Do beaver ponds alter mayfly fitness?

With increasing beaver populations worldwide it is important to understand the effects of beaver dams on stream ecosystems. We studied 10 montane beaver ponds of varying size and shape in western Colorado to evaluate their impacts on stream water temperatures and the size and timing of emergence of the common mayfly Baetis bicaudatus.  Downstream water temperatures varied predictably with pond morphology, being colder downstream of high dams and warmer downstream of low dams.  Additionally, pond morphology significantly predicted variation in Baetis size, with mayflies emerging at larger sizes downstream of high dams and smaller downstream of low dams.  However, there was no relationship between pond morphology and variation in timing of emergence between up and downstream reaches.  Furthermore, degree days during the emergence period did not definitively link water temperature to variation in mayfly size.  Our data have interesting implications for the effects of beaver ponds on Baetis fitness at the individual and population levels.  Because large Baetis females are more fecund, predictable size variation associated with beaver pond morphology makes it possible to predict where female Baetis will have greater individual fitness.  Furthermore, since trout prefer larger mayflies, catchments with ponds promoting larger mayfly sizes might be better fishing holes.