Discordance between living and death assemblages as evidence for anthropogenic ecological change

Abstract

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 ...

Differential responses of marine communities to natural and anthropogenic changes

“…The observed timing is also consistent with relatively recent shifts observed for molluscdominated communities in other regions, as suggested by discordances between live communities and surficial death assemblages [41,42].…”

Biology in the Anthropocene: Challenges and insights from young fossil records

“…47). A second key metaanalytic discovery is that strong live-dead discordance in species composition and relative abundance is a legacy of recent ecological change in the living assemblage: the living assemblage has apparently shifted away from its prior composition, which the time-averaged death assemblage remembers (53). This finding for marine mollusks overturns former assumptions that live-dead discordance is caused by poor postmortem preservation and seems to be general.…”

Marine sediments tell it like it was

“…The loss of such an important habitat and food source cascades throughout the ecosystem. As Kidwell (2) shows, using mismatches between the composition of living and death (i.e., dead remains sieved from the upper sediments) molluscan assemblages from lagoons and estuaries, before-and-after snapshot approaches can be used to reconstruct faunal changes associated with eutrophication. Such temporal data can then be used to show where and how much change has occurred because of human activities.…”