Ecological extinction caused by overfishing precedes all other pervasive human disturbance to coastal ecosystems, including pollution, degradation of water quality, and anthropogenic climate change. Historical abundances of large consumer species were fantastically large in comparison with recent observations. Paleoecological, archaeological, and historical data show that time lags of decades to centuries occurred between the onset of overfishing and consequent changes in ecological communities, because unfished species of similar trophic level assumed the ecological roles of overfished species until they too were overfished or died of epidemic diseases related to overcrowding. Retrospective data not only help to clarify underlying causes and rates of ecological change, but they also demonstrate achievable goals for restoration and management of coastal ecosystems that could not even be contemplated based on the limited perspective of recent observations alone.Few modern ecological studies take into account the former natural abundances of large marine vertebrates. There are dozens of places in the Caribbean named after large sea turtles whose adult populations now number in the tens of thousands rather than the tens of millions of a few centuries ago (1, 2).

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 990% of formerly important species, destroyed 965% 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. With recognition of their essential role for human and marine life, estuaries and coastal zones have become the focus of efforts to develop ecosystembased management and large-scale restoration strategies. To be successful, these approaches require historical reference points and assessments of the degree and drivers of degradation in an ecosystem context (8, 9).We reconstructed historical baselines and quantified the magnitude and causes of change in 12 temperate estuarine and coastal ecosystems in Europe, North America, and Australia from the onset of human settlement until today (Table 1). We used paleontologic, archaeological, historical, and ecological records (table S1) to quantify changes in 30 to 80 species per system standardized into 22 guilds and six taxonomic and seven functional groups, as well as seven water-quality parameters and species invasions (10). Species were selected for their economic, structural, or functional significance throughout history. We estimated relative abundance of each species over real time and across seven cultural periods reflecting the stage of cultural and market development rather than calendar dates (tables S2 and S3). Relative abundance was quantified as pristine (100%), abundant (90%), depleted (50%), rare (10%), or extinct (0%) (table S4). Recovery was quantified as partial or substantial when increasing from G10% to 910% and 950%, respectively (10). Our estimates are conservative compared with available absolute abundance records.

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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 cooccurring 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.

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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.ecological baseline ͉ eutrophication ͉ marine communities ͉ paleoecology H uman activities affect living systems in many ways, directly by means of harvesting and indirectly by means of processes ranging from habitat conversion to climate change. Many of these activities have deep roots in human history, but virtually all have intensified and become increasingly global in effect over the last two centuries and especially the last several decades (1-6). Acquiring baseline information on ecosystems before the onset of human activities of a particular type or intensity is thus essential to evaluating anthropogenic impacts and to developing targets for remediation. However, such baselines have been unobtainable in many settings where human impacts preceded biomonitoring. Sedimentary records can be a powerful means of reconstructing ecological and physical environmental changes in such situations, by using a variety of proxies to extend chronologies beyond the reach of available scientific observations (7). Such records are becoming more widely used to determine the historical trajectories of ecological change and to assess the likely role of humans as drivers (7-10). However, in nonvarved, estuarine and open-shelf sedimentary settings, time-averaging of biotic assemblages (the mixing of durable dead remains from multiple generations within the upper part of the sedimentary column) has the ...