. In contrast to reports for other abalone, the greatest proportion of H. asinina larvae are induced to settle and metamorphose (92.8% to 100% metamorphosis by 48 h postinduction) by articulated corallines of the genus Amphiroa. Comparison with field distribution data for different corallines suggests larvae are likely to be settling on the seaward side of the reef crest. We then compare the response of six different H. asinina larval families to five different coralline species to demonstrate that induction by the best inductive cue (Amphiroa spp.) effectively extinguishes substantial intraspecific variation in the timing of settlement.     

Predicting understorey structure from the presence and composition of canopies: an assembly rule for marine algae

Assembly rules provide a useful framework for predicting patterns of community assembly under defined environmental conditions. Habitat created by canopy-forming algae (such as kelps) provides a promising system for identifying assembly rules because canopies typically have a large and predictable influence on understorey communities. Across >1,000 km of subtidal South Australian coastline, we identified natural associations between assemblages of understorey algae and (1) monospecific canopies of Ecklonia radiata, (2) canopies comprised of E. radiata mixed with Fucales (Cystophora spp. and Sargassum spp.), and (3) gaps among canopies of algae. We were able to recreate these associations with experimental tests that quantified the assembly of understorey algae among these three habitat types. We propose the assembly rule that understorey communities on subtidal rocky coast in South Australia will be (1) monopolised by encrusting coralline algae beneath monospecific canopies of E. radiata, (2) comprised of encrusting corallines, encrusting non-corallines, and sparse covers of articulated corallines, beneath mixed E. radiata-Fucales canopies, and (3) comprised of extensive covers of articulated corallines and filamentous turfs, as well as sparse covers of foliose algae and juvenile canopy-formers, within gaps. Consistencies between natural patterns and experimental effects demonstrate how algal canopies can act as a filter to limit the subsets of species from the locally available pool that are able to assemble beneath them. Moreover, the subsets of species that assemble to subtidal rocky substrata in South Australia appear to be predictable, given knowledge of the presence and composition of canopies incorporating E. radiata.     

Consistency in a marine algal‐grazer interaction over multiple scales

Coralline algae are conspicuous members of many marine assemblages, especially those characterized by intense grazing pressure. We explored whether articulated species, especially Corallina vancouveriensis, depend on grazing invertebrates to both establish and flourish in an exposed rocky intertidal setting, and whether this plant–grazer relationship varied over more than three orders of magnitude (≈100–>300,000 μm). Three experimental manipulations, supplemented by observations on recruitment, demonstrated that (i) C. vancouveriensis failed to recover rapidly from disturbed areas when grazers were experimentally excluded; (ii) recruitment occurred in the presence of grazers; (iii) increasing surface texture of molded surfaces enhanced coralline recruitment more when grazers were present; and (iv) settlement occurred predominately in microtopographical low areas of a molded surface, whereas a competitively superior fleshy red alga tended to recruit to high areas. These results confirm that coralline algal establishment and persistence are enhanced by grazers and reveal that this relationship is consistent over a range of biologically relevant scales.     

珊瑚礁生态系统中珊瑚藻的生态作用研究进展

珊瑚藻是海洋红藻中的大型钙化藻类,全球分布623种,中国现有记录共77种。随着生态科学研究的广泛展开,人们越来越认识到,珊瑚藻在海洋生态系统中,尤其在维持珊瑚礁生态系统的生物多样性及生态功能中发挥着重要作用。目前,科研人员对有关珊瑚藻的初级生产力、钙化作用以及在诱导底栖无脊椎动物幼虫的附着与变态等方面已有多方面的研究和探索。然而,有关珊瑚藻生态功能的深层次机理问题有待进一步深入研究。文章着重围绕目前珊瑚藻研究中的一些热点问题,从近年来珊瑚藻在珊瑚礁生态系统中的生态功能方面的研究概况进行综述,以期加深人们对珊瑚藻的认识,并促进对珊瑚藻生态功能的进一步深入研究。     

COMPOSITION,DISTRIBUTION, AND ABUNDANCE OF DEEP‐WATER (>30 m) MACROALGAE IN CENTRAL CALIFORNIA1

The deep‐water macroalgal assemblage was described at 14 sites off the central California coast during 1999 and 2000 from SCUBA and remotely operated vehicle sampling. The stipitate kelp Pleurophycus gardneri Setchell & Gardner, previously thought to be rare in the region, was abundant from 30 to 45 m, forming kelp beds below the well‐known giant kelp forests. Macroalgae typically formed three broadly overlapping zones usually characterized by one or a few visually dominant taxa: 1) the upper “Pleurophycus zone” (30–45 m) of stipitate kelps and Desmarestia spp. with a high percent cover of corallines, low cover of uncalcified red algae, and rare green algae; 2) a middle “Maripelta zone” (40–55 m) with other uncalcified red algae and infrequent corallines and green algae; and 3) a zone (55–75 m) of infrequent patches of nongeniculate coralline algae. The green alga Palmophyllum umbracola Nelson & Ryan, not previously reported from the Northeast Pacific, was found over the entire geographical range sampled from 35 to 54 m. Year‐round profiles of water column irradiance revealed unexpectedly clear water with an average K₀ of 0.106·m ^{? 1} Received 18 January 2002. Accepted 16 December 2002. . The low percent surface irradiance found at the average lower macroalgal depth limits in this study (0.56% for brown algae, 0.12% for uncalcified red algae, and 0.01% for nongeniculate coralline algae) and lack of large grazers suggest that light controls the lower distributional limits. The ubiquitous distribution, perennial nature, and similar lower depth limits of deep‐water macroalgal assemblages at all sites suggest that these assemblages are a common persistent part of the benthic biota in this region.     

Larval settlement preferences of Acropora palmata and Montastraea faveolata in response to diverse red algae

Settlement specificity can regulate recruitment but remains poorly understood for coral larvae. We studied larvae of the corals, Acropora palmata and Montastraea faveolata, to determine their rates of settlement and metamorphosis in the presence of ten species of red algae, including eight species of crustose coralline algae, one geniculated coralline and one encrusting peyssonnelid. Twenty to forty percent of larvae of A. palmata settled on coralline surfaces of Hydrolithon boergesenii, Lithoporella atlantica, Neogoniolithon affine, and Titanoderma prototypum, whereas none settled and metamorphosed on Neogoniolithon mamillare. Larvae of M. faveolata had 13–25 % settlement onto the surface of Amphiroa tribulus, H. boergesenii, N. affine, N. munitum, and T. prototypum, but had no settlement on the surface of N. mamillare, Porolithon pachydermum, and a noncoralline crust Peyssonnelia sp. Some of these algal species were common on Belizean reefs, but the species that induced the highest rates of larval settlement and metamorphosis tended to be rare and primarily found in low-light environments. The shallow coral, A. palmata, and the deeper coral, M. faveolata, both had increased larval settlement rates in the presence of only a few species of red algae found at deeper depths suggesting that patterns of coral distribution can only sometimes be related to the distribution of red algae species.     

Latitudinal variation in intertidal algal community structure: the influence of grazing and vegetative propagation

Summary The hypothesis that sea urchin grazing and interactions with turf-forming red algae prevent large brown algae from forming an extensive canopy in the low intertidal zone of southern California was tested with field experiments at two study sites. Experimental removal of sea urchins resulted in rapid algal recruitment. Crustose coralline algae which typically dominate the substratum in areas with dense urchin populations were quickly overgrown by several species of short-lived green, brown and red algae. The removal of urchins also significantly increased the recruitment of two long-lived species of large brown algae (Egregia laevigata and Cystoseira osmundacea at one study site and E. laevigata and Halidrys dioica at the other). The experimental plots at both sites were eventually dominated by perennial red algae.A two-factorial experiment demonstrated that sea urchin grazing and preemption of space by red algae in areas where urchins are less abundant are responsible for the rarity of large brown algae in the low intertidal of southern California. The three dominant perennial red algae, Gigartina canaliculata, Laurencia pacifica and Gastroclonium coulteri, recruit seasonally from settled spores but can rapidly fill open space with vigorous vegetative growth throughout the year. These species encroach laterally into space created by the deaths of large brown algae or by other disturbances. Once extensive turfs of these red algae are established further invasion is inhibited. This interaction of algae which proliferate vegetatively with algae which recruit only from settled spores is analogous to those which occur between solitary and colonial marine invertebrates and between solitary and cloning terrestrial plants.It is suggested that a north-south gradient in the abundance of vegetatively propagating species, in grazing intensity and in the frequency of space-clearing disturbances, may account for latitudinal variation in intertidal algal community structure along the Pacific coast of North America.