Bears as Predators

Brown bears are omnivores and opportunistic, rather than obligatory, predators. They are powerful and can deliver a swift, lethal blow to smaller prey such as newborn calves but capturing and killing larger animals such as adult ungulates is not an easy task even for an adult bear. Thus, when bears hunt larger prey, they generally target weaker or more vulnerable individuals, for example, moose or other ungulates that are in poor condition following a long winter or females that are giving birth (1). Even still, our research suggests that bears only have a 9% success rate when hunting adult moose, meaning only 9% of attempted hunts end successfully for the bear (1). Interestingly, hunting success appears to be greater on the fringe of the core bear distribution as compared to the middle (1). Yet, we still know relatively little about how bears actually hunt in Scandinavia and or what their relative success is with their primary ungulate prey, neonate moose and reindeer. Even with small, vulnerable prey, successful hunts are not guaranteed. Understanding bear predatory behavior is important because those behaviors are the underlying mechanisms that drive bear predation patterns and their subsequent effect on prey population dynamics.

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Predation on moose

 

Bears in Scandinavia primarily prey on newborn moose and only occasionally kill adults; in general, yearling moose are more vulnerable to bear predation than older moose, and cows are more vulnerable than bulls (2-4). Three studies have been conducted to assess bear predation on moose in Scandinavia.

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An initial study was conducted between 1994 and 1998 in our Southern Study Area which followed radio-collared moose to assess cause-specific mortality for both calves and adults. At that time, brown bear density was estimated to be between 10 and 30 individuals per 1,000 km2 and moose density was estimated at between 400 and 1,340 per 1,000 km2. This study estimated that each individual bear (all older than 2 years old) takes about 6.5 moose calves per year, or approximately 26% of all moose calves born in the area (2). Females that lost their calves in spring produced more calves the following year (1.54 calves/female) than females that kept their calves (1.11 calves/female), which reduced the net loss of calves due to predation to about 22% (2). Of the moose calves killed, 93% were taken during their first four weeks of life. Bears killed 0.8% of radio-collared adult female moose per year, and no adult male moose, which represented 0.5-1.5% of the moose population (4).

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A second study was conducted between 2004 and 2006 in our Southern Study Area which followed radio-collared adult female bears (> 4 years old) to assess bear-moose kill rates. Bear densities were likely relatively similar to the previous study, while local winter moose densities were estimated at 500 individuals per 1000 km2, which is up to half of what was reported in the 1994-1998 study (3). This study estimated an average individual kill rate of 7.6 moose calves per bear per year, with a range between 6.1 to 9.4 calves per bear (3), which was more or less comparable to the first study. However, it is important to note that the 2004-2006 study only estimated the kill rate of adult female bears, and 67% of them (12 of 18) were solitary and had no cubs during the study year, the remaining 4 had yearlings. Thus, the estimate likely does not represent kill rates across the bear population as a whole.

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The most recent study was conducted in our Southern Study Area between 2014 and 2015 and followed a wider range of bear demographic classes, i.e., single adult females and males and both sexes of subadults, but still no females with cubs (5). Bear density in the area was likely similar or higher than the previous studies, while moose density was likely lower and at this point a couple of wolf packs had moved into the area. This study estimated that bears killed 4.25 moose calves per season but found quite a bit of variation in kill rates between individuals. For example, at the extremes, some bears killed up to 10 calves during the parturition period while others killed none (5). While adult single female and male bears were equally predatory, adults of both sexes killed more calves than younger, subadult bears. The lower average kill rate observed here could reflect shifting animal densities (i.e., more bears and fewer moose would decrease average kill rates). However, caution should be used as the lower average kill rate could also possibly be caused by the coarse methodologies used in the study, i.e., cluster searches were defined as ≥ 2 hourly GPS positions within 30 m of one another (5). Current research suggests that this GPS collar fix rate would likely result in missed predation events and underestimate bear kill rates (6).

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It should be noted that the estimates and results outlined should be extrapolated to present times with caution. Two of the above studies are over 20 years old, and the system and dynamics have changed (moose and bear densities have fluctuated and wolves have since recolonized in some of the bear range). The most recent estimates of kill rates were estimated using coarse metrics. A new study that began in 2023 in the Ljusdal and Härjedalen Municipalities (Gävleborg and Jämtland Counties, respectively) is attempting to quantify bear kill rates, and assess the combined impact of wolves and bears, in the context of the current multipredator system.

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Predation on reindeer

 

Our first study assessing bear predation on reindeer was conducted between 2010 and 2012 in our Northern Study Area in close collaboration with the two Sámi Siida of Udtja and Gällivare (7). These Sámi Siida are situated in the boreal forest, a habitat where Sámi reindeer herders reported high calf losses. This study placed GPS proximity collars on bears and UHF transmitters on reindeer to assess predation. The results suggest that, on average, bears visiting calving grounds killed 0.4 calves per day and 10.2 (with a range between 8.6 and 11.5) calves per bear, which accounted for between 39-62% of all reindeer calf mortality in these Siidas (7). The kill rates varied considerably among individual bears, primarily due to the duration of their presence on the calving grounds. The number of days spent in these areas emerged as the key factor influencing predation outcomes. Bears also killed some adult reindeer, although kill rates were low. Although females with cubs of the year do prey on moose calves, collared females with cubs of the year were not observed killing reindeer calves in this study.

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Sámi reindeer herders in mountain habitats have also reported high calf loss to bears, which resulted in a subsequent request to assess bear-reindeer predation in Sámi Siida that are situated in the mountains. Our group is currently conducting a study in collaboration with the Idre Sámi Siida in central Sweden (a temporary extension of our Southern Study Area) to assess bear-reindeer predation patterns including hunting behavior, kill rates, and predation rates. This project began in 2019 and the final field season is scheduled for 2024. Bear predation on semi-domestic reindeer is a key source of human-wildlife conflict in northern Sweden (7, 8), but less so in Norway where fewer bears inhabit reindeer herding areas (9).

 

Predation on sheep

 

Studies conducted in the late 1980s and early 1990’s suggests that bears in Norway prey heavily on free-ranging domestic sheep where they overlap (10, 11) and rely on them for a large proportion of their annual energy intake (12). It is well understood that sheep and lamb losses and compensations are both positively associated with bear density (13, 14). However, bear-sheep predation dynamics such as kill rates between bears and sheep in Norway have not been more recently assessed.

 

Bears as scavengers

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It should be noted that bears are also opportunistic scavengers who find meat from a wide range of sources including hunter-killed ungulate remains, carcasses from ungulates who died during winter, road-killed ungulates, and also by stealing kills from other carnivores, including other bears. There is some evidence that bears in Scandinavia may have a dominance hierarchy at carcasses, similar to other systems. For example, females with cubs of the year are much less likely to scavenge on hunter-killed carrion, likely to avoid negative interactions with other bears (15). Bears used >50% of wolf kills in our studies in central Sweden, whereas we did not record any wolf visit at bear kills (16).

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Bears and prey population dynamics

 

Compensatory versus additive predation explained: To understand prey population dynamics, it is first important to understand the difference between compensatory and additive predation. In compensatory predation, predation on one species tends to offset, or compensate, for other mortality factors affecting the prey population. Essentially, when predation is compensatory, it helps regulate the prey population by removing individuals that might have died from other causes like disease, starvation, or old age. This means that the total mortality rate of the prey population remains relatively stable, and predation serves to maintain the prey population at a sustainable level. On the other hand, additive predation occurs when the predation on a prey population is in addition to other sources of mortality. In this case, predation does not offset or compensate for other factors, rather it adds to the overall mortality of the prey population. This can result in a more significant reduction in the prey population, potentially leading to population declines or local extinctions if predation rates are high and not balanced by factors like prey reproduction.

 

The effect of bears on moose population dynamics:  Bears appear to generate only very minor fluctuations in moose population dynamics, while hunter harvest has an overwhelming effect on moose demography (17). Thus, although bear predation is known to be additive to other sources of predation with ungulates (18), humans likely usurp this effect, taking over as the primary driver of moose population dynamics in Scandinavia (17). Our research corroborates that while bears likely affect some moose population vital rates, like newborn survival, they are likely not the primary driver of long-term moose population dynamics in Scandinavia (19).

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First, bears do not typically kill adult moose and are therefore not an important source of adult mortality within the moose population (4). However, bears can affect calf survival and recruitment, which is an important driver of population growth (2, 3). For example, one recent study which used data from 2000 to 2017 suggests that moose calf/cow ratios, which are a loose index of calf recruitment, are negatively correlated with bear densities; calf/cow ratios decreased by 7% in northern Sweden where bears are their only main predator and by 18% in central Sweden where both bears and wolves prey on moose calves (19). Yet, long-term trends suggest that both calf/cow ratios and moose densities broadly declined over the last ~20 years across Sweden, including areas where wolves and bears were absent, and that these declines may have been driven by a variety of human-related factors (19). Furthermore, moose have an indirect compensatory mechanism to bear predation; moose that lose calves have a higher likelihood of having twins the following year, likely because of the energy conserved by not lactating the previous year (2).

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However, these results should be interpreted with caution as we do not truly have accurate estimates of the combined effect of predation (bears and wolves) and hunting on moose population dynamics. A new study that began in 2023 in the Ljusdal and Härjedalen Municipalities (Gävleborg and Jämtland Counties, respectively), where locals report high wolf and bear densities and low moose calf survival, attempts to estimate the kill rates of brown bears on moose calves across all sex and reproductive classes of bears. Importantly, local wolf kill rates and hunter harvest rates will also be assessed which will help provide a clearer picture of the role bears play in moose population dynamics. This study is further detailed in Future research for the SBBRP.