Weighing-in: universally applicable biometric conversion equations to support comparative assessment of invasive freshwater bivalves

Invasive bivalves continue to detrimentally impact freshwater ecosystems worldwide, with their ecological effects often being standardised by body size or biomass measurements. In their latest research, Coughlan and colleagues aim to derive universally applicable conversion equations to support reliable comparative assessment of bivalve driven ecological effects.

As dominant filter-feeders, most bivalves’ ecological impacts are a result of their filtration or particle clearance rates (i.e. the removal of suspended nutrients, contaminants and organic matter).

Although a considerable amount of data detailing bivalve filtration and clearance rates has been reported, a variety of different metrics for body size and biomass are frequently used as a means to standardise data. In particular, studies have tended to standardise their data in relation to grams of dry-weight and ash free dry-weight of the study species.

Whilst these data enable understanding of the dynamic processes shaping both community structure and biodiversity of invaded ecosystems, extracting data for comparative inter-study assessments can be exceedingly difficult, as knowledge of bivalve weight-to-weight conversion parameters is often limited, ambiguous or incomplete.

In our study, to facilitate improved comparative assessment amongst studies originating from disparate geographic locations, we report body size and biomass conversion equations for six invasive freshwater bivalves (or species complex members) worldwide: Corbicula fluminea, C. largillierti, Dreissena bugensis, D. polymorpha, Limnoperna fortunei and Sinanodonta woodiana. We also tested the reliability (i.e. precision and accuracy) of these equations.

We collected body size and biomass metrics of living-weight, wet-weight, dry-weight, dry shell-weight, shell free dry-weight and ash-free dry-weight from a total of 44 bivalve populations located across 14 countries in Asia, the Americas and Europe. Relationships between body size and individual biomass metrics (i.e. size-to-weight relationships), as well as proportional weight-to-weight conversion factors, were determined.

We then tested the reliability of our universal conversion equations by assessing the difference between the actual measurements of our samples and the values predicted by the equations. 

Sample site locations for populations of six invasive bivalve species

We found that for most species, although variation across sampled populations was detected, body size directional measurements were generally good predictors of all biomass metrics (size-to-weight relationships). Similarly, most proportional biomass relationships tended to be highly reliable (weight-to-weight).

Accordingly, our derived biomass prediction equations can be used to rapidly estimate the biologically active biomass of the assessed species, based on simpler biomass or body size measurements for a wide range of situations globally. This allows for the calculation of approximate average indicators that, when combined with density data, can be used to estimate biomass of bivalves per distinct geographic unit-areas and contribute to quantification of population-level effects. As such, our general equations will support meta-analyses, and also allow for comparative assessment of historic and contemporary data.

Graphical summarisation of variability in precision and accuracy for selected size-to-weight and proportional weight-to-weight conversion factors

From a management perspective, our derived equations can be used by conservation managers to determine bivalve driven effects and associated ecological risk, as well as the subsequent impact of any management interventions as part of cost-benefit assessments. For example, data from basic laboratory experiments describing bivalve clearance and nutrient cycling rates for a small number of specimens can be combined with density data from the invaded site, our equations will then allow bivalve driven effects to be scaled to the level of infestation observed at an invaded site.

Our equations will aid decision makers in prioritising the allocation of resources across multiple invaded sites. Further, the determination of biologically active biomass within invaded sites can also be used to predict nutrient release by purposefully killed bivalves, which can underpin a more strategic approach to control whereby only a portion of bivalves are actively killed at any one time to mitigate excess nutrient release. As such, our derived biometric conversion factors can provide a unifying platform for these comparative assessments, as well as management strategies.

Read the full Open Access paper Biometric conversion factors as a unifying platform for comparative assessment of invasive freshwater bivalves in Journal of Applied Ecology.

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