Causes and consequences of long-term defaunation in multi-species aggregations: Species traits predict freshwater mussel declines and decreases in biogeochemical storage and recycling

1. One of the foremost challenges in contemporary ecology is understanding the rapid loss of animal diversity and biomass, known as defaunation. The causes and consequences of defaunation depend on species traits that regulate which species are vulnerable to particular stressors and the ecosystem-level consequences of declines.
2. We used a >30-year long-term monitoring dataset and a trait-based framework to identify defaunation in multi-species freshwater animal aggregations (freshwater mussels). We quantified long-term trends in mussel density, species richness, and mussel-generated ecosystem function, and tested whether these declines were associated with drought—a key driver of freshwater defaunation. Finally, we determined whether defaunation varied among mussel taxa with contrasting ecophysiological (thermal tolerance guild) and life history traits.
3. Losses of mussel density were associated with biomass declines, and consequent losses of mussel-generated carbon and nutrient storage. Mussel aggregations became defaunated and lost function in rivers where drought conditions were more severe and/or frequent during the study. Unexpectedly, declines did not differ among species with different thermal tolerances—both thermally sensitive species and thermally tolerant species declined in defaunated streams. Rather, the taxa driving the declines in density and ecosystem function were those with life history traits adapted for stable hydrologic conditions (low fecundity, long lifespans, late age at maturity).
4. Our finding that declines differed among species based on life history traits, but not thermal tolerance guilds, contrasted with previous studies conducted at shorter time intervals. We suggest that the ecophysiological traits that impact individual survival may be more important in governing drought-induced defaunation at shorter time scales, while life history traits that govern population recovery may be more important at longer time scales.
5. The magnitude of lost biogeochemical storage and recycling associated with mussel declines was substantial when placed in the context of other studies. Given the significant role that river ecosystems play in global biogeochemical processes, freshwater defaunation may have major consequences if it continues unchecked.
6. More broadly, trait-based approaches show clear promise for advancing the predictive capacity of animal ecology. The growing availability of species trait databases for animal assemblages should be leveraged to advance the field.