With overwhelming evidence of change in habitats, biologists today must assume that few, if any, study areas are natural and that biological variability is superimposed on trends rather than ...stationary means. Paleobiological data from the youngest sedimentary record, including death assemblages actively accumulating on modern land surfaces and seabeds, provide unique information on the status of present-day species, communities, and biomes over the last few decades to millennia and on their responses to natural and anthropogenic environmental change. Key advances have established the accuracy and resolving power of paleobiological information derived from naturally preserved remains and of proxy evidence for environmental conditions and sample age so that fossil data can both implicate and exonerate human stressors as the drivers of biotic change and permit the effects of multiple stressors to be disentangled. Legacy effects from Industrial and even pre-Industrial anthropogenic extirpations, introductions, (de)nutrification, and habitat conversion commonly emerge as the primary factors underlying the present-day status of populations and communities; within the last 2 million years, climate change has rarely been sufficient to drive major extinction pulses absent other human pressures, which are now manifold. Young fossil records also provide rigorous access to the baseline composition and dynamics of modern-day biota under pre-Industrial conditions, where insights include the millennial-scale persistence of community structures, the dominant role of physical environmental conditions rather than biotic interactions in determining community composition and disassembly, and the existence of naturally alternating states.
Mismatches between the composition of a time-averaged death assemblage (dead remains sieved from the upper mixed-zone of the sedimentary column) and the local living community are typically ...attributed to natural postmortem processes. However, statistical analysis of 73 molluscan data sets from estuaries and lagoons reveals significantly poorer average "live-dead agreement" in settings of documented anthropogenic eutrophication (AE) than in areas where AE and other human impacts are negligible. Taxonomic similarity of paired live and dead species lists declines steadily among areas as a function of AE severity, and, for data sets comprising only adults, rank-order agreement in species abundance drops where AE is suspected. The observed live-dead differences in composition are consistent with eutrophication (anomalous abundance of seagrass-dwellers and/or scarcity of organic-loving species in the death assemblage), suggesting compositional inertia of death assemblages to recent environmental change. Molluscan data sets from open shelf settings (n = 34) also show higher average live-dead discordance in areas of AE. These results indicate that (i) live-dead discordance in surficial grab samples provides valuable evidence for strong anthropogenic modification of benthic communities, (ii) actualistic estimates of the ecological fidelity of molluscan death assemblages tend to be erroneously pessimistic when conducted in nonpristine settings, and (iii) based on their high fidelity in pristine study areas, death assemblages are a promising means of reconstructing otherwise elusive preimpact ecological baselines from sedimentary records.
Biologists increasingly appreciate the importance of community-level attributes in the functioning and temporal turnover of ecosystems, but data other than species richness are difficult to acquire ...over the habitat-toregional and decadal-to-millennial scales needed to recognize biodiversity change, discriminate between natural and anthropogenic drivers, and inform theoretical and applied ecology. Death assemblages (DAs)—the actively accumulating organic remains encountered in present-day seabeds and landscapes, as distinct from permanently buried fossil assemblages—are an underexploited source of historical information at precisely these scales. Meta-analyses, dynamic modeling, and individual case studies, particularly of mollusks and mammals, reveal that DAs differ from censused living assemblages (LAs) primarily because they are temporally coarse, time-averaged samples, contrary to concerns that postmortem bias dominates. Temporal pooling predictably damps the ability of DAs to detect small-scale variation, but promotes their ability to inventory rare species; estimate the abundance structure of the metacommunity; document range changes; evaluate historic habitat use; and identify now-absent species, community states, and anthropogenically shifted baselines.
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.
Increased coastal urbanization worldwide has resulted in increased nitrogen inputs to ecosystems, leading to eutrophication and other negative effects. We assessed δ15N in the dead-collected shells ...of three molluscan species in two estuaries in order to evaluate their ability to identify known gradients in wastewater nitrogen input, namely from private septic systems feeding directly into Waquoit Bay and from a groundwater plume reflecting wastewater injection at a municipal treatment plant in West Falmouth Harbor, Massachusetts, USA. Shells of a suspension-feeder (Geukensia demissa), a micro-algal grazer (Littorina littorea), and an omnivore (Nassarius obsoletus) were collected from lower intertidal sediments near the taxon's life habitat. We found that δ15N in dead-collected shells decreased significantly along the gradients of wastewater pollution in both estuaries, with similar results across all three trophic groups. These positive results demonstrate the potential of dead-shell assemblages for detecting spatial gradients of wastewater pollution.
•δ15N from dead bivalve shells capture gradients in wastewater.•Dead gastropod shells yield similar δ15N values but larger variance per site.•The disparate trophic groups of these molluscan taxa did not affect δ15N values.•Analysis reveals potential of dead-shell assemblages to track wastewater inputs.
Humans now play a major role in altering Earth and its biota. Finding ways to ameliorate human impacts on biodiversity and to sustain and restore the ecosystem services on which we depend is a grand ...scientific and societal challenge. Conservation paleobiology is an emerging discipline that uses geohistorical data to meet these challenges by developing and testing models of how biota respond to environmental stressors. Here we (
a
) describe how the discipline has already provided insights about biotic responses to key environmental stressors, (
b
) outline research aimed at disentangling the effects of multiple stressors, (
c
) provide examples of deliverables for managers and policy makers, and (
d
) identify methodological advances in geohistorical analysis that will foster the next major breakthroughs in conservation outcomes. We highlight cases for which exclusive reliance on observations of living biota may lead researchers to erroneous conclusions about the nature and magnitude of biotic change, vulnerability, and resilience.
The soft-sediment seafloor of the open continental shelf is among the least-known biomes on Earth, despite its high diversity and importance to fisheries and biogeochemical cycling. Abundant dead ...shells of epifaunal suspension-feeding terebratulid brachiopods (Laqueus) and scallops on the now-muddy mainland continental shelf of southern California reveal the recent, previously unsuspected extirpation of an extensive offshore shell-gravel ecosystem, evidently driven by anthropogenic siltation. Living populations of attached epifauna, which formerly existed in a middle- and outer-shelf mosaic with patches of trophically diverse muds, are restricted today to rocky seafloor along the shelf edge and to the sandier shelves of offshore islands. Geological age-dating of 190 dead brachiopod shells shows that (i) no shells have been produced on the mainland shelf within the last 100 years, (ii) their shell production declined steeply during the nineteenth century, and (iii) they had formerly been present continuously for at least 4 kyr. This loss, sufficiently rapid (less than or equal to 100 years) and thorough to represent an ecosystem collapse, coincides with intensification of alluvial-plain land use in the nineteenth century, particularly livestock grazing. Extirpation was complete by the start of twentieth-century urbanization, warming, bottom fishing and scientific surveys. The loss of this filter-feeding fauna and the new spatial homogeneity and dominance of deposit- and detritus-feeders would have altered ecosystem functioning by reducing habitat heterogeneity and seawater filtering. This discovery, attesting to the power of this geological approach to recent ecological transitions, also strongly increases the spatial scope attributable to the negative effects of siltation, and suggests that it has been under-recognized on continental shelves elsewhere as a legacy of coastal land use.
Estuarine and coastal transformation is as old as civilization yet has dramatically accelerated over the past 150 to 300 years. Reconstructed time lines, causes, and consequences of change in 12 once ...diverse and productive estuaries and coastal seas worldwide show similar patterns: Human impacts have depleted >90% of formerly important species, destroyed >65% of seagrass and wetland habitat, degraded water quality, and accelerated species invasions. Twentieth-century conservation efforts achieved partial recovery of upper trophic levels but have so far failed to restore former ecosystem structure and function. Our results provide detailed historical baselines and quantitative targets for ecosystem-based management and marine conservation.
Fossil assemblages of skeletal material are thought to differ from their source live communities, particularly in relative abundance of species, owing to potential bias from postmortem transport and ...time-averaging of multiple generations. However, statistical meta-analysis of 85 marine molluscan data sets indicates that, although sensitive to sieve mesh-size and environment, time-averaged death assemblages retain a strong signal of species' original rank orders. Naturally accumulated death assemblages thus provide a reliable means of acquiring the abundance data that are key to a new generation of paleobiologic and macroecologic questions and to extending ecological time-series via sedimentary cores.
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