Introduction
Diets of black bears (Ursus americanus Pallas, 1780; hereafter bear) vary geographically depending on foods available to bears (Pelton 2003). Knowledge of foods utilized by local bear populations provides insight into condition, reproductive rates, and habitat selection of bears, aiding effective management of these populations. One of the most commonly used techniques for assessing food habits is scat analysis. Food items from scats are identified and related to diets through frequency, relative density, or volume (e.g., Raine and Kansas 1990; Hellgren 1993; Kasbohm et al. 1995). Correction factors have also been developed for common food items to account for differences in digestibilities (Hewitt and Robbins 1996), with the use of these corrected values allowing for more accurate identification of important foods.
Identification of foods consumed does not necessarily indicate which foods are most beneficial to bears, whereas knowledge of nutritional gains from foods provides insight into which supply the greatest gains of energy and other key nutrients (Gluesing and Field 1986). The majority of past work has attempted to delineate these relationships by relating nutritional components of selected foods to quantities consumed (e.g., Elowe and Dodge 1989; Kasbohm et al. 1995). However, nutritional composition of plant species can vary substantially from plant to plant and even within the same plant (Holechek et al. 2004). Alternatively, nutritional components of fecal material from species with ruminant and monogastric digestive systems have been used to assess dietary quality (Studier et al. 1994; Hodgman et al. 1996; Magomedov et al. 1996; Felicetti et al. 2000; Codron et al. 2006). Fecal indices assume that as dietary intake of measured nutrients increases, so do fecal levels of those nutrients (Codron et al. 2006). This is particularly true for monogastrics, such as bears, because of higher dietary requirements and a digestive system that can process only highly digestible nutrients such as proteins, lipids, and simple sugars (Pritchard and Robbins 1990; Robbins 1993), which are highly digestible (Brody and Pelton 1988; Pritchard and Robbins 1990). For example, fecal nitrogen (FN) levels have been shown to vary directly with dietary nitrogen intake in several free-ranging omnivores as diverse as bats (Studier et al. 1994), baboons (Codron et al. 2006), and bears (Brody and Pelton 1988). Although factors other than dietary intake may affect FN levels, such as precipitation of proteins by tannins (Robbins 1993) that increase FN content, most simple monogastrics avoid such forages and presence of these forages can be identified in diets to determine any potential confounding effects. Other key nutrients for bears, such as lipids and energy, are less studied, but limited data indicate that fecal levels positively correlate with intake levels, although (like FN) fecal concentrations are lower owing to high digestibilities (Brody and Pelton 1988).
Thus, even though food sources have a variety of digestibilities with respect to nutritional components (Pritchard and Robbins 1990), estimates derived from consumed foods (i.e., scats and stomach content) likely represent as a minimum relative levels of nutritional components, and thereby reflect quality of diets consumed (Brody and Pelton 1988; Magomedov et al. 1996; Clark et al. 2003; Codron et al. 2006). Although limited evidence suggests that nutrient assimilation rates can vary seasonally in bears (Brody and Pelton 1988), this may simply reflect differences in nutrient availability (Pritchard and Robbins 1990), and regardless, differences are slight (Brody and Pelton 1988). Knowing which dietary items provide greatest nutrient gains to bears is important because consumption of energy-rich foods is critical for bears given their need to accrue large stores of fat for hibernation (Pelton 2003). High-energy foods include a variety of soft-mast-producing plants, as well as fat and proteinrich food sources such as hard mast, insects, and animal matter (Swenson et al. 1999; Rode and Robbins 2000; Inman and Pelton 2002), and high-protein diets can also result in increased mass gains and fecundity in bears (Tate and Pelton 1983; McLean and Pelton 1990; Beckmann and Berger 2003a).
Studies of food habits of bears from 1984 to 1991 in Rocky Mountain National Park (RMNP), Colorado, indicated greater use of animal matter compared with other localities (L.C. Zeigenfuss, United State Geological Survey, 2001 unpublished report), although direct comparisons with other studies was difficult because food habits were not assessed volumetrically, thus precluding the application of correction factors to more accurately represent foods consumed. RMNP's bear population is unique in that it is one of the highest elevation populations of bears in the United States, and such high elevations result in a substantially shorter growing season and a lack of hard-mast crops utilized by bears in other localities. These factors were believed to be the cause of the small size of bears (adult male = 74 kg, adult female = 60 kg; Baldwin 2008) reported in RMNP historically (1984-1991). However, recent observations (2003-2006, hereafter contemporary) indicated increased size (adult male = 99 kg, adult female = 68 kg; Baldwin 2008) and condition (mean body fat females--historic = 15%, contemporary = 24%; Baldwin and Bender 2009) over historic data, a result most parsimoniously related to changes in level of nutrition.
We hypothesized that contemporary diets of black bears would exhibit greater use of animal foods than in most other bear populations, that use of animal foods would be highest in summer and autumn, and that these animal foods would be significant sources of protein, fat, and energy. We predicted that contemporary diets would differ from historic diets given the increase in size and nutritional condition of bears over the last 15-20 years. Furthermore, given this increase in condition in bears, we predicted greater use of anthropogenic foods for the contemporary population given the high nutrient content of these foods (Stringham 1989). If correct, these predictions would underscore the importance of animal matter in diets of black bears in RMNP, and would indicate a need to modify current management of black bears in RMNP to reduce potential human-bear conflicts that are likely to arise when bears heavily utilize anthropogenic foods (Beckmann and Berger 2003b).
Study area
RMNP is a 1080 [km.sup.2] biosphere reserve located in the Rocky Mountain Front Range of north-central Colorado. Topography in RMNP was shaped by glaciations, and consists of high mountainous peaks interspersed with small sub-alpine meadows, lakes, streams, glaciers, and tundra at higher elevations. Elevations range from 2400 to 4345 m. The continental divide bisects RMNP, creating different climatic patterns and vegetation types to the east and west. Eastern RMNP is drier, with precipitation averaging 35.1 cm in the town of Estes Park, while western RMNP is more mesic, with precipitation averaging 50.8 cm in the town of Grand Lake. Seventy-five percent of precipitation falls from April to September. In Estes Park, mean daily high temperatures range from 7.2[degrees]C in February to 27.8[degrees]C in July, while in Grand Lake, mean daily high temperatures range from 0.0[degrees]C in December and January to 23.9[degrees]C in July.
Vegetation in RMNP consisted of >700 plant species. Community composition varied with more productive communities found on western slopes and at higher elevations (Beidleman et al. 2000). Montane forests and valleys west of the continental divide were comprised primarily of lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Wats.) and aspen (Populus tremuloides Michx.) interspersed with bunchgrass and sedge-dominated herbaceous meadows. Montane forests on the eastern slope included the same species, although drier sites were often dominated by ponderosa pine (Pinus ponderosa P. & C. Lawson) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco). Subalpine habitats varied less between western and eastern slopes and were dominated by Engelmann spruce (Picea engelmannii Parry ex Engelm.) and subalpine fir (Abies lasiocarpa (Hook.) …
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