Relationship of declining mussel biodiversity to stream-reach and watershed characteristics in an agricultural landscape

Freshwater mussels are among the most rapidly declining components of global biodiversity, but causes of local species disappearances are frequently unknown. We estimated decade-scale local extinction by resampling 118 stream reaches representing the best mussel habitat across a region that was once rich in species and is now mostly converted from prairies and riparian woodlands to intensive agriculture (Iowa, USA). Average species richness was reduced from >5 to <2 species, maximum richness was reduced from 22 to 15 species, and all mussel species were extirpated from 47% of the reaches since 1984 to 1985. More than half of the sites lost >75% of their species. Although 5 of the species were found at 20% to 140% more sites in 1998 than 1984 to 1985, 29 species (83%) decreased an average of 80% in geographic coverage, whereas 8 species were completely lost from these stream sites. Correlation analyses with reach and watershed characteristics determined using GIS and local sampling methods linked the greatest declines to rarity of streamside woodlands, high siltation, and most intensive agricultural land uses, i.e., where conditions had changed most from the historical land cover. The surveys indicated a very large extinction debt has been created by large-scale habitat modification over the last century and ongoing agricultural land uses.