Latitudinal variation in spongivorous fishes and the effectiveness of sponge chemical defenses

It has been proposed that predation pressure declines with increasing latitude and a positive correlation exists between predation intensity and the investment into chemical defenses. However, little direct evidence supports the idea that tropical species are better defended chemically than their temperate counterparts. Temperate reefs of the South Atlantic Bight (SAB) off Georgia, USA, provide a unique opportunity to study tropical sponges in a temperate environment. We documented sponge species richness and abundance, sponge predator density, and examined the ability of eight sponge species to chemically deter predation by fishes on two reefs in the SAB. We used rarefaction analysis and ANOVA to compare our results for sponge species richness and density, respectively, with similar published studies conducted on reefs of the sub-tropical Atlantic (i.e., Florida Keys). These analyses were combined with similar statistical comparisons for spongivorous fish species richness and density. Results showed that sponge species richness was lower, but sponge density was higher, on the temperate SAB reefs than on the subtropical reefs. Both spongivorous fish diversity and density were lower on the SAB reefs. The greater abundance of sponges and lower density of predators on SAB reefs suggest a lower frequency of predation on sponges on SAB reefs. Of the eight sponge species assayed from the SAB reefs, five possessed chemical extracts that were significantly less deterrent to fish predators than their tropical/subtropical conspecifics. When the results were combined across all sponge species, the chemical deterrence of fish predators was significantly lower for extracts obtained from the temperate sponge community as compared to the tropical/subtropical assemblage. These results support the more general hypothesis that a lower density and diversity of sponge predators occurs at high as compared to low latitudes in the western Atlantic and may contribute to decreased investment in chemical defenses.     

Coral community structure and risk assessment of high-latitude reefs at Sodwana Bay,South Africa

The importance of studying coral communities at different spatial scales is acknowledged in a growing volume of scientific literature, and principles of landscape ecology were thus used to elucidate the patterns in coral community structure on the high-latitude reefs in South Africa. These reefs are at the southernmost distribution of this fauna in Africa, are surprisingly species rich, and represent a biodiversity peak in this fauna south of the equator, regardless of the marginal nature of the environment. Coral community patterns were identified on and between the reefs at Sodwana Bay, justifying the grouping of reef areas in distinct zones. A number of landscape components were identified, ranging from the entire reef complex (10 km scale), individual reefs (1 km scales) and reef zones, to components that were separated using multivariate statistical analysis of transect data. These components transcended spatial similarities, e.g. the fore-reef on Five-mile Reef was not similar to the fore-reef on Seven-mile Reef, but was rather grouped with the reef flat on Two-mile Reef. This information was “translated” into an index of management intervention, based on risk assessment, and was generated using parameters that measure susceptibility to crown-of-thorns feeding, bleaching, diver-related damage and swell-induced breakage. We also assessed was the time elapsed since the last major disturbance and the proximity to the only boat launch site, a proxy measure of continuous disturbance. The risk assessment suggested that conservation management is most needed in the stable and “climax” coral communities that are usually characterised by a near-equal mix of hard and soft corals at maximal coral species diversity.     

Chaetognaths of the United States South Atlantic Bight: Distribution, abundance and potential interactions with newly spawned larval fish

Chaetognaths are one of the most numerous organisms in the zooplankton community off the coast of North Carolina. During two years of offshore sampling in the late winter to early spring, sixteen chaetognath species were identified, four of which had not previously been reported in the waters of the United States South Atlantic Bight. Offshore samples were dominated by Sagitta enflata Grassi, 1881, one of the larger species, which contributed > 61% of total chaetognath abundance while dominant coastal species were S. tenuis Conant, 1896 and S. hispida Conant, 1895. Abundances, body sizes and spatial distributions were determined for the most abundant chaetognath species along with the overall abundance of three common co-occurring larval fish species (spot, Atlantic croaker and Atlantic menhaden). In addition, laboratory feeding experiments were conducted using S. tenuis and S. hispida to estimate the potential impact of chaetognath predation on representative North Carolina larval fish which spawn offshore and subsequently migrate into local estuaries. Feeding rates (no. prey items day^− 1) varied with prey type and the condition (starved/fed) of the chaetognath. Weight specific daily rations (SDR) were found to vary inversely with chaetognath size, decreasing exponentially with an increase in chaetognath length. The observed abundance and distribution data indicate that wintertime chaetognath populations in offshore waters of Onslow Bay, North Carolina have the potential to interact with recently spawned larval fish and may simultaneously act as competitors, predators, or prey. Furthermore, calculations using published values of chaetognath abundance, zooplankton standing crops, and our SDR estimates indicate that chaetognaths in a representative North Carolina estuary would require a minimum of 5.96 cal m^− 3 day^− 1 to sustain their biomass. Allowing for the reported spatial and temporal variability in zooplankton abundance in these systems, chaetognaths should consume 4.4%-20.9% of the estimated total zooplankton production day^− 1. This further emphasizes the role of chaetognaths not only as predators, but also as competitors with larval fish for zooplankton food stocks in southeastern United Sates estuaries.     

Prey nutritional quality and the effectiveness of chemical defenses against tropical reef fishes

Summary Many coral-reef seaweeds and sessile invertebrates produce both secondary chemicals and mineral or fibrous skeletal materials that can reduce their susceptibility to consumers. Although skeletal materials often have been assumed to function as physical defenses, their deterrent effectiveness may derive from their reduction of prey nutritional quality as well as from noxiousness of the skeletal material itself. To test the relative importance of prey nutritional quality and chemical defenses in susceptibility to predation, we offered reef fishes on Guam a choice of artificial foods varying in nutritional quality (4% versus 22% protein) and in secondary chemistry (spanning approximately natural concentration ranges). Field feeding assays were performed with pachydictyol A from the pantropical brown seaweed genus Dictyota, manoalide from the Micronesian sponge Luffariella variabilis, and a brominated diphenyl ether from the Micronesian sponge Dysidea sp. The results indicated that chemical defenses were less effective in high- than in low-quality foods. In paired assays with metabolite-free controls, all three compounds at natural concentrations significantly reduced feeding by reef fishes only in assays using low-quality food, and not in assays with high-quality food. When fishes were offered an array of artificial foods varying in both food quality and metabolite concentration, food quality significantly affected fish feeding in all three cases, while secondary chemistry was significant in only one. Thus differences in nutritional quality, within the natural range among reef organisms, can be comparable to or greater in importance than secondary chemistry in affecting feeding preferences of their consumers. Reduced nutritional quality may be an important selective advantage of producing indigestible structural materials, in addition to their roles as physical support and defense, in coral reef organisms.     

Vertical and lateral variations in coccolithophore community structure across the subtropical frontal zone in the South Atlantic Ocean

In this study the coccolithophore compositions of 11 plankton depth stations along a N–S transect from the South Atlantic Subtropical Gyre to the Subantarctic Zone were examined qualitatively and quantitatively. The lateral and vertical distribution patterns of not only the most abundant taxa but also of the morphotypes of distinct species complexes, such as Calcidiscus leptoporus, Emiliania huxleyi, and Umbellosphaera tenuis were the focus. Geographic variation among morphotypes mirrors different ecological affinities of the members of a species complex. Multivariate statistics were used to infer the relationship between a set of known environmental data and species concentrations. The results of the detrended Canonical Correspondence Analysis (CCA) revealed the presence of distinct species assemblages. The Subtropical Gyre assemblage within the upper 50 m of the photic zone is mainly composed of Umbellosphaera irregularis, U. tenuis types III and IV, Discosphaera tubifera, Rhabdosphaera clavigera, S. pulchra and E. huxleyi var. corona, adapted to warm and oligotrophic conditions. In the deeper photic zone abundant Florisphaera profunda, Gephyrocapsa ericsonii and Oolithotus spp. are encountered, benefiting from higher nutrient concentrations in the vicinity of the nutricline. A well-defined Subtropical Frontal Zone (STFZ) association is clearly dominated by E. huxleyi types A and C throughout the upper 100 m of the water column. Secondary contributors in the upper photic zone are Syracosphaera spp. (mainly S. histrica, S. molischii), Michaelsarsia elegans, Ophiaster spp. and U. tenuis type II. This assemblage is associated with cooler, nutrient-rich waters. E. huxleyi type B is found deeper in the water column. Here it is accompanied by Algirosphaera robusta, G. muellerae, and S. anthos indicating a tolerance of lower light availability in environments with elevated productivity. C. leptoporus spp. leptoporus shows relatively high cell numbers in all sampled water levels throughout the STFZ. Interestingly, its coccoliths are often smaller 5 μm in lith diameter. The mean coccolithophore assemblages of a station were compared to the underlying surface sediment assemblages. For the most part, the distribution of the morphotypes is reflected in the sedimentary archive, thus proving their potential as paleoecological proxies.     

Ecological structure of assemblages of coral reef fishes on isolated patch reefs

A 9-year study of the structure of assemblages of fish on 20 coral patch reefs, based on 20 non-manipulative censuses, revealed a total of 141 species from 34 families, although 40 species accounted for over 95% of sightings of fish. The average patch reef was 8.5 m² in surface area, and supported 125 fish of 20 species at a census. All reefs showed at least a two-fold variation among censuses in total numbers of fish present, and 12 showed ten-fold variations. There was also substantial variation in the composition and relative abundances of species present on each patch reef, such that censuses of a single patch reef were on average about 50% different from each other in percent similarity of species composition (Czekanowski's index). Species differed substantially in the degree to which their numbers varied from census to census, and in the degree to which their dispersion among patch reefs was modified from census to census. We characterize the 40 most common species with respect to these attributes. The variations in assemblage structure cannot be attributed to responses of fish to a changing physical structure of patch reefs, nor to the comings and goings of numerous rare species. Our results support and extend earlier reports on this study, which have stressed the lack of persistant structure for assemblages on these patch reefs. While reef fishes clearly have microhabitat preferences which are expressed at settlement, the variations in microhabitat offered by the patch reefs are insufficient to segregate many species of fish by patch reef. Instead, at the scale of single patch reefs, and, to a degree, at the larger scale of the 20 patch reefs, most of the 141 species of fish are distributed without regard to differences in habitat structure among reefs, and patterns of distribution change over time. Implications for general understanding of assemblage dynamics for fish over more extensive patches of reef habitat are considered.     

DNA barcoding of coastal ichthyofauna from Bahia,northeastern Brazil,South Atlantic: High efficiency for systematics and identification of cryptic diversity

In spite of the utility of DNA barcoding in discriminating marine fish species, few studies using this technique are available along the Brazilian coast, which encompasses nearly 9000 km, distinct biogeographic regions and endemism rates. Therefore, in the present work, we provided the first DNA barcode library based on sequencing of the mitochondrial gene cytochrome c oxidase subunit I (COI) for Actinopterygii fish from northeastern coast of Brazil. The database included 459 specimens from 54 genera, 31 families and 13 orders, with mean interspecific genetic distance of and mean intraspecific distance of 0.15%. As a result, the neighbor-joining (NJ) tree resulted in the molecular identification of 78 taxa. Cryptic species were indicated in Eucinostomus (mojarras) since the 82 specimens morphologically identified as Eucinostomus melanopterus or Eucinostomus sp. formed five clusters with intraspecific genetic distances up to 0.31% and lowest interspecific distance of 11.14%. Eucinostomus jonesii and Eucinostomus harengulus were identified for the first time in ichthyofauna inventories from northeastern Brazil. The present work has also added new records to the Barcode of Life Data System (BOLD), thereby favoring the identification of other specimens at different life stages or in forensic studies. The present dataset reinforces the reliability of DNA barcoding to estimate the overlooked richness of marine fish in Brazilian coast with application in conservation, ecology, systematics and fisheries.     

Effects of coral transplantation and giant clam restocking on the structure of fish communities on degraded patch reefs

Active restoration is being practiced to supplement conservation activities for the purpose of reversing the trend of reef degradation. In the last decade, the feasibility of different restoration approaches such as coral transplantation and restocking of other marine biota has been the focus of research and relatively few have examined experimentally its effects on the resultant communities. In this study, coral transplantation and giant clam restocking were applied on 25 degraded patch reefs (~ 25 m²) inside a marine sanctuary in Pangasinan, northwestern Philippines to examine their effects on the community structure of reef fishes. Five interventions or treatments were employed: 1) “coral” consisted of transplantation of a combination of Acropora spp. and Pocillopora spp. on concrete blocks; 2) “clam” consisted of restocking of Tridacna gigas; 3) “clam+coral” consisted of restocking of T. gigas with Acropora spp. transplanted on their shells; 4) “shell” consisted of deployment of T. gigas shells; and 5) “control” consisted of no intervention. Fish communities on the patch reefs were monitored monthly for 3 months before the intervention and were monitored further for 11 months after the intervention, including 1 recruitment season. After the intervention, the coral cover and the “other biota” category increased in the coral and clam+coral treatments, due to the transplanted corals and deployed giant clams. Consequently, the complexity of the substrate was enhanced. A month after the intervention, a rapid increase in the abundance and species richness of reef fishes on the coral, clam+coral and clam treatments was observed compared to the shell and control treatments. A change in species composition of reef fish assemblage was also apparent in the coral and clam+coral treatments relative to the clam, shell and control, especially 4 months after the intervention. The present experiment demonstrates the feasibility of improving the condition of degraded patch reefs, which can subsequently enhance the fish community. Results also show the importance of the underlying substratum and the abundance of live corals and clams to reef fishes.     

Biotic structure and morphology of patch reefs from South China (Ningqiang Formation, Telychian, Llandovery, Silurian)

Summary Ningqiang Formation (late Telychian, Llandovery, Silurian), characterized by nearly 3000 m of shales in tercalated with carbonates, is situated between Ningqiang (S. Shaanxi Province) to Guangyuan (N. Sichuan Province) adjacent to the northwest margin of the Yangtze Platform. The high diversity “Xiushan Fauna”, and abundant reef development, illustrate a relatively warm and persistent shallo marine environment in these early Silurian sediments. The sequence shows reef radiation after recovery from the end Ordovician mass extinction envents. Multiple horizons of reef-building occurred within a relatively short geological interval and resulted in more than 30patch reefs up to 200 m in diameter and 1–50 m vertically, composed of abundant fossils. Reef biota include frame-building corals, stromatoporoids, bryozoans, and microbialites, and reef-associated oranisms such as crinoids, brachiopods, trilobites, gastropods, nautiloids and ostracods. Three reefrelated biotic associations are recognised: a) reefs dominated by framework with crinoids and microbia; b) reefs dominated by only crinoids and microbia; and c) crinoiddomainated facies. Seven representative reef examples illustrate different morphologies and growth styles. A high terrigenous debris input and shallow epicontinental ramp, which lacked obvious topographic variation, were major controls which resulted in rather simple reefs; sedimentation was apparently the main constraint on lateral and vertical extension of reefs, and prevented large-scale reef complexes developing.     

Blooms of benthic dinoflagellates of the genus Ostreopsis; an increasing and ecologically important phenomenon on temperate reefs in New Zealand and worldwide

Blooms of benthic dinoflagellates belonging to the tropical genus Ostreopsis are an increasingly common phenomenon in temperate regions worldwide. This is reflected in the rapid upsurge of publications on Ostreopsis from temperate regions since 2000. Relatively little is known about these blooms or their effects on other organisms. An unprecedented bloom of Ostreopsis siamensis occurred on shallow reefs in northern New Zealand in 2004 providing an opportunity to examine the dynamics of an O. siamensis bloom and its effect on community structuring sea urchins (Evechinus chloroticus). The bloom occurred following a period of calm sea conditions with warmer than average water temperatures. The cover of O. siamensis was highly ephemeral and strongly related to temporal and spatial variation in wave action. Blooms were most prevalent at sites protected from prevailing swells where O. siamensis covered 30–60% of the reef with the concentrations on macroalgae reaching 1.4 × 10⁶ cells g⁻¹ wet weight, some of the highest recorded worldwide. Surveys of the health of sea urchins in relation to the cover of O. siamensis suggested strong negative effects on this ecologically important herbivore and urchin densities declined by 56–60% at bloom sites over the study period. Further research is needed to examine the factors controlling the distribution and intensity of this new phenomenon, and into the ecological effects of such blooms on marine communities and the potential mechanisms responsible.     

Composition and community structure of the coralline algal reefs from Atol das Rocas, South Atlantic, Brazil

 Coralline-algal frameworks from Atol das Rocas reefs were sampled along windward and leeward transects, and multivariate statistical analysis was used to investigate the composition and community structure of the encrusting community. The following components of the reefs were identified in slabbed and polished reef blocks sampled along each transect: encrusting coralline algae dominated by Porolithon cf. pachydermum, vermetid gastropods, Homotrema rubrum, acervulinid foraminiferans, molluscs, corals, polychaete tubes, clionid sponge borings, polychaete/sipunculid borings, cemented sediments, and growth cavities. Q-mode cluster analysis correctly classified 78% of all windward samples and 69% of all leeward samples, and R-mode separated reef components adapted to high wave energy environments from those adapted to low wave energy. Separate classification and ordination of samples from each transect indicate that seriation breakdown occurs in the windward reef between 25 and 45 m from the reef front. In the leeward reef between 75 and 90 m from the leeward reef front, seriation breakdown was not found to be significant. These results confirm that seriation (zonation) is best developed in the windward reef, as has been documented in coral-dominated reefs. Accepted: 28 February 2000     

Status,population trend and genetic structure of South American fur seals,Arctocephalus australis,in southwestern Atlantic waters

The South American fur seal (SAFS), Arctocephalus australis, was severely depleted during the 18th century. This work updates information on distribution, abundance, seasonal movements, and genetic structure along the southwestern South Atlantic. Its distribution in Argentine waters includes nineteen rookeries on the Patagonian coast, Isla de los Estados, and islets of the Beagle Channel. Censuses were carried out in rookeries from Isla Escondida to Isla de los Estados from 1996 to 2014. The largest concentrations are found in Chubut, (4,500–15,500 seals) and Isla de los Estados (4,500). Pups were recorded on Isla Escondida, Rasa, and Isla de los Estados. An increasing number of SAFS were recorded in northern Patagonia, including seals marked in Uruguay. Skin samples were taken from southern Brazil to Tierra del Fuego in order to evaluate population structure and demographic trends. Genetic results indicated no population subdivisions (Φ_st = ?0.0292, F_st = ?0.00716, P > 0.05). Bayesian skyline plots constructed for the entire data set indicated evidence of rapid population expansion discernible in Nef between the last 20,000 and 17,000 yr. Genetics results and observation data from marked fur seals support the hypothesis of ancient gene flow and a single Atlantic population. Therefore, SAFS should be managed under international and integrated conservation policies.     

Effects of monsoon-driven wave action on coral reefs of Guam and implications for coral recruitment

Benthic cyanobacteria can respond rapidly to favorable environmental conditions, overgrow a variety of reef organisms, and dominate benthic marine communities; however, little is known about the dynamics and consequences of such cyanobacterial blooms in coral reef ecosystems. In this study, the benthic community was quantified at the time of coral spawnings in Guam to assess the substrate that coral larvae would encounter when attempting settlement. Transects at 9, 18, and 25-m depths were surveyed at two reef sites before and after heavy wave action driven by westerly monsoon winds. Communities differed significantly between sites and depths, but major changes in benthic community structure were associated with wave action driven by monsoon winds. A shift from cyanobacteria to crustose coralline algae (CCA) accounted for 44% of this change. Coral recruitment on Guam may be limited by substrate availability if cyanobacteria cover large areas of the reef at the time of settlement, and consequently recruitment may in part depend upon wave action from annual monsoon winds and tropical storms which remove cyanobacteria, thereby exposing underlying CCA and other substrate suitable for coral settlement.     

华南地区典型抽水型水库后生浮游动物群落的种类组成与结构

大镜山水库位于广东省珠海市,是一座富营养抽水型中型水库,为了解富营养过程中抽水型水库后生浮游动物群落的种类组成与结构特征,于2006年3～12月在水库敞水区进行每月1次或2次调查.共检到轮虫32种、枝角类4种和桡足类9种.其中,臂尾轮属为轮虫优势属,角突臂尾轮虫(Brachionus angularis)和剪形臂尾轮虫(B. forficula)为最主要的优势种.长额象鼻溞 (Bosmina longirostris)为枝角类的优势种,中华窄腹剑水蚤(Limnoithona sisnensis)为桡足类第1优势种,温中剑水蚤(Mesocyclops thermcyclopoides)为次优势种.中华窄腹剑水蚤个体较小(成体体长为0.3～0.5mm),对咸水有较强的适应能力,该种是通过河流调水引入大镜山水库.后生浮游动物的个体体重分布范围为0.01～81.92 μg,种类主要分布的个体体重区间为0.08～0.32 μg,在该区间的浮游动物主要为轮虫.浮游动物的丰度与生物量以桡足类占优势,丰度峰值出现在个体体重为0.08～0.16 μg的区间上,并以无节幼体为主;生物量的峰值出现在2.56～5.12 μg的等级上,峰值为46.43 μg/L,主要由桡足类构成.水库中后生浮游动物的丰度和生物量范围分别为21.9～876 ind./L和17.47～1089.73 μg/L,丰水期丰度较高,丰度和生物量的峰值分别出现在5月和9月份.与同地区的其它水体相比,大镜山水库浮游动物种类数低,与水体咸度高和鱼类养殖导致的高强度捕食压力有关.抽水入库量、降雨量是影响后生浮游动物丰度的主要因子,它们明显地削弱了温度的作用;在水温下降的枯水期,鱼类捕食压力下降导致了枝角类丰度的增加.     

Palaeoecological preferences and geographical distribution of Late Maastrichtian deep-sea ostracods in the South Atlantic

The distributional patterns of deep-sea ostracods in the terminal Cretaceous South Atlantic were examined from seven DSDP/ODP holes (356, 525A, 527, 528, 529, 698A and 689B). Many of the genera recorded in this study are previously known from Campanian-Maastrichtian deep-sea sediments. However, the recovery of Aversovalva, Hemiparacytheridea, Profundobythere, Mayburya and Legitimocythere reveals considerable similarities with the Tertiary deep-sea ostracod fauna of the Pacific and the North Atlantic. The underlying causes controlling the variation in faunal density among the various sites are likely to be related to variation in food supply and, to a lesser extent, to variation in oxygen levels, since all sites quantitatively assessed are regarded to represent oligotrophic, well-oxygenated conditions. A Q -mode principal component analysis with varimax rotation yielded a first component largely dominated by Bythocypris and significantly correlated with palaeotemperature; there is an increasing abundance of Bythocypris towards higher latitudes with a maximum abundance at Site 689 where the palaeotemperature was around 9°C. A species of the thermophilic genus Cytherelloidea occurs also at this site, probably as a result of a slow adjustment to colder conditions in the absence of a thermocline at that time. The second component is largely dominated by Krithe and significantly correlated with palaeotemperature and palaeodepth. Among the sites studied, Krithe is most common at Site 356, where the palaeotemperature was around 15°C; today, most representatives of this genus are psychrospheric and adapted to temperatures below 10°C.     

Relationships between the spread of Caulerpa filiformis and fish communities on temperate rocky reefs

The previously sub‐dominant native marine macrophyte Caulerpa filiformis is now dominant on many sub‐tidal rocky reefs in New South Wales (NSW), Australia and is expanding its distribution. As C. filiformis is highly chemically defended and structurally different to co‐occurring habitat‐forming macrophytes, two key attributes that govern fish assemblages, we hypothesized that fish assemblages, particularly herbivorous fishes, would be different at sites where C. filiformis occurred from where it was previously absent and within sites, fish community structure would be correlated to the cover of C. filiformis. We investigated these hypotheses by determining reef‐associated fish assemblage attributes (assemblage structure, species richness, total abundance, Shannon‐Weiner diversity, abundance of herbivorous species) along transects within sites where C. filiformis was present and absent. Surprisingly, despite large patches and very high densities of C. filiformis on the reefs we sampled, at larger spatial scales (i.e., among sites) no fish assemblage metrics differed between sites with large stands of C. filiformis and sites without the alga. Moreover the abundance of one dominant herbivore, the rock cale Aplodactylus lophodon, was greater at sites within large beds of C. filiformis. At smaller spatial scales, however, i.e. within sites where C. filiformis was present, fish assemblages did vary as a function of C. filiformis cover along transects, although this was not consistent across sampling times. Overall, our results suggest that the potential effects of the spread of this alga on faunal communities warrants further investigation.     

Comparison of co-occurrence structure of temperate forest herb-layer communities in 1949 and 2000

When species presence–absence data are analyzed across a range of sites, many communities show a “checkerboard distribution” in which two or more species do not co-occur despite sharing geographic ranges and habitat requirements. It is not clear how the loss of native species might shift the underlying checkerboard structure. I tracked changes in the incidence of 175 vascular plant species across 59 sites in northern Wisconsin that have declined in species density between 1950 and 2000. Based on C-scores and the number of perfect checkerboard distributions, there was significant co-occurrence structure in both time periods. The nature of that structure depended on the particular null model was constructed. When only row values are fixed, plant species showed greater degrees of co-occurrence than the null model predicted, while fixing both rows and columns yielded less co-occurrence than found in the null model. Changes in species density did not strongly influence co-occurrence patterns, reflecting relative stability of co-occurrence patterns over a 50-year interval. This suggests that minor losses of species will not necessarily lead to fundamental changes in co-occurrence structure.     

The role of disturbance in the establishment of indigenous and alien plants at Inaccessible and Nightingale Islands in the South Atlantic Ocean

Disturbances play an important role in the establishment of vegetation at Inaccessible and Nightingale Islands in the South Atlantic Ocean. Type, altitude and age of disturbance all influenced the species richness, life-form and abundance of seedlings. A few indigenous species appear to be dependent on small disturbances, such as the clearings around albatross nests, for their perpetuation, e.g. Acaena sarmentosa and Gnaphalium thouarsii. Alien forbs and grasses, e.g. Conyza albida, Holcus lanatus, Pseudognaphalium luteo-album and Rumex obtusifolius, tended to dominate large recent disturbances such as soil-slips and depressions, and to colonize man-modified sites. Alien plants were less common on the plateau at Inaccessible Island than on the lowlands. Their success appears to be enhanced by large-scale disturbances. The highest density and diversity of seedlings occurred on large soil-slips where mineral soil was exposed and vegetation cover was minimal.