Anthropogenic effect recorded in the live-dead compositional fidelity of land snail assemblages from San Salvador Island, Bahamas

Terrestrial malacofaunas that inhabit islands are vulnerable to human activities. Habitat destruction, introduction of exotic species, predators, etc. are distorting the composition and distribution of indigenous snail communities. Specifically, the taxonomic discordance between live and dead assemblages may be the consequence of anthropogenic disturbances rather than natural post-mortem processes. Live-dead fidelity may hence reflect the degree of human alteration in a given locality. This approach was used to study the relative abundance of live and dead land snails from pristine and urbanized localities around San Salvador Island (Bahamas). Thirteen species were encountered from 64 samples containing 5,343 specimens. Taxonomic composition was significantly different between areas of negligible anthropogenic disturbance and those impacted by humans. Although which species of land snails are endemic to San Salvador is unknown, all identified species are native to the Caribbean region and invasive species were not encountered. However, some synanthropic species, found exclusively in urban-developed areas, have not been recovered from the local fossil record, pointing to their relatively recent introduction possibly from nearby islands. Non-metric MDS, Jaccard-Chao index, and Spearman correlation analyses indicated that dead assemblages displayed a good compositional correspondence to live communities at pristine sites, whereas considerable disparity was sometimes observed at human-modified areas. Urbanized areas probably favored the proliferation of synanthropic species whereas dead assemblages may contain anthropophobic taxa that lived there prior to recent human modification. These findings are consistent with previous live-dead fidelity studies of marine molluscan and terrestrial small-mammal assemblages, and suggest that the taxonomic discordance between live and dead assemblages of land snails may be indicative of recent anthropogenic alteration.     

Relative abundances of living and dead molluscs in two Californian lagoons

Assemblages of living benthic invertebrates (predominantly bivalve molluscs) from the sand-channel habitat of two Southern California (U.S.A.) lagoons were sampled on ten occasions over a 37-month period. A one-time sampling of the corresponding assemblages of accumulating dead remains made possible a contrast of living and dead assemblages designed to assess the biasing effects of post-mortem transportation, shell dissolution, and time-averaging. Species-by-species comparisons of the living and dead molluscs found together in the same samples strongly suggested that post-mortem transportation is insignificant within this high-energy habitat. A similar conclusion arose from contrasting the pattern of spatial heterogeneity of the living community with that of the dead assemblage. Species presence-and-absence comparisons were generally more reliable than comparisons of relative abundances. Adjustments for experimentally determined rates of post-mortem shell dissolution proved significant and further decreased the correspondence in relative abundances between living and dead assemblages. Greater temporal variability of living populations at Mugu Lagoon, probably caused by a more harsh physical environment, increased the differences in composition between living and dead assemblages, which suggests that correspondence in relative abundances between living and dead assemblages generally should be expected to decrease as the life environment becomes more harsh.     

Fidelity of molluscan life and death assemblages on sublittoral hard substrata around granitic islands of the Seychelles

This study quantified the degree of coincidence between living and dead molluscan faunas in a shallow-water coral reef environment in the Indian Ocean. The results were compared with those from a similar life:death study in the northern Red Sea, and with those published for reef corals and soft substrata molluscs. The proportions of quantitatively important taxa are robust to sampling intensity, but fidelity indices and rank-order correlations are strongly influenced by quantitatively unimportant taxa. Distinct differences between life and death assemblages were recognized, which are due to distinct biases in the death assemblage. Bivalves that lived in close contact with living corals are preferentially overgrown after death and should provide considerable temporal and ecological information in a potential fossil record, as they will be preserved within a rapidly growing reef framework. Some gastropod taxa are preferentially transported into surrounding soft substrata postmortem. Here they will be affected by time-averaging and taphonomic disintegration typically occurring in sediments resulting in the associated loss of much temporal information. Most gastropod shells, however, are inhabited by hermit crabs postmortem, which may strongly alter the fossil gastropod community structure. The results are similar to a comparable study in the northern Red Sea, with a major exception being the strong dominance of hermit crab-inhabited gastropod shells in the death assemblage of the Seychelles. Comparison of life:death assemblages between hard and soft substrata, in keeping with the northern Red Sea study, showed the strong dominance of dead shells in the soft substrata with the converse on the hard substrata. This results from different accumulation conditions for dead shells in soft substrata. Fidelity indices are well suited to demonstrate that sedimentary death assemblages are typically remarkably robust reflections of local community composition but they do not record the strong biases in death assemblages of coral reef associated hard substrata molluscs and are therefore unsuitable for comparisons of life and death assemblages in reef environments.     

The high fidelity of the cetacean stranding record: insights into measuring diversity by integrating taphonomy and macroecology

Stranded cetaceans have long intrigued naturalists because their causation has escaped singular explanations. Regardless of cause, strandings also represent a sample of the living community, although their fidelity has rarely been quantified. Using commensurate stranding and sighting records compiled from archived datasets representing nearly every major ocean basin, I demonstrated that the cetacean stranding record faithfully reflects patterns of richness and relative abundance in living communities, especially for coastlines greater than 2000 km and latitudinal gradients greater than 4°. Live-dead fidelity metrics from seven different countries indicated that strandings were almost always richer than live surveys; richness also increased with coastline length. Most death assemblages recorded the same ranked relative abundance as living communities, although this correlation decreased in strength and significance at coastline lengths greater than 15,000 km, highlighting the importance of sampling diversity at regional scales. Rarefaction analyses indicated that sampling greater than 10 years generally enhanced the completeness of death assemblages, although protracted temporal sampling did not substitute for sampling over longer coastlines or broader latitudes. Overall, this global live-dead comparison demonstrated that strandings almost always provided better diversity information about extant cetacean communities than live surveys; such archives are therefore relevant for macroecological and palaeobiological studies of cetacean community change through time.     

Autochthonous death assemblages from chemoautotrophic communities at petroleum seeps: Palaeoproduction,energy flow,and implications for the fossil record

Death assemblages produced by chemoautotrophic communities at cold seeps represent a type of autochthonous accumulation that is difficult to differentiate from other heterotrophic autochthonous communities using taphonomic characteristics. We test the hypothesis that cold‐seep assemblages can be discriminated by unique biological or community attributes rather than taphonomic attributes. To test this hypothesis, we compared several cold seeps on the Louisiana upper continental slope to heterotrophic sites on the Louisiana slope and to a putative seep site in the middle‐late Campanian Pierre Shale near Pueblo, Colorado. Seep assemblages are characterized by a unique tier and guild structure, size‐frequency composition, and animal density that together identify the palaeoenergetics structure of these communities and distinguish them from the other assemblages of the shelf and slope. All seep assemblages were dominated by primary consumers, whereas the heterotrophic assemblage was dominated by carnivores. Carnivore dominance seems to be typical of shelf (or euhaline) death assemblages. Seep assemblages, in contrast, retain the theoretically‐expected rarity of predaceous forms in fossil assemblages. Epifauna and semi‐infauna dominate the tier structure of the heterotrophic assemblage as is typical for continental shelf and slope assemblages. The infaunal tier was unusually well represented in most petroleum seep assemblages. Local enrichment of food resources and the dominance of shelled primary consumers explain the guild and tier structure of seep assemblages. Hindcasting of energy demand (palaeoingestion) and an estimate of sedimentation rate confirms that energy demand by the community exceeds the supply from planktonic rain in seep communities. Thus, seep assemblages can be recognized using biological attributes where taphonomic analysis is ambiguous.     

Time‐averaging and fidelity of modern death assemblages: building a taphonomic foundation for conservation palaeobiology

Ecosystems today are under growing pressure, with human domination at many scales. It is difficult, however, to gauge what has changed or been lost – and why – in the absence of data from periods before human activities. Actualistic taphonomic studies, originally motivated to understand preservational controls on deep‐time fossil records, are now providing insights into modern death assemblages as historical archives of present‐day ecosystems, turning taphonomy on its head. This article reviews the past 20 years of work on the temporal resolution and ability of time‐averaged skeletal assemblages to capture ecological information faithfully, focusing primarily on molluscs from soft‐sediment seafloors. Two promising arenas for ‘applied taphonomy’ are then highlighted: (1) using live‐dead mismatch – that is, observed discordance in the diversity, species composition, and distribution of living animals and co‐occurring skeletal remains – to recognize recent anthropogenic change, and (2) using time‐averaged death assemblages as windows into regional diversity and long‐term baselines, as a supplement or substitute for conventional live‐collected data. Meta‐analysis and modelling find that, in unaltered habitats, live‐dead differences in community‐level attributes can be generated largely or entirely by time‐averaging of natural spatial and temporal variability in living assemblages, on time frames consistent with the range of shell ages observed in death assemblages. Time‐averaging coarsens the temporal and spatial resolution of biological information in predictable ways; by comparison, taphonomic bias of information arising from differential preservation, production and transport of shells is surprisingly modest. Several challenges remain for basic taphonomic research, such as empirical and analytical methods of refining the temporal resolution of death assemblages; assessing the fate of resolution and fidelity with progressive burial; and expanding our understanding of the dynamics of skeletal accumulation in other groups and settings. Rather than shunning human‐impacted areas as inappropriate analogues of the deep past, we should capitalize on them to explore the fundamental controls on skeletal accumulation and to develop robust protocols for bringing time‐averaged death assemblages into the toolkits of conservation biology and environmental management.     

Ghosts of yellowstone: multi-decadal histories of wildlife populations captured by bones on a modern landscape

Natural accumulations of skeletal material (death assemblages) have the potential to provide historical data on species diversity and population structure for regions lacking decades of wildlife monitoring, thereby contributing valuable baseline data for conservation and management strategies. Previous studies of the ecological and temporal resolutions of death assemblages from terrestrial large-mammal communities, however, have largely focused on broad patterns of community composition in tropical settings. Here, I expand the environmental sampling of large-mammal death assemblages into a temperate biome and explore more demanding assessments of ecological fidelity by testing their capacity to record past population fluctuations of individual species in the well-studied ungulate community of Yellowstone National Park (Yellowstone). Despite dramatic ecological changes following the 1988 wildfires and 1995 wolf re-introduction, the Yellowstone death assemblage is highly faithful to the living community in species richness and community structure. These results agree with studies of tropical death assemblages and establish the broad capability of vertebrate remains to provide high-quality ecological data from disparate ecosystems and biomes. Importantly, the Yellowstone death assemblage also correctly identifies species that changed significantly in abundance over the last 20 to ～80 years and the directions of those shifts (including local invasions and extinctions). The relative frequency of fresh versus weathered bones for individual species is also consistent with documented trends in living population sizes. Radiocarbon dating verifies the historical source of bones from Equus caballus (horse): a functionally extinct species. Bone surveys are a broadly valuable tool for obtaining population trends and baseline shifts over decadal-to-centennial timescales.     

Paleoecology of some molluscan assemblages from the badenian (Miocene) marine sandy facies of Poland

Three molluscan assemblages from the Badenian (Miocene) marine sandy facies of Poland are described in terms of their taxonomic composition, diversity, and trophic structure. The structural variation between the molluscan assemblages seems to be largely controlled by a gradient in water energy which in turn results in gradients in substrate mobility and organic content of bottom sediments. Together with a subordinate factor of local sedimentation rate, these gradients control the distribution of lucinoid mucus tube feeders vs. deep burrowing siphonate suspension feeders, of browsers, and of deposit feeders vs. suspension feeders. Some structural features of the molluscan assemblages are regarded as related to biotic factors. The graphs of cumulative species frequencies vs. the logarithm of cumulative individual frequencies are used as a measure of community organization. These graphs indicate distinct differences in ecological maturity between the molluscan assemblages. On this basis, two different stages of ecological succession in an offshore sandy bottom environment are recognized. The evidence from the comparison of the Miocene molluscan assemblages with their recent counterparts may support the view that the evolutionary stability of species populations of a given community, and long-term stability of the community structure are independent. High environmental variability and stress seem to be correlated with evolutionary stability of species populations rather than with long-term permanence of community structure.     

Influence of setting,sieve size,and sediment depth on multivariate and univariate assemblage attributes of coral reef-associated mollusc death assemblages from the Gulf of Aqaba

Measures of diversity and ecology of marine invertebrate assemblages depend on a variety of factors including environmental conditions and methodological decisions. In this study, the influence of such factors on multi- and univariate assemblage parameters of molluscan death assemblages from the Gulf of Aqaba (Red Sea, Jordan) was evaluated. Sediment samples were collected at two coral reef types, a patch reef at 13 m of water depth characterized by fine-grained sediments and a Millepora-fringing reef with coarse-grained sediments at 5 m of water depth. The upper and lower 10 cm of the sediment column were separately removed and sieved with mesh sizes of 1 and 2 mm. A large dataset of 6400 bivalve and gastropod shells was compiled to evaluate how setting, sediment depth, and sieve size influenced taxonomic composition and species richness, species-abundance patterns and the Shannon–Wiener index, the number of drilled shells per species and drilling frequency (DF) of the assemblage. Setting had the strongest impact on all aspects, followed by sieve size, but sediment depth was insignificant, probably due to complete homogenization of the sediments by reworking and bioturbation. Multivariate assemblage parameters distinguished much better between categories (setting, sieve size) than univariate measures. Sieve size-related disagreements recognized between the two higher taxa are mostly due to the underlying difference in body-size distribution of bivalve and gastropod assemblages. We conclude that species richness and other ecological characteristics of molluscan death assemblages in coral reef-associated sediments will most strongly reflect habitat complexity of the sites chosen, are significantly influenced by methodological decisions (i.e., sieve size), will only poorly preserve temporal patterns, and the results will differ between bivalves and gastropods.     

Factors controlling upper Jurassic ammonite assemblages in north-central Mexico

Lower Kimmeridgian to Lower Tithonian (Upper Jurassic) sections studied at Sierra de Palotes (Durango) and Sierra de Catorce (San Luis Potosí), Mexico, show low-energy deposits in which the composition of fossil macroinvertebrate assemblages, including megabenthos, reflects biostratinomic control. Monotonous siltstones provide continuous records of ammonite assemblages and reflect dominant deposition of shells in living areas; meanwhile, discontinues records were forced by episodic post-mortem transportation of shells, which was especially accentuated under storm influence. Rhythmic marly-silty limestones and marls illustrate a fossil record probably determined by minor transgressive-regressive pulses. The major changes in lithofacies are reflected by condensed silty and phosphatic mudstones deposited during significant floodings affecting areas under dominant terrigenous sedimentation. These changes determined more or less significant variations in the composition of fossil assemblages according to their relation to changing ecological conditions. However, shifting ecospaces exhibit no direct relationship to changes in lithofacies. Post-mortem transportation, operating in relation to both marine floodings and changes in the pattern of upper-water currents, was the main biostratinomic factor affecting the areal distribution of ammonite populations. Shell transportation and sedimentation rate controlled preservation and ultimately influenced diversity in recorded ammonite assemblages. The post-mortem behaviour (interpreted from shell structure and preservation), and therefore distribution, of ammonite shells points to shallow-water environments during the Kimmeridgian - Early Tithonian in areas (such as SE Durango and San Luís Potosí) close to the changing boundary between dominant carbonate and terrigenous sedimentation. No reworking affecting ammonite biostratigraphy has been identified in the sections studied.     

Remembrance of things past: modelling the relationship between species' abundances in living communities and death assemblages

Accumulations of dead skeletal material are a valuable archive of past ecological conditions. However, such assemblages are not equivalent to living communities because they mix the remains of multiple generations and are altered by post-mortem processes. The abundance of a species in a death assemblage can be quantitatively modelled by successively integrating the product of an influx time series and a post-mortem loss function (a decay function with a constant half-life). In such a model, temporal mixing increases expected absolute dead abundance relative to average influx as a linear function of half-life and increases variation in absolute dead abundance values as a square-root function of half-life. Because typical abundance distributions of ecological communities are logarithmically distributed, species' differences in preservational half-life would have to be very large to substantially alter species' abundance ranks (i.e. make rare species common or vice-versa). In addition, expected dead abundances increase at a faster rate than their range of variation with increased time averaging, predicting greater consistency in the relative abundance structure of death assemblages than their parent living community.     

Linking taphonomy to community-level abundance: Insights into compositional fidelity of the Upper Triassic shell concentrations (Eastern Alps)

Although actualistic live/dead comparisons lead to robust estimates of fidelity of modern death assemblages, quantitative evaluation of fidelity of fossil assemblage remains uncertain. In this paper, effects of storm reworking on compositional fidelity of the Upper Triassic shell concentrations (Eastern Alps, Austria) are evaluated. An exploratory approach is based on comparison of reworked and non-reworked assemblages in ordination analyses. Non-reworked assemblages of one or more communities provide a baseline for evaluation of fidelity of reworked assemblages. In siliciclastic-rich intervals of the Kössen Formation, shell concentrations are represented by (1) packstones with small, shallow infaunal bivalves, (2) floatstones and pavements with large semi-infaunal bivalves, and (3) bioclastic marlstones. In carbonate-rich intervals, bioclastic floatstones with bivalves and brachiopods occur. Analyzing all shell concentrations, eight sample groups sharing similar species composition are discriminated. Limited effect of storm reworking on composition of shell concentrations is indicated by (1) a general persistence of six sample groups when only non-reworked assemblages are analyzed, (2) similarity in composition between reworked and non-reworked assemblages within sample groups, and (3) compositional segregation between non-reworked assemblages of distinctive sample groups, mostly without any reworked assemblages of intermediate composition.Depth-related variations in dead-shell production, shell destruction and body size governed preservation and distribution of the shell concentrations along onshore-offshore gradient in the Kössen Basin. First, at times when environmental conditions were unfavorable for shell producers, coupled with high background shell destruction rates, limestone beds formed during storm events were shell-poor. Second, less common shell concentrations in upper than in lower parts of siliciclastic intervals can be related to higher environmental stress in shallower habitats. Third, the difference between shell concentrations dominated by small and large bivalves is driven by between-habitat differences in body size and is not due to a differential sorting of small and large shells. Combining community analysis based on species abundances with taphonomic analysis can thus be helpful in tracking fidelity of fossil assemblages.     

Response of nematode communities after large‐scale ice‐shelf collapse events in the Antarctic Larsen area

Owing to large‐scale ice‐shelf disintegration events, the Antarctic Larsen A and B areas recently became ice‐free. During the ANT‐XXIII/8 Polarstern campaign, this region was sampled for the first time. Our study is the first to investigate benthic communities in this area and their response to the collapse of ice shelves in the Antarctic. The nematofauna appears to be strongly influenced by the sudden ice‐cover removal, although its response differs from that of the macro‐ and megabenthos. Our results indicate that precollapse, sub‐ice communities were impoverished and characterized by low densities, low diversity and high dominance of a few taxa. This might still be visible at a station located deep inside the Larsen B embayment, where Halomonhystera was dominant. Post‐collapse recolonization of the ‘inner’ stations, i.e. those located furthermost from the former ice‐shelf edge, is believed to be a long‐time process. At the time of sampling, community structure at the inner stations was not or only slightly influenced by colonization, and might be structured by local environmental conditions. Our results indicate that a locally increased food supply after ice‐cover removal could provoke a faster, local response of the nematode assemblages compared with the response due to recolonization. Thalassomonhystera is recognized as an opportunist, taking advantage of increased food supply at inner stations A_South and B_North. Communities living close to the former ice‐shelf edge are believed to be at an intermediate or late stage of succession, with a dominance of Microlaimus, a common Antarctic genus and quick colonizer. Densities here were comparable with those at other Antarctic stations, whereas they were considerably decreased at the inner stations. In general, the collapse of the Larsen ice shelves initially has a positive effect on the shelf nematode fauna in the area, both in terms of abundance and diversity.     

MODERN LAND MOLLUSCA FROM MAIDEN CASTLE, DORSET, AND THEIR RELEVANCE TO THE INTERPRETATION OF SUBFOSSIL ARCHAEOLOGICAL ASSEMBLAGES

To aid in the interpretation of molluscan assemblages from archaeologicalsites, modern land mollusc faunas were analysed from 43 turfsamples on a transect of the ramparts of Maiden Castle, Dorset,in chalk grassland. Distinct faunas in terms of species composition,molluscan abundance and species diversity as defined by theShannon and Brillouin indices were present. They were relatedto habitat factors, especially lime, structural diversity andwarmth. The results allow the identification of different grasslandtypes and show that distinct assemblages can occur in closeproximity, delineated by sharp boundaries. However, interpretationof subfossil assemblages from archaeological contexts is bestmade in terms of general relations of relevance between assemblagesand environment because matching by species composition betweenmodern faunas and subfossil assemblages is generally impossible. (Received 17 November 1993; accepted 1 February 1994)     

Differential taphonomy of modern brachiopods (San Juan Islands, Washington State): effect of intrinsic factors on damage and community-level abundance

The differences in shell structure and population turnover between organic-poor, impunctate (Hemithiris) and organic-rich, punctate brachiopod (Terebratalia) in a mixed-bottom, siliciclastic setting (San Juan Islands, WA) lead to different taphonomic damage and fidelity with respect to community-level abundance in death assemblages. In comparing shell interiors of similar-sized specimens, Terebratalia is predominantly affected by fibre detachment and shows almost no microbioerosion at the SEM scale; whereas, Hemithiris shows less marked fibre detachment at the SEM scale and is more intensely affected by microbioerosion both at SEM and light microscope (LM) scales. Fibre detachment related to rapid, microbially-induced organic matter decay appears to be the main destructive process acting on Terebratalia. Higher bioerosion levels in Hemithiris at SEM and LM scales are probably related to a combination of a low maceration rate and a preferential settlement by borers. From their vastly different abundances in life assemblages it can be deduced that Terebratalia produces dead shells at a much higher rate than Hemithiris. Therefore, the proportion of altered Terebratalia, relative to Hemithiris, is expected to be decreased due to its higher production of recently dead cohorts. That Terebratalia is also characterized by high damage levels shows that differential population turnover alone is not responsible for the differences in taphonomic damage. This shows that organic-rich and organic-poor shells are characterized by differential post-mortem durability. Although very few Hemithiris are present in the life assemblages, high durability ensures its relative over-representation in death assemblages. Terebratalia is not strongly under-represented in death assemblages, despite its high destruction rate, because of large production of recently dead shells. Even with the biasing effect of differential durability, the good fidelity reported in previous live-dead studies can be enhanced by higher population turnover of numerically dominant taxa, leading to constant input of recently dead shells into death assemblages.     

Acidic episodes retard the biological recovery of upland British streams from chronic acidification

We tested two predictions required to support the hypothesis that anthropogenic acidic episodes might explain the poor biological response of upland British streams otherwise recovering from acidification: (i) that invertebrate assemblages should differ between episodic and well-buffered streams and (ii) these effects should differentiate between sites with episodes caused by anthropogenic acidification as opposed to base-cation dilution or sea-salt deposition. Chronic and episodically acidic streams were widespread, and episodes reflected acid titration more than dilution. Nonmarine sulphate (16–18% vs. 5–9%), and nitrate (4–6% vs. 1–2%) contributed more to anion loading during episodes in Wales than Scotland, and Welsh streams also had a larger proportion of total stream sulphate from nonmarine sources (64–66% vs. 35–46%). Sea-salts were rarely a major cause of episodic ANC or pH reduction during the events sampled. By contrast, streams with episodes driven by strong anthropogenic acids had lower pH (5.0±0.6) and more dissolved aluminium (288±271 μg L⁻¹) during events than where episodes were caused by dilution (pH 5.4±0.6; 116±110 μg Al L⁻¹) or where streams remained circumneutral (pH 6.7±1.0; 50±45 μg Al L⁻¹). Both biological predictions were supported: invertebrate assemblages differed among sites with different episode chemistry while several acid-sensitive species were absent only where episodes reflected anthropogenic acidification. We conclude that strong acid anions – dominantly nonmarine sulphate – still cause significant episodic acidification in acid-sensitive areas of Britain and may be a sufficient explanation for slow biological recovery in many locations.     

The dead do not lie: using skeletal remains for rapid assessment of historical small-mammal community baselines

Conservation and restoration efforts are often hindered by a lack of historical baselines that pre-date intense anthropogenic environmental change. In this paper I document that natural accumulations of skeletal remains represent a potential source of high-quality data on the historical composition and structure of small-mammal communities. I do so by assessing the fidelity of modern, decadal and centennial-scale time-averaged samples of skeletal remains (concentrated by raptor predation) to the living small-mammal communities from which they are derived. To test the power of skeletal remains to reveal baseline shifts, I employ the design of a natural experiment, comparing two taphonomically similar Great Basin cave localities in areas where anthropogenic land-use practices have diverged within the last century. I find relative stasis at the undisturbed site, but document rapid restructuring of the small-mammal community at the site subjected to recent disturbance. I independently validate this result using historical trapping records to show that dead remains accurately capture both the magnitude and direction of this baseline shift. Surveys of skeletal remains therefore provide a simple, powerful and rapid alternative approach for gaining insight into the historical structure and dynamics of modern small-mammal communities.     

Bryozoans of the Weddell Sea continental shelf,slope and abyss: did marine life colonize the Antarctic shelf from deep water,outlying islands or in situ refugia following glaciations?

Aim At the height of glaciations such as the Last Glacial Maximum (LGM), benthic life on polar continental shelves was bulldozed off nearly all of the Antarctic shelf by grounded ice sheets. The origins of the current shelf benthos have become a subject of considerable debate. There are several possible sources for the current Antarctic shelf fauna, the first of which is the continental slope and deep sea of the Southern Ocean. The high levels of reported eurybathy for many Antarctic species are taken as evidence supporting this. A second possible source for colonists is the southern margins of other continents. Finally, shelves could have been recolonized from refugia on the continental shelves or slopes around Antarctica. The current study investigates whether the patchily rich and abundant biota that now occurs on the Antarctic continental shelf recolonized from refugia in situ or elsewhere. Location Weddell Sea, Antarctica. Methods We examined bryozoan samples of the BENDEX, ANDEEP III and SYSTCO expeditions, as well as the literature. Using similarity matrices (Sørensen coefficient), we assessed similarities of benthos sampled from around Antarctica. By assessing numbers of species shared between differing depths and adjacent shelf areas, we evaluated the origins of cheilostome bryozoan communities. Results Bryozoans decreased from 28, 6.5 and 0.3 colonies per trawl, and 0.16, 0.046 and 0.0026 colonies per cm² of hard surface from shelf to slope to abyssal depths. We found little and no support for recolonization of the Weddell Sea shelf by bryozoans from the adjacent slope and abyss, in the scenario of LGM faunal wipe‐out. The Weddell Sea shelf bryozoan fauna was considerably more similar to those on other Antarctic shelves than to that of the adjacent (Weddell Sea) continental slope. The known bryozoan fauna of the Weddell Sea shelf is not a subset of the Weddell Sea slope or abyssal faunas. Main conclusions We consider that the composition of the current Weddell Sea bryozoan fauna is most easily explained by in situ survival. Thus we consider that at least some of the Weddell Sea fauna persisted throughout the LGM, although not necessarily at the same locations throughout, to recolonize the large area currently occupied.     

Patterns in the distribution of soft corals across the central Great Barrier Reef

Distribution patterns of soft coral genera were examined at 11 reefs situated in a broad transect from inshore to the Coral Sea in the central region of the Great Barrier Reef. Twenty-five genera representing the Orders Alcyonacea and Stolonifera were recorded, and the survey also included one genus of the Order Gorgonacea. Total living soft coral cover is greatest on outershelf reef slopes, and is often less than and inversely related to the cover by stony corals. Soft coral diversity is generally low on reef flats, where soft coral cover is low or nil except in protected, inshore areas. The most diverse assemblages occur on reef slopes in midshelf and outershelf areas, where Efflatounaria and nephtheid genera predominate, and widely distributed alcyoniid genera are common. These richer assemblages are less well represented in the Coral Sea, while innershelf reefs support a less diverse fauna of somewhat different generic composition. Distribution patterns of soft corals across the transect broadly match similar variations in the distributions of stony corals and fishes, inshore reefs being generally depauperate. Such variations across the continental shelf are closely associated with changes in prevailing environmental conditions, but further research will be required to elucidate the effects of environmental parameters on benthic community structure.     

Effects of Chiton granosus (Frembly, 1827) and other molluscan grazers on algal succession in wave exposed mid-intertidal rocky shores of central Chile

Molluscan grazers can have important effects on the abundance, colonization rates, and successional pathways of algal assemblages and the entire intertidal community. In general, early successional algae are more readily consumed than corticated algae and kelps, which usually get established later in the community succession. To generalize, however, the effect of different grazers on algal assemblages must be examined on different coasts and under different scenarios. This information could help us understand the mechanisms of ecosystem processes and situations in which general models do not apply. Along the coast of Chile, humans harvest large keyhole limpets, which seem to be the only invertebrate grazers capable of controlling the dominant corticated alga Mazzaella laminarioides, a canopy-forming species that can cover extensive areas of the mid intertidal zone. In this scenario, where large limpets are harvested, the overall effects of the diverse molluscan assemblage of limpets, chitons and snails on algal succession and on corticated algae in particular are not clear. We conducted a 26-month-long experiment to evaluate the effects of molluscan grazers on mid-intertidal algal succession and to isolate the effects of Chiton granosus, the most conspicuous member of the assemblage at these tidal elevations. At sites heavily impacted by humans the molluscan grazer assemblage had strong negative effects on colonization and abundance of green algae such as ulvoids and Blidingia minima. In doing so, the grazer assemblage had a strong negative indirect effect on the establishments of chironomid fly larvae, which were only observed on green algal mats and rarely on bare rock. No significant effects were detected on epilithic microalgae, and effects on sessile invertebrates were highly variable over space and time. C. granosus also had significant negative effects on green algae but did not account for the total grazing pressure exerted by the guild. Limited foraging excursions (ca. 35 cm) from refuges and moderate site (crevice) fidelity in this species may contribute to the patchiness in green algal distribution observed in the field. Nearly 13 months after rock surface were experimentally cleared, M. laminarioides appeared in all experimental plots, but increased over three times faster in enclosures containing C. granosus than in exclosures plots or controls, suggesting that moderate levels of herbivory could actually facilitate the establishment of this alga in the succession and that the green algal cover found in the absence of grazers may delay its establishment.