Estimates of Abundance and Predation - The Population Ecology of Beaver in Isle Royale National Park

Romanski, Mark

Chapter 1: An important method for estimating the abundance of wildlife is the double-count method, which involves comparing numbers of organisms seen by two independent observers. We used the double-count method to estimate sightability and number of North American beaver (Castor canadensis) colonies in Isle Royale National Park (Lake Superior, North America). In 1990 and 2002, we used the method with two observers flying in a DeHavilland Beaver. In 2006, 2007, and 2008, we used two observers in separate, smaller aircraft that flew transects and circles at lower altitudes. Unexpectedly, sightability was lower from the smaller aircraft. Specifically, sightability had been 0.84 (95% CI = [0.78, 0.90]) for the smaller aircraft and 0.96 (95% CI = [0.92, 0.99]) for larger aircraft. However, other evidence suggests sightability had been grossly overestimated for the larger aircraft. Sightability when flying higher and straighter, required in the larger aircraft, may have been ~0.50. Overestimation arose from violating an assumption of the double-count method – the assumption that all lodges are equally sightable. Disturbingly, that violation became apparent only after surveys from the smaller aircraft. The results are an object lesson where, the double-count method gave reason to think observers had a high rate of detecting colonies, when in fact it had been much lower.Chapter 2: Predation, a top-down process, is often implicated as a strong control in ecosystem function and trophic cascades. Conversely, the influences of forest succession, or more generally, “bottom-up” processes, are thought to be the primary regulatory agent in beaver (Castor sp.) population dynamics. We took advantage of spatial-temporal variability in North American beaver (Castor canadensis) colony abundance on Isle Royale National Park (Lake Superior, North America) to explore the influences of top-down, bottom-up and biotic processes on the population ecology of beaver. Beaver colony abundance for each of three, distinct, forested regions on Isle Royale is well correlated. However, the forest succession trajectories in these regions are not synchronized. Consequently, on Isle Royale, beaver population dynamics are not entirely explained by forest succession. We conducted aerial surveys to estimate active beaver colony abundance, and census the wolf (Canis lupis) and moose (Alces alces) populations from 1962-2009. Additionally, we used occurrence of beaver in wolf feces as a surrogate for predator diet to estimate annual predation rates, the proportion of the annual beaver population consumed by wolves. Beaver colony abundance importantly explained the proportion of wolf diet that was beaver, which averaged ~14% for the period. Mean predation rate was ~0.16 and was significantly explained by wolf abundance. Climate data, mean annual precipitation and mean annual stream flow as surrogates for water availability representing abiotic processes, did not influence beaver population dynamics for the period. Interestingly, beaver population dynamics were an important, albeit short-lived, bottom-up influence for wolf population dynamics. Interactions between wolves and its focal prey, moose, and alternate prey, beaver, may represent apparent competition that could have long-term, negative impacts for beaver population dynamics. In a larger context, beaver populations are importantly influenced both from the top-down and the bottom-up and these influences can be more aptly interpreted when applied to an appropriate temporal scale.