Discussion
In this study, we found that calf survival during both summer and winter
was influenced by several factors including the presence of predators,
landscape and climatic features, human activities such as hunting, and
behavioural traits such as the migratory strategy of the mother.
Specifically, we observed that summer calf survival decreased with
increasing bear density and varied significantly between years, giving
support to one of the two predation hypotheses and to theclimate hypothesis . During the summer season, brown bears
are one of the main predators of moose calves. Previous research has
shown that moose calves can constitute a significant portion, ranging
from 36% to 44% of the total energy content in the diet of brown bears
(Opseth, 1998). In south-central Sweden, studies estimated a predation
rate of 26% for moose calves by brown bears, with individual female
bears killing an average of 6.8 calves per year (Swenson et al., 2007).
The majority (92%) of brown bear predation on moose calves occurs when
the calves are less than one month old (Swenson et al., 2007; Rauset et
al., 2012). Our study revealed that the risk of calf mortality is more
than twice as high in areas with high bear density compared to areas
with low bear density. These findings align with a previous study that
found a negative association between bear density and calf/cow ratio
(i.e., the number of observed calves per female moose in autumn) in
Sweden (Tallian et al., 2021). The lower summer survival rates and
decreased autumn calf/cow ratios observed with increasing bear density
likely reflect the impact of bear predation on neonate moose.
While we do not have information on cause-specific mortality for each
moose calf included in the study, we were able to confirm at least two
documented bear predation events during the first study year: one in the
northernmost part of the study area, where a pair of twin calves were
killed by bears, and one in the easternmost part where the female moose
and her calf were observed being followed by a female bear and her two
yearling cubs, and the calf was not seen again the following day.
Overall, these findings highlight the significant influence of bear
predation on the survival of moose calves during the summer season.
In ungulates, the survival of adult females is a crucial driver of
population growth, but calf survival can account for up to 75% of the
variation in population growth rates (Gaillard et al., 2000, Raithel et
al., 2007). Previous research from Sweden asserted that bear predation
was not an influential driver of moose population dynamics (Haglund
1974) but that study was made during a time with a much lower brown bear
population size and did not have access to the modern tools of
GPS-technology for field checks. Instead, our study supports the notion
that bear predation on moose neonates is likely an influential factor
driving population dynamics of moose (Swenson et al. 2007, Rauset et al.
2012, Tallian et al. 2021). This is likely to become even more relevant
in the future as the brown bear population has been expanding in their
southern range (Kindberg et al. 2011; Kindberg 2010) and now overlaps
with the core areas of the wolf population.
In Scandinavia, wolves are important predators of new-born moose calves,
which make up approximately 90% of wolf kills during the summer season
(Sand et al., 2008, Tallian et al., 2017). Surprisingly, we did not
detect a relationship between wolf presence and summer calf survival.
The lack of such an association might be related to the spatial location
of the collared female moose and wolf space use and behaviour during
denning time. During the summer season, breeding wolves exhibit more
restricted space use compared to winter, focusing their activities
around the den area where they care for their pups (Fritts and Mech,
1981; Jędrzejewski et al., 2001; Walton et al., 2001; Zimmermann et al.,
2019). During the denning period, Scandinavian female and male wolves
had a mean attendance at the den site of 68% and 70%, respectively
(Alfréeden, 2006). Consequently, the likelihood of a moose calf being
killed by wolves may vary within wolf territories, with higher predation
risk closer to the den site compared to farther away.
In our study, we characterized wolf presence based on whether a moose
home range was inside or outside a wolf home range. By using this broad
classification, we might have overlooked the spatial variation in
predation risk within wolf home ranges. This could have resulted in a
failure to detect a correlation between wolf presence and calf survival,
which may have been influenced by a bias introduced by the sampling of
collared females. In other words, our data might have included an
overrepresentation of collared female moose that utilized areas with low
wolf activity during the summer season.
We offer several potential ad hoc explanations for the lack of
support for our forage opportunity hypotheses (habitat
productivity and proportion of young forests) in our analysis of summer
survival. Previous studies have indicated that during the summer season,
the composition of moose home ranges tends to resemble the overall
landscape, suggesting that moose utilize a greater variety of habitats
compared to winter (Hjeljord et al., 1990; Nikula et al., 2004). This
may indicate that although clearcuts and young forests are important
habitats for moose, the more homogenous distribution of high-quality,
nutritional plant forage during summer allows moose to find optimal
feeding patches in different habitats, also explaining why productivity
was unrelated to calf mortality. Moreover, Nicholson et al. (2014) found
that during summer, females with calves had lower selection strength for
young forests and tended to select older forests, which may also explain
why we did not find a correlation between calf mortality and the
proportion of young forests.
The observed annual variation in summer survival of neonates is likely
an effect of differing climatic conditions. Neonate growth and survival
can be negatively affected by hot and dry summers due to nutritional
constraints (Cook et al. 2004), but also by the environmental conditions
experienced by their mothers in the preceding summer and during
pregnancy in the preceding winter and spring (Forchhammer et al. 2001;
Eacker et al. 2016; Bastille‐Rousseau et al. 2016).
During autumn-winter, calf survival exhibited a negative association
with increasing snow depth in the presence of wolves
(predation*snow hypothesis ) and an increasing proportion of young
forests in the mother’s home range (habitat hypothesis ). In
addition, calf survival was higher for stationary moose calves compared
to migratory calves, contrary to our initial prediction (migration
hypothesis ). Deep snow can hinder the movement of both wolves and
moose, but since the former have a lighter foot loading, they can often
travel on top of the snow crust (Peterson 1974). Hence, deeper snow
generally increases the vulnerability of moose to predation and
therefore increases the hunting success of wolves (Kolenosky 1972;
Peterson and Allen 1974; Haber 1977) and kill rate (Nelson and Mech
1986; Huggard 1993; Post et al. 1999). Contrary to expected, we found
that deeper snow was positively correlated with calf survival the
absence of wolves. Although surprising, this unexpected result may be
confounded by spatial variations in snow depth within our study area: in
the northernmost part, where wolves are absent, snow depth averaged 45
cm, whereas average snow depth in the central and southern parts (where
wolves are present) was only 22cm. Thus, the absence of wolves and the
generally higher snow depths in the northernmost region might be
contributing to the observed positive relationship between calf survival
and snow depth.
As food resources become scarcer during winter, moose commonly aggregate
in young forest plantations to feed (Gundersen 2003), with Scots pine
being the quantitatively most important food source (Månsson, 2007). In
our study, we observed a negative relationship between calf survival and
the proportion of young forests during autumn-winter. This finding
supports the notion that clearcuts and young forests represent riskier
habitats for moose, both in terms of predation by wolves (Gervasi et
al., 2013; Ausilio et al., 2022) and hunting by humans (Ausilio et al.,
2022).
Game harvesting has become the leading cause of mortality in many
ungulate species (Allendorf et al. 2008; Darimont et al. 2015), with
moose calves accounting for about 40% of total harvest of moose in
Scandinavia (www.algdata.se; www.ssb.no). We did not find a relationship
between harvest density and moose calf survival, but we found a
significant negative correlation between hunting risk and autumn-winter
survival: calves exposed to high hunting risk had twice the risk of
mortality compared to calves exposed to low hunting risk. The lack of a
relationship between harvest density and survival is most likely a
result of the spatial scale at which both harvest statistics and harvest
density were calculated, which is the moose management unit (MMU). Moose
management units are larger (average ± SE = 1830 ± 140
km2,Wikenros et al. (2020)) than an average moose home
range (68 ± 7 km2), which means that one MMU can
include several hundreds of moose home ranges. Moose management units
are in turn made up by many hunting teams. Unfortunately, harvest
density at the hunting team level was not available, so we decided to
average harvest density at the MMU-level, which may be a too coarse
spatial scale to identify a significant correlation between harvest
density and calf survival. Hunting risk was, on the other hand,
estimated at a much finer spatial scale (hunting team level, Ausilio et
al 2022) and was negatively correlated to calf survival, providing
support for the hunting part of our predation-hunting hypothesis .
In our study area, the moose population is partially migratory, with
some individuals moving between high-altitude summer ranges and
low-altitude winter ranges. Migration is usually the product of
balancing the costs and benefits of remaining in a range all year-round
or moving to new areas. We found that the migratory strategy of females
was unrelated to summer calf survival, while during autumn-winter,
migratory females were associated with a lower probability of calf
survival compared to stationary ones. This outcome was opposite to ourmigration hypothesis . A possible explanation is that migratory
moose in our study area moved from summer ranges, that were often
located in Norway where the hunting season stopped already in December,
to winter ranges in Sweden where the hunting season continued to
February. It is also possible that migratory moose were forced to select
sub-optimal habitats that increased their exposure to hunting and wolf
predation compared to stationary ones, which can select optimal habitats
based on the perceived risk of hunting and foraging needs.
Both the wolf and brown bear populations have increased during the last
decades and expanded their geographical distribution in Scandinavia
(Liberg et al., 2012; Wabakken et al., 2011; Kindberg et al., 2011), and
in many areas these two predators have overlapping ranges (Kindberg et
al. 2011). A future scenario likely to arise is therefore increased
predation pressure on moose, which will intensify the competition with
human hunters for this shared prey (Jonzén et al. 2013). In areas where
wolves and bears coexist with human hunters, the additive effects of two
large carnivores and human harvest will most likely result in greatly
reduced calf survival, which may influence population dynamics. In order
to ensure a sustainable harvest of the moose population, calf survival
is a crucial demographic parameter to take into consideration. The
coexistence of wolves, brown bears, and human hunters presents
challenges that can lead to reduced calf survival, ultimately
influencing population dynamics. To ensure a balanced and sustainable
harvest of the moose population, comprehensive management strategies
need to account for the intricate interplay between predation, hunting,
habitat conditions, and climatic factors.