Extinction Risk of Western North American Freshwater Mussels: Anodonta Nuttalliana, the Anodonta Oregonensis/Kennerlyi Clade, Gonidea Angulata, and Margaritifera Falcata

Abstract

The recent declines in eastern North American species of freshwater mussels have been well documented, but the status of western species has been comparatively understudied. However, various local and regional studies and anecdotal observations indicate that western mussels are also declining, suggesting the need for range-wide assessments of extinction risk and changes in freshwater mussel distributions. Using historic (pre-1990) and recent (1990-2015) occurrence data from across western states and incorporating observations of recent population dynamics, we assessed the extinction risk of western freshwater mussels according to the categories and criteria of the International Union for Conservation of Nature (IUCN) Red List. Percent change in occupied watersheds (by area) between historic and recent time periods was evaluated against IUCN-established thresholds. Additionally, we considered whether evidence of declines was also supported by reported observations of changes in abundance or occurrence in studied water bodies, watersheds, or regions. We also assessed the proportion of watersheds that have reduced species richness as compared with historic levels. We evaluated four western freshwater mussel taxonomic entities: three currently recognized species and one clade consisting of two currently recognized species. Of the four entities assessed, two are Vulnerable (Anodonta nuttalliana and Gonidea angulata), one is Near Threatened (Margaritifera falcata), and one is Least Concern (Anodonta oregonensis/kennerlyi clade). Freshwater mussel richness declined 35% across western watersheds by area, and among the most historically diverse watersheds, nearly half now support fewer species/clades. Future research and conservation efforts should prioritize identifying the proximate causes for these declines and preserving existing habitat and populations.

Reassessing Enigmatic Mussel Declines in the United States

“…Approximate Onset of Decline Source assessment of Pacific coast river systems showed mussel declines in some areas (Blevins et al 2017), but the characteristics of these declines, and the extent to which they are enigmatic or attributable to specific factors, remain unclear.…”

Freshwater mussels in Mediterranean‐climate regions: Species richness, conservation status, threats, and Conservation Actions Needed

“…Despite the greater overall richness in the MB, most individual river basins in that region contain between one and four species only (Lopes‐Lima et al, 2017b; Gomes‐dos‐Santos et al, 2019). Range declines in CCali have resulted in little overlap in the distribution of most species, such that there is often only a single species per basin where mussels still occur (Howard et al, 2015; Blevins et al, 2017). Similarly, only two species commonly occur in SA.…”

“…Based on Williams et al (2017), both Anodonta species are recognized in this review, but they are discussed together owing to the difficulty of disentangling their individual distributions. Although there are historical records of additional Anodonta species in the region ( Anodonta oregonensis and Anodonta kennerlyi ), recent studies suggest that these are erroneous (Mock et al, 2010; Blevins et al, 2017). Museum records and published literature show that mussels once occurred throughout California, in many cases being highly abundant (Howard et al, 2015).…”

“…Museum records and published literature show that mussels once occurred throughout California, in many cases being highly abundant (Howard et al, 2015). Recent surveys have found that they have declined substantially, and appear to have been extirpated in much of CCali (Howard et al, 2015; Blevins et al, 2017).…”

“…Based on morphology, the two species co‐occur; however, individuals of the two morphotypes from the same river are genetically more similar to one another than to individuals with similar morphology from other rivers (Mock et al, 2010), highlighting the difficulty in assigning species‐level classification to mussels using morphology alone. Anodonta nutalliana (including the synonymous A. californiensis ) appears to have been extirpated from most of southern California (Howard et al, 2015; Blevins et al, 2017).…”