Stream Fragmentation Thresholds for a Reproductive Guild of Endemic Great Plains Fishes

We present a mechanistic pathway by which human alteration of streams has caused the decline of a unique reproductive guild of Great Plains stream-dwelling fishes.  Stream fragmentation related to impoundments, diversion dams, and stream dewatering has effectively created a mosaic of large river fragments throughout the Great Plains of central North America.  We analyzed longitudinal fragment lengths (measured in river km) and literature accounts of population status for eight species among 60 fragments to derive species-specific thresholds in stream length associated with population persistence.  Fragment lengths were a strong predictor of population status among species and differed (F_2,21 = 30.14, P < 0.01) according to population status for extirpated (mean = 136 ± 21 km), declining (226 ± 69), and stable (458 ± 137) populations.  Fragment length explained 71% of reported extirpations and estimated minimum thresholds in fragment length explained 67% of variation in population persistence among eight species.  Literature accounts suggest this pattern exemplifies declines among at least 14 North American species that occur throughout a third of the contiguous United States and have shown declines over the past 60 years.  By this account stream fragmentation presents a substantial challenge to the conservation of biodiversity in North America.  Conservation management insights inferred from our findings suggest measures that restore connectivity within fragmented river systems will likely benefit imperiled pelagic-spawning fishes.