ENGINEERS OF OPPORTUNITY: BEAVER ABILITY TO EXPLOIT HEADWATER STREAMS ON LANDSLIDES IN STEEP TERRAIN

During our previous work in the southeastern San Juan Mountains, we noted that headwater streams on landslides were typically characterized by the presence of beaver dams whereas other headwater tributaries typically lacked beaver dams. This trend led us to hypothesize that landslide topography, including hummocky topography, low stream gradient, and internally drained basins, would attract beavers because landslides provide favorable terrain for dam emplacement. More specifically, we hypothesized that 1) beaver would prefer to build beaver dams in headwater streams on the hummocky topography of landslides and 2) headwater streams on landslides would have significantly lower gradients than headwater streams away from landslides. In order to test our hypothesis, we visited 10 landslide sites within the field area and collected data including location, elevation, and evidence of beaver activity. In addition, we visited 15 adjacent headwater streams that showed no evidence of previous landslide activity. We then compared the landslide and non-landslide headwater streams using the G-test, a statistical test that assesses whether or not variables are independent of one another. We were able to reject the null hypothesis with a significant confidence interval (P=.003). Thus, there is very likely a relationship between beaver presence and landslides. We infer that this relationship is due to decreased stream gradient on landslides, but we need to continue our work to show if the relationship is statistically relevant. If a relationship exists, not only can we further our understanding of landslide topography and its effect on wildlife, but we can further specify a topographic type that is preferential for the establishment of beaver colonies.