Death Assemblages Research Articles

Radiocarbon dating supports bivalve-fish age coupling along a bathymetric gradient in high-resolution paleoenvironmental studies

Abstract Studies of paleocommunities and trophic webs assume that multispecies assemblages consist of species that coexisted in the same habitat over the duration of time averaging. However, even species with similar durability can differ in age within a single fossil assemblage. Here, we tested whether skeletal remains of different phyla and trophic guilds, the most abundant infaunal bivalve shells and nektobenthic fish otoliths, differed in radiocarbon age in surficial sediments along a depth gradient from 10 to 40 m on the warm-temperate Israeli shelf, and we modeled their dynamics of taphonomic loss. We found that, in spite of the higher potential of fishes for out-of-habitat transport after death, differences in age structure within depths were smaller by almost an order of magnitude than differences between depths. Shell and otolith assemblages underwent depth-specific burial pathways independent of taxon identity, generating death assemblages with comparable time averaging, and supporting the assumption of temporal and spatial co-occurrence of mollusks and fishes.

A U‐Th Dating Approach to Understanding Past Coral Reef Dynamics and Geomorphological Constraints on Future Reef Growth Potential; Mazie Bay, Southern Great Barrier Reef

AbstractReconstructing coral reef histories provides a window of understanding into reef response to changing environmental and climatic conditions over various temporal scales. Here we present the results of 117 U‐Th dates from emergent reef flat and slope cores and surface death assemblages, combined with previously published fossil microatoll data, to capture the entire sequence of reef growth at Mazie Bay, inshore Southern Great Barrier Reef (GBR). Coral U‐Th dates indicate that Mazie Bay reef initiated ~6,900 years before present (yr. BP) quickly filling accommodation space. While rates of vertical reef accretion (5.3 ± 1.0 mm year−1) were comparable to the GBR average during the mid‐Holocene (~5.0 mm year−1), reef flat progradation occurred at a rate 1.5‐ to 6‐fold previous GBR rates until 5,100 yr. BP (~70.4 cm year−1). Average progradation slowed to ~7.1 cm year−1 in the subsequent ~4,000 years and reef slope cores indicate this reef had largely “turned‐off” by 400 yr. BP, with modern coral communities existing as a veneer over the largely senescent framework. Death assemblage dates highlight coral disturbance and recovery regimes in response to increased cyclone activity 1960–1985 AD and recent extreme sea surface temperature and flood events post 2000 AD. U‐Th dating of mid‐Holocene to modern coral deposits from Mazie Bay reef provides a unique insight into past reef development, response to recent disturbance regimes, and potential for future reef growth. In the case of Mazie Bay, our data suggest limited accommodation space and increased occurrence of sea surface temperature extremes will restrict future reef growth at this site.

The overlooked role of taphonomy in ecology: post‐mortem processes can outweigh recruitment effects on community functions

Biogenic structures that persist post‐mortem are ubiquitous, but rarely considered as key ecological features. Post‐mortem structures in many ecological systems exert community‐level effects and thus the dynamics of their degradation (i.e. taphonomy) become important in affecting community functions and ecosystem services, and these often‐overlooked effects may rival the influence of recruitment and typical post‐recruitment processes. Moreover, in some highly researched habitat types, post‐mortem structures may be mistaken for living organisms, thus introducing significant error into our understanding of population processes. We examined widespread patterns and processes in marine assemblages with calcareous tubeworms and brackish‐water assemblages with bryozoans, in order to test the importance of taphonomy for populations and communities. In tubeworm assemblages, taphonomic variability caused differential accumulation of post‐mortem structures, exerting community‐level influence, and importantly it negated effects from recruitment on the community functions. For tubeworms and bryozoans, proportions of post‐mortem structures varied between sheltered and exposed habitats with overall distributional variability being caused by the share of post‐mortem structures, not living animals. Population‐level inferences in such systems would be extremely problematic without careful differentiation of dead/living structures. Dynamics emerging from the interaction of taphonomic processes and recruitment ecology are relevant for many other important biogenic habitats, especially coral reefs with death assemblages of bleached corals; a failure to consider variability in processes acting on the death assemblages, alongside the living, may severely distort understanding of patterns and processes in populations and communities.

Profundal Testate Amoebae (Arcellacea) of Lake Superior and Lake Michigan

Abstract Sediment surface death assemblages of recent testate amoebae (Arcellacea) are reported from nine sites in Lake Superior and Lake Michigan. These are the first profundal sediment-water interface samples of testate amoebae described from either of the Great Lakes which provide valuable insight on deep-water, large-lake assemblages. Centropixid strains were present to abundant in shallower, nearshore sites (up to 66 m water depth). Assemblages at depths >40 m were dominated by Difflugia oblonga “tenuis.” The shallowest sample (26 m) was dominated by Centropyxis aculeata “discoides” and Difflugia oblonga “tenuis.” Over 100 tests per sample were observed from >100 m. Density of tests appears to be constrained by lithology rather than water depth. The deepest site (325 m) yielded low foraminiferal abundances. This pilot study provides a first step towards documenting the distribution of testate amoebae in the Great Lakes.

Population fluctuations of the fungiid coral Cycloseris curvata, Galápagos Islands, Ecuador.

Fungiid corals (Cnidaria: Anthozoa: Scleractinia) occur at isolated locations scattered throughout the eastern tropical Pacific. They can be reef-associated but are often found on sand and rubble substrata distant from reef coral habitat. Cycloseris curvata is known in this region from the southern Gulf of California, through Mexico, Costa Rica, and Panamá, and with the southern-most populations occurring in the Galápagos Islands, Ecuador. During Archipelago-wide surveys (1988-2019), living individuals of Cycloseris curvata were observed at only two locations, Devil's Crown (near Floreana Island) and Xarifa Island (near Española Island). The Devil's Crown population was observed from 1988 to 2017, whereas living individuals in the Xarifa population were observed from 2005 to 2009. In 2012 a death assemblage (dead skeletons) was discovered at Darwin Island, at the northern-most extent of the Archipelago. At Devil's Crown, visual surveys were performed annually or biennially from 1990 to 2012, with two more surveys in 2017 and 2019. The living Cycloseris curvata population consisted of 15 individuals in 1990 that gradually increased to 78 individuals by 1995. Over 200 individuals were observed in 1996, and high numbers persisted through 1998 with 335 individuals. Live tissue surface area per polyp ranged from 0.5 to 95.0cm2. The population decreased to 112 individuals in 1999 (following warming associated with the 1997-98 El Niño), with further declines to 20 in 2009 (following cooling associated with the 2007 La Niña) and a rebound to 91 in 2012. After a 5y break in data collection, only one individual (28.3cm2) was observed in 2017, and in 2019 none were observed. Although undetected living Cycloseris curvata populations may exist, and renewed recruitment provides some hope for population reestablishment, it is possible that this fungiid coral species is now extirpated from the Galápagos Archipelago.

Ocean quahogs (Arctica islandica) and Atlantic surfclams (Spisula solidissima) on the Mid-Atlantic Bight continental shelf and Georges Bank: The death assemblage as a recorder of climate change and the reorganization of the continental shelf benthos

The degree to which evidence of range shifts in shelf biomass dominants, Atlantic surfclams and ocean quahogs, as a function of climate change is examined by comparing the dissimilarity in distribution of the living community and the record of habitation ensconced in the death assemblage. Comparison of the distribution of live animals and dead shells reveals for both species that live animals are rarely found where dead shells were not collected, but dead shells were collected at a substantively larger number of sites than where live animals were found. The geographic footprint resolved by the dead shells was much broader than that of live animals. Extensive spatially-coherent regions exist inshore and offshore of the present-day habitat of occupation where evidence of previous occupation exists. Sites where ocean quahog shells were found without live ocean quahogs included a portion of the continental shelf inshore of the present-day habitable range, an area only habitable under colder climatic conditions than observed today. Surfclam shells were found offshore of the present-day habitable region, in deeper water: additional warming of bottom waters offshore would be necessary for surfclams to occupy the region now occupied solely by dead shells. The death assemblage shows that these biomass dominants have repositioned themselves across the continental shelf during the past and this repositioning certainly is driven by changes in bottom water temperature. Moreover, the distribution of surfclam shells indicates that the death assemblage may provide a possible window into the future, as movement of the present-day population is towards these deeper-water previously-occupied regions. Overall, the death assemblage may provide an important source of information on range shifts sparing the need to carry out extensive and frequent benthic surveys and may permit reconstruction of ongoing shifts in community composition as a product of climate change when adequate survey data are not available.

Environmental controls on shallow subtidal molluscan death assemblages on San Salvador Island, The Bahamas

Ecological studies that span multiple marine depositional environments frequently find that benthic invertebrate assemblages are correlated primarily with an onshore-offshore gradient. Variability within the same depositional environment is inferred to not result from water depth differences, but other parameters such as substrate consistency and shear stress. Modern studies focusing on assemblage variations within the same depositional environment can identify which variables affecting assemblage compositions are independent of water depth. This study examines the environmental controls on the composition of molluscan death assemblages from the shallow subtidal environments of San Salvador Island. Molluscan assemblages were tabulated from twenty-nine bulk samples collected from four locations around San Salvador Island representing seagrass meadows, open sandflats, and patch reef systems common in the shallow subtidal. In addition, quantitative environmental data for each sample was characterized, including seagrass density, water depth, temperature, pH, dissolved oxygen, and grain-size, as well as qualitative observations of wave energy and bioturbation. Cluster analysis and canonical correspondence analysis (CCA) were performed to identify environmental gradients associated with molluscan assemblage composition. Cluster analysis and CCA Axis 1 identify seagrass coverage and substrate consistency as the primary environmental variables describing molluscan death assemblage compositions. CCA Axis 2 identifies wave energy, bioturbation, and dissolved oxygen as the secondary environmental variables, though the small range of dissolved oxygen values (17–21 mg/L) collected using single, instantaneous measurements make it unclear whether the oxygen gradient is ecologically significant for our time-averaged assemblages. Our analysis demonstrates the variability of assemblages within a single depositional environment, as well as the utility of fine-resolution studies for identifying secondary environmental variables that control the composition of marine invertebrate assemblages.

High-latitude benthic bivalve biomass and recent climate change: Testing the power of live-dead discordance in the Pacific Arctic

Time-averaged molluscan death assemblages sampled from tropical to temperate open continental shelves commonly disagree in species composition with local living communities only in areas that have changed in response to anthropogenic eutrophication and other locally intense human stresses, providing a means of recognizing shifted baselines. In contrast, the ability of live-dead discordance to resolve the spatially heterogeneous effects of human-induced climate change has not been tested in high-latitudes, where climate change entails substantial changes in nutrient cycling with consequences for benthic biomass and where cold waters are antagonistic to carbonate shell preservation. North Pacific Arctic and Subarctic seabeds offer ideal conditions for testing the resolving power of molluscan live-dead discordance, using well-documented ecologic changes in nutrient cycling and benthic biomass in response to reduced sea ice. Ecosystem monitoring since 1980 has established that the boundary between the Arctic and the Subarctic on the Bering Sea continental shelf, maintained by ice-influenced bottom water, shifted northward between 1998 and 2001. The benthic community in the transitioned area now experiences new pelagic predators, more variable quantity and quality of deposited food, and altered sediment grain size, and macrofaunal dominance has shifted from diverse communities of specialized suspension or deposit feeders to facultative deposit feeding guilds. We find that in habitats where either Subarctic or Arctic conditions have persisted, bivalve death assemblages agree closely with counterpart living communities in taxon and guild composition and are not subject to significant post-mortem bias. Significant live-dead discordance occurs only in areas with documented changes in carbon delivery, sediment grain size, and community composition over the last several decades; there, death assemblages are mixtures of shells from pre- and post-transition communities, as confirmed by monitoring data. This spatial pattern is robust to both numerical abundance- and biomass-based measures of community composition. In fact, biomass is especially powerful in revealing fine, station-level discordance that is strongly tied to known sites within habitats where new carbon deposition levels, grain size, or benthos have occurred since 1980. Live-dead discordance can thus reliably differentiate between stable and rapidly changing habitats in cold, high-latitude settings, relevant to evaluating climate change, and biomass-based currencies of community composition are as robust as numerical abundance data, and in fact, improve spatial resolution.

Ecological fidelity and spatiotemporal resolution of arthropod death assemblages from rodent middens in the central Atacama Desert (northern Chile)

Evaluating the magnitude and direction of biases affecting the ecological information captured by death assemblages is an important prerequisite for understanding past, present, and future community-environment relationships. Here, we establish the ecological fidelity and spatiotemporal resolution of an overlooked source of fossil remains: the soil arthropod assemblages found in rodent middens (that span from the present to >44,420 cal yr BP) collected in the central Atacama Desert of northern Chile. We evaluated the “live-dead agreement” across four sources of soil arthropod data; two contemporary surveys of live communities (i.e., live), and two sources of death assemblages (i.e., dead). Although live-dead agreements and diversity indices are highly variable among samples (live and dead assemblages), our results consistently demonstrate that an average fossil midden (i) better captures the structure and composition of living communities than species richness per se; (ii) offers a spatially-resolved picture of those communities at local scales; and (iii) is only weakly affected by time-averaging. The fine spatiotemporal resolution of fossil midden records in the Atacama, and most likely other areas of the world where rodent middens occur offers ecological information on the structure and composition of fossil arthropod assemblages potentially over many thousands of years. This information is reliable enough to establish historical baselines before past and ongoing anthropogenic impacts.

Algal reefs (Coralligenous) from glacial stages: Origin and nature of a submerged tabular relief (Hyblean Plateau, Italy)

In the framework of the Italian multidisciplinary research project “Mud volcanoes Ecosystem study - Sicily Channel”, cores MV02_GC01 and MV03_GC02 were recovered from a tabular relief at 140 m water depth, to investigate the nature of such uncommon shallow seep-related morphology trough micro- and macro-paleontological, mineralogical, sedimentological and geochronological analyses.Both cores record muddy surficial sedimentation, hosting a strictly circalittoral molluskan death assemblage typical of mobile substrate, coherent with the present-day seafloor depth and environment. The core bottom recovered a biogenic framework, in which the molluskan fossil assemblage is characteristic of shallow-water, possibly infralittoral, representing a past sea level low stand. The biogenic framework shows structure and composition that are similar to present day algal reefs indicated as coralligenous build-ups. Both cores recorded the Holocene transgression. Core MV02_GC01 is longer and its basal section records cobbles and pebbles of both terrigenous and biogenic origin partially covered by a dark patina, and finely preserved Halimeda sp. fragments. The dark patina reveals a fibrous nature, often containing framboid pyrite crystals, ankerite, but no organic matter. We interpret the dark patina level as the last lowstand, possible forming in a lagoon-like paleoenvironment. Below such level, another coralligenous build-up occurs, U/Th dated to MIS 6, in agreement with the available seismic interpretation. The tabular relief lays up to 10 m above the surrounding seafloor and reveals a complex inner architecture, in which at least two generations of coralligenous build-ups shaped its geomorphological development, and the local tectonic activity contributed to its uplift.

Mummified and skeletal southern elephant seals (Mirounga leonina) from the Victoria Land Coast, Ross Sea, Antarctica

AbstractWe report on an accumulation of mummified southern elephant seals (Mirounga leonina) from Inexpressible Island on the Victoria Land Coast (VLC), western Ross Sea, Antarctica. This accumulation is unusual, as elephant seals typically breed and molt on sub‐Antarctic islands further north and do not currently occupy the VLC. Prior ancient DNA analyses revealed that these seals were part of a large, Antarctic breeding population that crashed ~1,000 yr ago. Radiocarbon dates for Inexpressible Island mummies range from 380 to 3,270 yr before present, too old to have been created by Scott's Northern Party in 1912 and varying too widely in age to represent a catastrophic death assemblage. Skeletal measurements reveal that most Inexpressible Island mummies are adult or subadult males. The presence of male elephant seals on Inexpressible Island until several hundred years ago suggests that, at a minimum, it served as a haul‐out site for the large Antarctic population and may have hosted a breeding colony. The conditions that allowed this Antarctic population to use the Ross Sea, the factors spurring its decline, and the implications for the adaptability and sensitivity of the species to environmental change all merit further study.

PATTERNS OF UNREPAIRED SHELL DAMAGE IN RECENT BRACHIOPODS FROM FIORDLAND (NEW ZEALAND)

In order to provide quantitative data concerning patterns of shell breakage and repair in rhynchonelliform brachiopods, we studied undisturbed death assemblages from a New Zealand fiord complex where three species of terebratulide and one rhynchonellide occur in dense mixed patches on the near vertical walls. Proportions of damaged (both repaired and non-repaired) individuals varied between both taxa and sampling site. However, the general observation was that few individuals show signs of having been able to repair damage but the proportion of individuals showing unrepaired, and hence presumably fatal, breakages was higher (up to 76% in Magasella sanguinea from one sample from Tricky Cove in Doubtful Sound). Damage was mostly concentrated around the anterior margins and affected both valves and is consistent with having been clamped between a set of either jaws or claws. Potential culprits include fish (wrasse), rock lobsters and echinoids. As yet it is unclear whether the damage results from deliberate feeding activity or as collateral damage from grazers feeding on other organisms on the fiord walls which may allow secondary predation by asteroids. The net effect is, however, the same, in that the damage appears to have been fatal. More structured sampling is now required to understand the spatial variation in this damage and mortality, and also to establish the culprits with more certainty.

BIOGEOGRAPHIC SHELL SHAPE VARIATION INTROPHON GEVERSIANUS(GASTROPODA: MURICIDAE) ALONG THE SOUTHWESTERN ATLANTIC COAST

Broad-scale latitudinal morphological trends in gastropods along the southwestern Atlantic coast are scant, since the majority of studies have focused on local scales. Here, we evaluate biogeographic shell shape variation in the marine gastropod Trophon geversianus across most of its distributional range, covering 14 degrees of latitude. Samples come from death assemblages which have the potential to unveil biogeographic patterns along spatio-temporal scales and are not affected by short-term volatility in comparison with living assemblages. We performed morphometric analyses on shells from death assemblages, and compared shape variation between mid-Holocene and modern shells from one southern site. Multivariate analyses identified two morphotypes matching the biogeographic regions of the Argentine Sea that segregates a warm-temperate from a cold-temperate zone. The Magellan province morphotype is characterized by a larger shell, lower spire height, and higher aperture length than the Argentinean province morphotype. This change in shell shape is significantly correlated to sea surface temperature, even after accounting for spatial autocorrelation, which could be indirectly influencing intraspecific morphoclines via shifts in growth rates. On the other side, shell size and shape variations were also detected (size increase over recent geological time) between mid-Holocene and modern specimens at the Beagle Channel, which could be attributed to paleoenvironmental changes and to shifts in predator-prey relationships. Our study illustrates the usefulness of death assemblages for revealing large-scale patterns of shell-shape variability in mollusk species, and highlights the spatial coincidence of intraspecific morphological differentiation with the transition zone between biogeographic provinces of the Argentine Sea.

QUANTIFYING THE DIGESTIVE FINGERPRINTS OF PREDATORS ON THE BONES OF THEIR PREY USING SCANNING ELECTRON MICROSCOPY

Paleoecological reconstruction relies on accurately determining the taphonomic origin of fossil deposits. Predation is a common mechanism by which skeletal remains become concentrated over time, leading to the formation of modern and fossil prey death assemblages. Skeletal element representation and breakage patterns within such death assemblages can be used to infer the identity of the responsible predator. However, assemblage-level metrics cannot be used to infer if a single fossil specimen is predator-derived. Microscopic digestive etching on individual bones can also indicate past predation events because acidic gastric fluids create distinctive micrometer-scale fissures in cortical bone. Here we establish a quantitative approach to predator identification from small mammal prey remains using microscopic digestive damage patterns. To do this, we collected mandibles from rodents digested by 13 predator species from local wildlife rehabilitation centers, and imaged them using an FEI Quanta 200 SEM. Results indicate that bones exposed to gastric fluids show clear digestive fissures, and that owl-digested specimens can be readily distinguished from specimens that were digested by diurnal raptors and mammalian carnivores. Specifically, owl-digested specimens are characterized by a high density of small and short digestive fissures. Within the owls, digestive fissure patterns appear to scale with owl body size. Finally, we used linear discriminant analysis to build a classification scheme from our modern data and applied it to Holocene mouse fossils from Two Ledges Chamber, Nevada. We found that the fossil specimens display the digestive fingerprints of owls. Quantification of microscopic digestive fissures thus offers a promising new approach for elucidating the taphonomic history of individual fossil specimens.

Life span bias explains live–dead discordance in abundance of two common bivalves

AbstractLife span bias potentially alters species abundance in death assemblages through the overrepresentation of short-lived organisms compared with their long-lived counterparts. Although previous work found that life span bias did not contribute significantly to live–dead discordance in bivalve assemblages, life span bias better explained discordance in two groups: longer-lived bivalve species and species with known life spans. More studies using local, rather than global, species-wide life spans and mortality rates would help to determine the prevalence of life span bias, especially for long-lived species with known life spans. Here, we conducted a field study at two sites in North Carolina to assess potential life span bias between Mercenaria mercenaria and Chione elevata, two long-lived bivalve species that can be aged directly. We compared the ability of directly measured local life spans with that of regional and global life spans to predict live–dead discordance between these two species. The shorter-lived species (C. elevata) was overrepresented in the death assemblage compared with its live abundance, and local life span data largely predicted the amount of live–dead discordance; local life spans predicted 43% to 88% of discordance. Furthermore, the global maximum life span for M. mercenaria resulted in substantial overpredictions of discordance (1.4 to 1.6 times the observed live–dead discordance). The results of this study suggest that life span bias should be considered as a factor affecting proportional abundances of species in death assemblages and that using life span estimates appropriate to the study locality improves predictions of discordance based on life span compared with using global life span estimates.

Mollusk shell assemblages as archives of spatial structuring of benthic communities around subtropical islands

Surficial assemblages of mollusk shells provide minimally invasive, quantitative data that are potentially adequate for assessing spatial organization of local benthic ecosystems. This study evaluates distribution and ecological characteristics of mollusk-dominated benthic communities around San Salvador Island (Bahamas). A total of 60 bulk samples collected along 12 onshore-offshore transects around the island yielded 20,301 specimens representing 180 mollusk species. Radiocarbon dating of individual shells (n = 30) indicated that these surficial accumulations represented a multi-millennial (time-averaged) record of local communities. Indirect multivariate ordinations (NMDS) separated mollusk death assemblage samples by locality, local habitat types, and regional context. Even in the case of comparable habitats within the same bay, samples clustered by transect location suggesting that surficial shell accumulations faithfully archive local differences in mollusk communities despite time-averaging and post-mortem shell transport. At the regional scale, a clear faunal separation is observed between windward and leeward sides of the island, suggesting that water energy represents an overriding regional driver that controls local composition of mollusk assemblages. Within each water energy regime, the faunal composition appeared to be primarily controlled by habitat type (seagrass versus sand habitats). Moreover, consistent with theoretical predictions that structured habitats enhance faunal stability of local communities, death assemblages from seagrass habitats were less variable in faunal composition than open sand habitats. Results indicate that San Salvador Island benthic communities are characterized by a predictable spatial organization controlled primarily by regional physical and, secondarily, biological processes. Also, the fact that extensively time-averaged shell assemblages have retained an interpretable spatial pattern suggests that regional physical processes, local habitat distribution, and structuring of mollusk associations must have been stable over multi-centennial time scales. This case study suggests that non-invasive sampling of dead mollusks can be an effective strategy for assessing prevalent long-term patterns in spatial distribution of regional faunal associations and examining processes that drive spatial structuring of marine communities.

Spatial, temporal and taxonomic scaling of richness in an eastern African large mammal community

AbstractAimEcological patterns and process change across spatial, temporal and taxonomic scales. This confounds comparisons between modern and fossil communities, which are sampled across very different scales, especially temporal ones. We use a recent bone dataset (i.e., “death assemblages”) from a modern ecosystem to explore spatial, temporal and taxonomic scaling in biodiversity assessments. Our ultimate goal is to create a model based on these scaling relationships to facilitate meaningful comparisons between modern and fossil communities.LocationAmboseli National Park, southern Kenya.Time periodMid‐1960 s to present day.Major taxa studiedLarge mammals (>1 kg).MethodsWe implemented a random placement null model and used model selection methods to investigate how species richness at Amboseli scales as a function of time and area [i.e., the species–time–area relationship (STAR) model]. We then analysed how the model coefficients change at different taxonomic scales (i.e., genus, family, order).ResultsIn agreement with previous studies, we find species richness scales positively with time and area but with a negative interaction between the two. Rates of richness turnover decrease as taxonomic scale increases.Main conclusionsWe hypothesize that decreasing rates of turnover with increasing spatial and/or temporal scale are caused by taking progressively larger samples from a species pool that is changing at a slower rate relative to turnover at the scale of sampling. Because increasing area and time are simply alternative ways of uncovering the species pool, increased time‐averaging of communities results in a more spatially averaged ecological signal. Increasing taxonomic scale causes turnover rates to decrease because of how lower‐level taxa are aggregated into coarser, higher‐level ones. The STAR model presents a framework for extrapolating and comparing richness between small‐scale modern and large‐scale fossil communities, as well as a means to understand the general processes involved with changing scale.

Testing for human impacts in the mismatch of living and dead ostracode assemblages at nested spatial scales in subtropical lakes from the Bahamian archipelago

AbstractNaturally time-averaged accumulations of skeletal remains—death assemblages—provide reliable, albeit temporally coarse, information on the species composition and structure of communities in diverse settings, and their mismatch with local living communities usually signals recent human-driven ecological change. Here, we present the first test of live–dead mismatch as an indicator of human stress using ostracodes. On three islands along a gradient of human population density in the Bahamas, we compared the similarity of living and death assemblages in 10 lakes with relatively low levels of human stress to live–dead similarity in 11 physically comparable lakes subject to industrial, agricultural, or other human activities currently or in the past. We find that live–dead agreement in pristine lakes is consistently excellent, boding well for using death assemblages in modern-day and paleolimnological biodiversity assessments. In most comparison of physically similar paired lakes, sample-level live–dead mismatch in both taxonomic composition and species’ rank abundance is on average significantly greater in the stressed lakes; live–dead agreement is not lower in all samples from stressed lakes, but is more variable. When samples are pooled for lake-level and island-level comparisons, stressed lakes still yield lower live–dead agreement, but the significance of the difference with pristine lakes decreases—species that occur dead-only (or alive-only) in one sample are likely to occur alive (or dead) in other samples. Interisland differences in live–dead agreement are congruent with, but not significantly correlated with, differences in human population density. This situation arises from heterogeneity in the timing and magnitudes of stresses and in the extent of poststress recovery. Live–dead mismatch in ostracode assemblages thus may be a reliable indicator of human impact at the sample level with the potential to be a widely applicable tool for identifying impacted habitats and, perhaps, monitoring the progress of their recovery.

Live-Dead Mismatch of Molluscan Assemblages Indicates Disturbance from Anthropogenic Eutrophication in the Barnegat Bay-Little Egg Harbor Estuary

Previous research combining data on the abundance of benthic invertebrates living in the northernmost section of the Barnegat Bay-Little Egg Harbor estuary with data on water quality suggested a community composition consistent with anthropogenic eutrophication. This part of the estuary, however, experiences both high-nutrient and low-salinity environmental conditions, which are both ecologically stressful to sensitive species and favorable to opportunistic species, complicating assessments of the direct effects of anthropogenic eutrophication. Here, geohistorical baselines were derived from molluscan death assemblages from four localities along a north-south gradient in the bay—two northern low-salinity, high-nutrient sites and two southern high-salinity, low-nutrient sites—to disentangle the effects of salinity and nutrient enrichment on the composition of benthic communities. Disturbance in the northern section of the estuary from freshwater input has been relatively consistent for thousands of years, but nutrient enrichment is a more recent stressor, developing only in the past few decades. The cause of the disturbed benthic community is unlikely to be anthropogenic eutrophication, if there is little discordance between the living and the death assemblage in the northern, low-salinity section of the bay. Comparisons of rank-order abundance (Spearman's rho) and taxonomic similarity (Jaccard-Chao index) of the living and death assemblages along the north-south gradient, however, indicate that compositional shifts occur at the northern sites only. A multivariate index analysis, M-AMBI, which integrates the AZTI Marine Biotic Index, a biotic index based on species sensitivity/tolerance to disturbances, with Shannon diversity and species richness, shows a similar pattern: ecological status shifts (from undisturbed to disturbed) in the northern but not southern sites along the north-south salinity gradient. These independent lines of evidence corroborate the inference that it is most likely anthropogenic eutrophication (and not salinity) disturbance driving observed patterns in benthic community composition. Further sampling and incorporation of geohistorical data into ongoing benthic monitoring in the estuary is recommended to establish ecological baselines where none exist, guide restoration planning, and assess the success of restoration efforts.

Ecophenotypy, temporal and spatial fidelity, functional morphology, and physiological trade-offs among intertidal bivalves

AbstractEcophenotypic variation in populations is driven by differences in environmental variables. In marine environments, ecophenotypic variation may be caused by differences in hydrodynamic conditions, substrate type, water depth, temperature, salinity, oxygen concentration, and habitat heterogeneity, among others. Instances of ecophenotypic variation in modern and fossil settings are common, but little is known about the influences of time averaging and spatial averaging on their preservation. Here we examine the shell morphology of two adjacent populations, both live collected and death assemblages, of the infaunal, suspension-feeding, intertidal bivalve Leukoma staminea from the well-studied Argyle Creek and Argyle Lagoon locations on San Juan Island, Washington. Individuals in the low-energy lagoon are free to burrow in the fine-grained substrate, while clams in the high-energy creek are precluded from burrowing in the rocky channel. Our results demonstrate variation in size and shape between the adjacent habitats. Lagoon clams are larger, more disk-shaped, and have relatively larger siphons than their creek counterparts, which are smaller, more spherical in shape, and have a relatively shallower pallial sinus. This ecophenotypy is preserved among death assemblages, although with generally greater variation due to time averaging and shell transport. Our interpretation is that ecophenotypic variation, in this case, is induced by differing hydrodynamic regimes and substrate types, cumulatively resulting in physiological trade-offs diverting resources from feeding and respiration to stability and shell strength, all of which have the potential to be preserved in the fossil record.