Jennifer K. Fortin

Trophic cascades from wolves to grizzly bears in Yellowstone

We explored multiple linkages among grey wolves (Canis lupus), elk (Cervus elaphus), berry-producing shrubs and grizzly bears (Ursus arctos) in Yellowstone National Park. We hypothesized competition between elk and grizzly bears whereby, in the absence of wolves, increases in elk numbers would increase browsing on berry-producing shrubs and decrease fruit availability to grizzly bears. After wolves were reintroduced and with a reduced elk population, we hypothesized there would be an increase in the establishment of berry-producing shrubs, such as serviceberry (Amelanchier alnifolia), which is a major berry-producing plant. We also hypothesized that the percentage fruit in the grizzly bear diet would be greater after than before wolf reintroduction. We compared the frequency of fruit in grizzly bear scats to elk densities prior to wolf reintroduction during a time of increasing elk densities (1968-1987). For a period after wolf reintroduction, we calculated the percentage fruit in grizzly bear scat by month based on scats collected in 2007-2009 (n = 778 scats) and compared these results to scat data collected before wolf reintroduction. Additionally, we developed an age structure for serviceberry showing the origination year of stems in a northern range study area. We found that over a 19-year period, the percentage frequency of fruit in the grizzly diet (6231 scats) was inversely correlated (P < 0·001) with elk population size. The average percentage fruit in grizzly bear scats was higher after wolf reintroduction in July (0·3% vs. 5·9%) and August (7·8% vs. 14·6%) than before. All measured serviceberry stems accessible to ungulates originated since wolf reintroduction, while protected serviceberry growing in a nearby ungulate exclosure originated both before and after wolf reintroduction. Moreover, in recent years, browsing of serviceberry outside of the exclosure decreased while their heights increased. Overall, these results are consistent with a trophic cascade involving increased predation by wolves and other large carnivores on elk, a reduced and redistributed elk population, decreased herbivory and increased production of plant-based foods that may aid threatened grizzly bears.

Maternal condition determines birth date and growth of newborn bear cubs

Abstract The number, size, and survival of bear cubs emerging from winter dens depend on maternal condition prior to entering the den. We hypothesized that delayed implantation provides flexibility in timing of birth such that pregnant females are able to track environmental or body conditions long after conception to optimize reproductive output in a changing environment. We tested the hypotheses that causative links between maternal condition and size of newly emerging brown bear (Ursus arctos) cubs were females in superior condition give birth earlier and, thereby, lactate longer in the den than females in poorer condition; and females in superior condition produce more milk or higher quality milk, which accelerates cub growth relative to females in poorer condition. No brown bear with a body fat content ≤ 20% produced cubs even though breeding occurred. Brown bears that were fat gave birth earlier than those that were lean. Cubs nursing from fat mothers grew faster than those nursing from lean mothers. The combination of an earlier birth date and faster growth by cubs produced from fat mothers increased mass of brown bear and polar bear (U. maritimus) twins at den emergence by 330–360 g for each unit increase in percent maternal body fat content when entering hibernation.

Grizzly bear predation links the loss of native trout to the demography of migratory elk in Yellowstone

The loss of aquatic subsidies such as spawning salmonids is known to threaten a number of terrestrial predators, but the effects on alternative prey species are poorly understood. At the heart of the Greater Yellowstone ecosystem, an invasion of lake trout has driven a dramatic decline of native cutthroat trout that migrate up the shallow tributaries of Yellowstone Lake to spawn each spring. We explore whether this decline has amplified the effect of a generalist consumer, the grizzly bear, on populations of migratory elk that summer inside Yellowstone National Park (YNP). Recent studies of bear diets and elk populations indicate that the decline in cutthroat trout has contributed to increased predation by grizzly bears on the calves of migratory elk. Additionally, a demographic model that incorporates the increase in predation suggests that the magnitude of this diet shift has been sufficient to reduce elk calf recruitment (4–16%) and population growth (2–11%). The disruption of this aquatic–terrestrial linkage could permanently alter native species interactions in YNP. Although many recent ecological changes in YNP have been attributed to the recovery of large carnivores—particularly wolves—our work highlights a growing role of human impacts on the foraging behaviour of grizzly bears.

Dietary and spatial overlap between sympatric ursids relative to salmon use

Abstract We hypothesized that there would be minimal dietary overlap between sympatric brown bears (Ursus arctos) and American black bears (U. americanus) relative to salmon (Oncorhynchus spp.) utilization when alternative foods (e.g., fruits) are abundant. To maximize the chance that we would reject this hypothesis, we examined the diets of brown and black bears known to have visited salmon streams. Species, sex, and individual identification of bears visiting salmon streams were determined by DNA analysis of hair and feces collected in 2002–2004 along those streams. Diets were estimated from fecal residues and stable isotope analyses of hair. Assimilated diets of brown bears were 66.0% (SD  =  16.7%) salmon, 13.9% (SD  =  7.5%) terrestrial animal matter, and 20.1% (SD  =  17.2%) plant matter. Assimilated diets of black bears were 8.0% (SD  =  5.4%) salmon, 8.4% (SD  =  9.7%) terrestrial animal matter, and 83.6% (SD  =  7.7%) plant matter. Male and female brown bears did not differ in either the proportion of dietary salmon, terrestrial animal matter, or plant matter. The relative amounts of fruit residues in the feces of brown bears (87.0%, SD  =  15.2%) and black bears (91.8%, SD  =  7.2%) did not differ. Both sexes of brown bears visited salmon streams and consumed significant amounts of salmon, but only male American black bears visited streams and then consumed minimal amounts of salmon. Thus, brown bears were largely carnivorous and black bears were largely herbivorous and frugivorous. This reduced dietary overlap relative to salmon and fruit use is understandable in light of the concentrated, defendable nature of salmon in small streams, the widely dispersed, non-defendable nature of abundant fruits, the dominance of brown over black bears, the higher energy requirement of the larger brown bear, and, therefore, the differing ability of the species to efficiently exploit different food resources.

Nutritional consequences of experimentally introduced tourism in brown bears.

Abstract Although numerous studies have documented behavioral effects of nature-based tourism on wildlife populations, few studies have determined whether behavioral changes translate to effects on individual condition and population health. This issue is currently a concern for wildlife managers in Alaska, USA, and Canada where bear viewing is a rapidly growing industry expanding into previously undisturbed bear habitats. Rather than record observations at long established tourism sites, we experimentally introduced bear viewing into 2 relatively undisturbed brown bear (Ursus arctos) populations in south-central Alaska. We examined the nutritional consequences of behavioral changes induced by the presence and activity of bear viewers for bears feeding on early summer vegetation and late-summer salmon (Oncorhynchus kisutch and O. nerka). We used Global Positioning System collars, monitored food resource availability, and quantified individual resource use and condition for a year prior to and during the introduction of bear viewing. Though bear viewing altered spatiotemporal resource use in all treatments, total resource use declined only when we exposed bears to 24-hour daily human activity. Energy expenditure, indexed as daily travel distances, was significantly higher when bears responded by altering spatial rather than temporal resource use. However, body weight and composition were unaffected by all treatments as bears shifted their foraging to other locations or times. Managers can minimize nutritional impacts of bear-viewing programs by avoiding spatial displacement and providing predictable time periods when bears can access food resources free of human activity. Bears in this study exhibited a high degree of behavioral plasticity, which may be an important factor in identifying flagship species for sustainable ecotourism programs.

Impacts of Human Recreation on Brown Bears (Ursus arctos): A Review and New Management Tool

Increased popularity of recreational activities in natural areas has led to the need to better understand their impacts on wildlife. The majority of research conducted to date has focused on behavioral effects from individual recreations, thus there is a limited understanding of the potential for population-level or cumulative effects. Brown bears (Ursus arctos) are the focus of a growing wildlife viewing industry and are found in habitats frequented by recreationists. Managers face difficult decisions in balancing recreational opportunities with habitat protection for wildlife. Here, we integrate results from empirical studies with expert knowledge to better understand the potential population-level effects of recreational activities on brown bears. We conducted a literature review and Delphi survey of brown bear experts to better understand the frequencies and types of recreations occurring in bear habitats and their potential effects, and to identify management solutions and research needs. We then developed a Bayesian network model that allows managers to estimate the potential effects of recreational management decisions in bear habitats. A higher proportion of individual brown bears in coastal habitats were exposed to recreation, including photography and bear-viewing than bears in interior habitats where camping and hiking were more common. Our results suggest that the primary mechanism by which recreation may impact brown bears is through temporal and spatial displacement with associated increases in energetic costs and declines in nutritional intake. Killings in defense of life and property were found to be minimally associated with recreation in Alaska, but are important considerations in population management. Regulating recreation to occur predictably in space and time and limiting recreation in habitats with concentrated food resources reduces impacts on food intake and may thereby, reduce impacts on reproduction and survival. Our results suggest that decisions managers make about regulating recreational activities in time and space have important consequences for bear populations. The Bayesian network model developed here provides a new tool for managers to balance demands of multiple recreational activities while supporting healthy bear populations.

Dietary breadth of grizzly bears in the Greater Yellowstone Ecosystem

Abstract Grizzly bears (Ursus arctos) in the Greater Yellowstone Ecosystem (GYE) are opportunistic omnivores that eat a great diversity of plant and animal species. Changes in climate may affect regional vegetation, hydrology, insects, and fire regimes, likely influencing the abundance, range, and elevational distribution of the plants and animals consumed by GYE grizzly bears. Determining the dietary breadth of grizzly bears is important to document future changes in food resources and how those changes may affect the nutritional ecology of grizzlies. However, no synthesis exists of all foods consumed by grizzly bears in the GYE. We conducted a review of available literature and compiled a list of species consumed by grizzly bears in the GYE. We documented ≥266 species within 200 genera from 4 kingdoms, including 175 plant, 37 invertebrate, 34 mammal, 7 fungi, 7 bird, 4 fish, 1 amphibian, and 1 algae species as well as 1 soil type consumed by grizzly bears. The average energy values of the ungulates (6.8 kcal/g), trout (Oncorhynchus spp., 6.1 kcal/g), and small mammals (4.5 kcal/g) eaten by grizzlies were higher than those of the plants (3.0 kcal/g) and invertebrates (2.7 kcal/g) they consumed. The most frequently detected diet items were graminoids, ants (Formicidae), whitebark pine seeds (Pinus albicaulis), clover (Trifolium spp.), and dandelion (Taraxacum spp.). The most consistently used foods on a temporal basis were graminoids, ants, whitebark pine seeds, clover, elk (Cervus elaphus), thistle (Cirsium spp.), and horsetail (Equisetum spp.). Historically, garbage was a significant diet item for grizzlies until refuse dumps were closed. Use of forbs increased after garbage was no longer readily available. The list of foods we compiled will help managers of grizzly bears and their habitat document future changes in grizzly bear food habits and how bears respond to changing food resources.

Assessing Nutritional Parameters of Brown Bear Diets among Ecosystems Gives Insight into Differences among Populations

Food habit studies are among the first steps used to understand wildlife-habitat relationships. However, these studies are in themselves insufficient to understand differences in population productivity and life histories, because they do not provide a direct measure of the energetic value or nutritional composition of the complete diet. Here, we developed a dynamic model integrating food habits and nutritional information to assess nutritional parameters of brown bear (Ursus arctos) diets among three interior ecosystems of North America. Specifically, we estimate the average amount of digestible energy and protein (per kilogram fresh diet) content in the diet and across the active season by bears living in western Alberta, the Flathead River (FR) drainage of southeast British Columbia, and the Greater Yellowstone Ecosystem (GYE). As well, we estimate the proportion of energy and protein in the diet contributed by different food items, thereby highlighting important food resources in each ecosystem. Bear diets in Alberta had the lowest levels of digestible protein and energy through all seasons, which might help explain the low reproductive rates of this population. The FR diet had protein levels similar to the recent male diet in the GYE during spring, but energy levels were lower during late summer and fall. Historic and recent diets in GYE had the most energy and protein, which is consistent with their larger body sizes and higher population productivity. However, a recent decrease in consumption of trout (Oncorhynchus clarki), whitebark pine nuts (Pinus albicaulis), and ungulates, particularly elk (Cervus elaphus), in GYE bears has decreased the energy and protein content of their diet. The patterns observed suggest that bear body size and population densities are influenced by seasonal availability of protein an energy, likely due in part to nutritional influences on mass gain and reproductive success.

Temporal niche switching by grizzly bears but not American black bears in Yellowstone National Park

Abstract Grizzly bears (Ursus arctos) have been reported as either nocturnal or diurnal in various studies, but have not been known to switch between the 2 times unless disturbed by humans. Black bears (Ursus americanus) are almost solely diurnal in studies unless human influences occur. Because human disturbance is often difficult to control, the relative temporal niche of both species remains ill-defined. Thus, the present study examined bears in Yellowstone National Park (Wyoming) where hunting does not occur, human activities are relatively benign, and bear species are sympatric to determine if niche occupancy was a stable feature of the species. Onset of activity was anticipatory of both sunrise or morning civil twilight (illumination sufficient for human vision) for individuals of either species. The peak hour of activity in black bears was consistently midday, but fluctuated in grizzly bears from midday during early spring, late summer, and fall to evening during late spring and early summer. Black bears did not temporally avoid the times when the more dominant grizzly bears were active. Mean activity levels were higher for male black bears than for both male and female grizzly bears. Together, results suggest that the foraging needs of black bears necessitate ingestion of less-digestible, lower-quality foods requiring longer foraging time during daytime hours, whereas grizzly bears adapt their diet to seasonally available food sources, necessitating greater temporal flexibility.

Wolves trigger a trophic cascade to berries as alternative food for grizzly bears

This is a Forum article in response to: Barber-Meyer, S. (2015) Trophic cascades from wolves to grizzly bears or changing abundance of bears and alternate foods? Journal of Animal Ecology, 83, doi: 10.1111/1365-2656.12338. We used multiple data sets and study areas as well as several lines of evidence to investigate potential trophic linkages in Yellowstone National Park. Our results suggest that a trophic cascade from wolves to elk to berry production to berry consumption by grizzly bears may now be underway in the Park.