Native freshwater mussels are a persistently overlooked component of freshwater systems in North America despite the valuable ecosystem services they provide. This trend is especially prevalent in the Pacific Northwest, where species distribution maps are still being established and baseline population data is scarce to non-existent in most watersheds.
Preliminary inquiries into native mussel population dynamics have found some beds lacking juveniles, indicative of impending extinction debt phenomenon. In the summer of 2018, | resurveyed 8 sites in the South Umpqua Basin, Oregon, where western pearlshell (Margaritifera falcata) mussel presence was documented within the past 30 years.
Western pearlshell were present at each site, but the density and spatial variability of animals differed dramatically - from as few as 4 animals in a 0.85 km stretch of river to a mussel bed with greater than 85,000 animals in a 950 m? area. In total, 2 western pearlshell beds and 1 floater mussel bed (Anodonta sp.) were sampled to determine whether reproduction was occurring and to document baseline population structures.
These beds were located in either the upper S. Umpqua River near Tiller, OR, or in Cow Creek near Riddle, OR. No western pearlshell beds meeting the definition of this project were found in the lower S. Umpqua River sites; native mussels at these sites were few and far between. Instead, high densities of invasive Asian clams (Corbicula fluminea) were omnipresent and likely outcompeting native mussels. The relationship between invasive and native bivalves merits further research in Pacific Northwest streams, especially as warming waters expand the fundamental niche of Asian clams into historically cold-water streams and rivers.