Jansen A. Smith

Fossil clam shells reveal unintended carbon cycling consequences of Colorado River management

Water management that alters riverine ecosystem processes has strongly influenced deltas and the people who depend on them, but a full accounting of the trade-offs is still emerging. Using palaeoecological data, we document a surprising biogeochemical consequence of water management in the Colorado River basin. Complete allocation and consumptive use of the rivers flow has altered the downstream estuarine ecosystem, including the abundance and composition of the mollusc community, an important component in estuarine carbon cycling. In particular, population declines in the endemic Colorado delta clam, Mulinia coloradoensis, from 50--125 individuals m−2 in the pre-dam era to three individuals m−2 today, have likely resulted in a reduction, on the order of 5900–15 000 t C yr−1 (4.1–10.6 mol C m−2 yr−1), in the net carbon emissions associated with molluscs. Although this reduction is large within the estuarine system, it is small in comparison with annual global carbon emissions. Nonetheless, this finding highlights the need for further research into the effects of dams, diversions and reservoirs on the biogeochemistry of deltas and estuaries worldwide, underscoring a present need for integrated water and carbon planning.

Mollusk Assemblages As Records of Past and Present Ecological Status

AMBI and Bentix are widely used benthic indices for guiding remediation decisions under two major pieces of environmental legislation in Europe — the Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD). These indices usually incorporate all marine benthic invertebrates in a sample. Some recent studies, however, have applied these benthic indices to only mollusk species due to the ease of identifying only one taxonomic group to the species level and because death assemblages (accumulated dead mollusk shells in sediments) may be valuable sources of data for assessing baseline conditions. Although they found that ecological status differences can be detected by applying AMBI and Bentix to mollusks, these studies did not test whether mollusk-only index values, and the ecological statuses indicated by them, are equivalent to those calculated from the whole benthic community. To test this assumption, we performed a meta-analysis of data from 12 European benthic community studies comparing mollusk-only index values with whole-community values. Using five mollusk-only data sets, we also assessed whether application of AMBI and Bentix to molluscan death assemblages can be used to detect changes in ecological status over time. We show that the application of AMBI and Bentix to only the molluscan taxa in benthic communities is a viable method for determining the ecological status of water bodies. Our results also suggest that the application of benthic indices to molluscan death assemblages has great potential to 1) establish baseline conditions for assessing ecological status under the WFD and 2) estimate the natural range of variation of ecosystem attributes for defining sustainability thresholds under the MSFD. We outline three recommendations for the future use of mollusk-only AMBI and Bentix based on our results: 1) mollusk-only index values should be adjusted to facilitate comparisons with whole-community studies; 2) local ecological group assignments should be used if possible; and 3) we encourage collaboration between paleoecologists and benthic ecologists to facilitate interpretations of index values from death assemblages. We conclude that mollusk-only benthic index assessments of molluscan death assemblages have the potential to be a powerful tool for guiding management decisions under the WFD and MSFD.

ON DRILLING FREQUENCY AND MANLY'S ALPHA: TOWARDS A NULL MODEL FOR PREDATOR PREFERENCE IN PALEOECOLOGY

Abstract In the paleoecological literature, drilling frequency—the percent of specimens in a prey taxon with complete drill holes—is commonly interpreted as an indication of a predators preference. Such taxon-specific drilling frequencies are often compared with one another and related to underlying prey characteristics such as cost-benefit ratio or body size. Although this approach can demonstrate a predators relative preference for one prey type over another, it fails to consider whether predation on any prey type is greater than would be expected by a predator without preference. Here we develop a null model for evaluating predator preference in paleoecology by considering drilling frequency in the framework of Manlys alpha, which is a well-established model in the ecological literature that has been used to evaluate predator preferences. In effect, Manlys alpha normalizes taxon-specific drilling frequencies with all potential prey types in the community, allowing for a statistically rigorous test of the null hypothesis that drilling predation, and therefore predator preference, on any given prey type is equivalent to all other available prey types in the community. After discussing the statistical basis for the model, we demonstrate the models utility by applying it to a published dataset of drilling predation on Pliocene bivalves from Langenboom, Netherlands. The Manlys alpha approach, which uses the same data (i.e., drilling frequencies) that are commonly collected by paleoecologists, provides a null hypothesis to more rigorously assess predator preferences and inherently includes community context for predator-prey interactions in the fossil record.

Effects of dams on downstream molluscan predator–prey interactions in the Colorado River estuary

River systems worldwide have been modified for human use and the downstream ecological consequences are often poorly understood. In the Colorado River estuary, where upstream water diversions have limited freshwater input during the last century, mollusc remains from the last several hundred years suggest widespread ecological change. The once abundant clam Mulinia modesta has undergone population declines of approximately 94% and populations of predators relying on this species as a food source have probably declined, switched to alternative prey species or both. We distinguish between the first two hypotheses using a null model of predation preference to test whether M. modesta was preyed upon selectively by the naticid snail, Neverita reclusiana, along the estuarys past salinity gradient. To evaluate the third hypothesis, we estimate available prey biomass today and in the past, assuming prey were a limiting resource. Data on the frequency of drill holes—identifiable traces of naticid predation on prey shells—showed several species, including M. modesta, were preferred prey. Neverita reclusiana was probably able to switch prey. Available prey biomass also declined, suggesting the N. reclusiana population probably also declined. These results indicate a substantial change to the structure of the benthic food web. Given the global scale of water management, such changes have probably also occurred in many of the worlds estuaries.

Conceptions of Long-Term Data Among Marine Conservation Biologists and What Conservation Paleobiologists Need to Know

Marine conservation biologists increasingly recognize the value of long-term data and the temporal context they can provide for modern ecosystems. Such data are also available from conservation paleobiology, but the enormous potential for integration of geohistorical data in marine conservation biology remains unrealized. The lack of a common language for data integration and a tendency in each field to measure different variables, at scales that may differ by orders of magnitude, make integration difficult. To better understand how conservation paleobiology can maximize its potential, we conducted a survey of marine conservation biologists working in the United States.