Does Allometric Scaling Improve Estimates of Population Abundance Based on Environmental DNA?

Despite the popularity of environmental DNA (eDNA) analysis for non-invasive, cost-effective monitoring of aquatic biodiversity, its application for population abundance estimation is in its infancy. One of the uncertainties in eDNA-based abundance estimation surrounds the process of eDNA production: large individuals may produce less eDNA per unit body mass than smaller conspecifics, and there may be an allometric (power-law) relationship between the amount of eDNA released and body mass. Although integrating allometric scaling in eDNA production could refine eDNA-based abundance estimation, this theoretical framework may have a complex relationship with observed eDNA concentrations, especially in natural environments. We conducted a literature search to summarise previous studies that estimated population abundance using a combination of eDNA concentrations and allometric scaling frameworks, and found that allometry improved abundance estimates in only 5 of 12 studies. This did not seem to depend on the type of environment being studied, or the type of eDNA assay that was used. We discuss biological and technical factors that may help to explain the inconsistent allometric relationship between eDNA production and population abundance. We suggest that allometric scaling may be helpful when (i) the target populations exhibit substantial body size variation and (ii) species-specific scaling coefficients are available. However, our review shows that knowledge gaps remain in our understanding of abundance estimates based on eDNA, regardless of whether allometric relationships are factored into analyses.