Beaver pond morphology as a tool for predicting changes downstream
Abstract
As natural discontinuities of stream networks, beaver ponds have the potential to influence the structure and function of stream ecosystems. After declining during the extensive fur trade in North America, beaver populations have recently grown in some areas, especially montane regions. While previous studies have documented a variety of effects of beaver ponds on stream tail waters, little is known about the mechanisms driving those effects and no consistent generalities have emerged. I studied 18 montane beaver ponds of varying size and shape in western Colorado near the Rocky Mountain Biological Laboratory. Our goal was to test whether variation in beaver pond morphology (pond size and shape) explains downstream changes in stream temperature, mayfly size and timing of emergence. Downstream water temperatures varied predictably with pond morphology, being colder downstream of high-head dams and warmer downstream of low-head dams. Pond morphology also significantly predicted variation in size of mature Baetis bicaudatus (the most abundant mayfly), with larger mayflies emerging downstream of high-head dams and smaller mayflies downstream of low-head dams. There was no relationship between pond morphology and variation in timing of emergence of Baetis between up- and downstream 2 reaches. Multiple linear regression showed that none of the potential mechanisms tested (predatory stonefly abundance, food resource availability and stream temperature) explained the relationship between pond morphology and mayfly size. Our results have interesting implications for the effects of beaver ponds on Baetis individual fitness, because large Baetis females are more fecund. Furthermore, predictable female size variation associated with beaver pond morphology makes it possible to model the effects of beaver activity on local contributions of Baetis to the regional pool of reproductive adults at the catchment scale.
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References (64)
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