Historical ecological baselines from dead marine molluscs reveal natural and human-related changes in benthic fauna

Background/Question/Methods

Ecologists need historical baselines to understand the variability inherent to a given community. Communities do vary and reference conditions can clarify when natural variability becomes significant change. However, while some communities have reference conditions for several years or decades, not all areas have historical data. Without accurate and confident knowledge of reference conditions environmental managers could improperly characterize the health, or lack thereof, for a community. Death assemblages, the dead remains of formerly living communities in an area, can provide reference conditions for communities where workers have not observed modern conditions for very long. Naturally accumulating death assemblages represent historical information that is temporally averaged over years to decades and gives a coarser picture of how much a community has varied, or not varied, through time. Using marine molluscs from Newport, Oregon we compare modern observations with historical conditions as recorded by death assemblages. If death assemblages capture the same taxonomic and rank abundance patterns as living assemblages then they are a good source for reference information. If death assemblages do not agree with live assemblages then multiple hypotheses were evaluated to test for explanations related to sampling or taphonomic biases, time averaging, or ecological change. 

Results/Conclusions

Modern and historical samples are strongly and positively correlated in the community traits of taxonomic composition and rank abundance structure across all depths but are strongest at deeper sites. Some biases from sampling and taphonomy do exist at shallow depths and in specific groups (under-sampling of deep burrowing Pharid bivalves alive and removal of Tellinid bivalves from historical samples due to destruction).  But, biases affecting the ability for historical samples to capture modern species gradients are not found consistently across all samples and do not erase depth gradients recorded by historical samples. Differences in modern and historical samples were strongest at a site where a known impact exists: the dumping of modern dredge spoil. Additional differences in modern and historical samples suggested the natural shifting of relic mud habitats at shallow depths. Both modern and historical samples reveal that faunal changes correlate with dominant wave base depth and this physical mechanism may be a main driver for spatial distribution of species in this habitat. The ability of death assemblages to capture modern gradients suggests they are a strong source for reference information and should be tested for use in environmental monitoring situations.