Carcass monitoring and grizzly bear scavenging across two management jurisdictions of the northern Yellowstone winter range (1997-2012)
Regan, Brooke Sierra
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Spring ungulate carcasses are an important food source for grizzly bears (Ursus arctos) on the Northern Yellowstone Winter Range (NYWR). The objectives of this study were to: 1) provide a 1997 - 2012 update on spring carcass monitoring surveys across the NYWR, 2) compare grizzly bear use of carcasses in the spring between the Gallatin National Forest (GNF) and Yellowstone National Park (YNP), and 3) compare detection rates for two carcass survey techniques implemented on the GNF in 2006, 2008, and 2009. Carcasses occurred on the NYWR at low quantities (x= 31 carcasses per year), with the exception of 'pulse' events in 1997, 2006, 2008, and 2011 (x = 152 carcasses per year). On average, 76% of the carcasses on the NYWR were elk, and 57% were classified as adults. Wilcoxon rank sum tests indicated that both the proportion of carcasses used by grizzly bears and the number of carcasses used per kilometer of transect was less (P = 0.010 and P = 0.018, respectively) on the GNF than YNP in 'pulse years' and did not differ (P = 0.470 and P = 0.550) in years characterized by low carcass counts. Direct evidence of human activity was documented at 80% of all mature elk carcass sites on the GNF, and was estimated by YNP management to not exceed 1% of all carcass sites in YNP, although no data was collected. Density of roads was higher (P < 0.001) on the GNF than in YNP. I used a multiple logistic regression framework to assess the correlates of grizzly bear carcass use and found that the only significant parameter of ecological interest to predict grizzly bear use of carcasses was road density. The odds that grizzly bears scavenged on a given survey area in a given year decreased 83% for every 1 km/6.15 km 2 increase in road density. A Wilcoxon rank sum test of carcass detection rates for strategically and systematically placed transects revealed no differences or higher detection rate ranks for the less resource intensive strategic method. Managers of multi-use ungulate winter ranges may consider spring road closures that limit human activity, in order to enhance foraging opportunities for grizzly bears.