Introduction
Survival is a fundamental parameter influencing the demography and
population dynamics of ungulates (Gaillard et al., 1998). Variations in
survival rates can lead to fluctuations in population density (Loison
and Langvatn, 1998; Gaillard et al., 1998). Typically, ungulates exhibit
a hump-shaped pattern of age-specific survival, with the lowest and most
variable rates observed among juveniles, high and stable rates in prime
adulthood, and declining rates associated with senescence (Caughley,
1966; Gaillard et al., 1998, 2000; Loison et al., 1999). In the absence
of human hunting, adult survival rates remain relatively constant from
year to year, while juvenile survival rates show high variability
(Gaillard et al., 1998). Thus, the temporal variation and
unpredictability associated with juvenile survival make it a critical
demographic trait affecting population dynamics and trajectories
(Houston, 1982; Gaillard et al., 1997, 2000; Raithel et al., 2007).
Juvenile survival in ungulates is influenced by multiple factors,
including predation by predators (Linnell et al., 1995; Swenson et al.,
2007), exposure to pathogens (Grobler, 1981), access to food resources
(Eberhardt, 2002; Scornavacca et al., 2016), and climate conditions
(Singer et al., 1997; Ericsson et al., 2002; Ciach and Pęksa, 2019).
Predation often represents the primary source of mortality for neonate
ungulates during summer (Linnell et al., 1995; Swenson et al., 2007).
The availability of food resources and the nutritional status of
individuals strongly influence vital rates in ungulates, and these
factors are mostly determined by density-dependent effects associated
with the environment (Pettorelli et al., 2005; Toïgo et al., 2006). In
boreal systems, ungulates frequently utilize clearcuts and young forests
early in the growing season due to higher forage density and the
presence of high-quality plants (Hjeljord et al., 1990; Boyce et al.,
2003; Månsson, 2009; Hebblewhite et al., 2008). Plant productivity has
been shown to positively affect the survival of both adult and juvenile
ungulates (Sims, 2017; Hurley et al., 2017).
Winter severity represents an important factor impacting the survival of
juvenile ungulates (Loison and Langvatn, 1998; Garrott et al., 2003).
Snow conditions and cold temperatures restrict movement and increase
energy expenditure, potentially making ungulates more vulnerable to
predation (Smith et al., 2004; Hebblewhite, 2005; Garrott et al., 2008).
In ungulate species subjected to human hunting, hunters often constitute
the most significant source of mortality for both adults and juveniles
(Festa‐Bianchet et al., 2003; Apollonio et al., 2010). Hunting can have
substantial impacts on demography in harvested populations (Langvatn and
Loison, 1999; Ginsberg and Milner‐Gulland, 1994). The effects of large
carnivores and hunting on ungulate population dynamics depend on whether
predation is additive or compensatory, the specific sex and age classes
targeted by these mortality sources, and the cumulative impact of
predation-induced mortality in relation to harvest (Bischof et al.,
2008). However, the effects of many of these factors remain largely
unknown. Therefore, despite the challenges associated with their
assessment (Caughley, 1977; Eberhardt, 1985; Lebreton et al., 1993),
survival rates are crucial parameters for understanding the mechanisms
affecting population dynamics and for effectively managing harvested
populations (Caughley, 1966; Raithel et al., 2007). Thus, investigating
changes in survival rates of juvenile ungulates and identifying the
factors influencing them is vital for population management, including
the estimation of sustainable harvest rates (Porath, 1980; Rohm et al.,
2007).
Moose (Alces alces ) holds significant economic and recreational
value across Scandinavia (Storaas et al., 2001; Lavsund et al., 2003;
Boman et al., 2011). Over the past two decades, calf/cow ratios and
moose densities have generally declined in both areas with and without
large carnivores in Sweden (Wikenros et al., 2020; Tallian et al.,
2021). In areas with large carnivores, moose face multiple mortality
sources that vary in importance over space and time. Brown bear
(Ursus arctos ) predation is often the primary cause of mortality
for neonatal moose in early summer (Swenson et al., 2007). As summer and
autumn progress, bear predation declines in significance, while hunting
becomes the primary mortality factor for both adult and juvenile moose
during early and late autumn (Cederlund and Sand, 1991; Lavsund et al.,
2003). Wolves (Canis lupus ) also pose a significant mortality
risk to moose throughout the year, particularly to calves (Sand et al.,
2005; Sand et al., 2008). As expected, winter mortality of moose calves
in Norway was higher in wolf territories compared to areas lacking
wolves (Saether et al., 1996; Sivertsen et al., 2012). The spatial and
temporal variations in the presence of hunters and large carnivores
underscore the importance of investigating the impact of human-related,
abiotic, and biotic factors on moose calf mortality patterns. Given the
declining moose densities (Tallian et al., 2021), understanding the
factors that influence moose demography, such as calf survival, becomes
increasingly important as carnivore populations expand and in the
context of climate change, which may lead to increased thermoregulation
costs (Murray et al., 2006; Lenarz et al., 2009). This knowledge is
essential for providing management strategies that can mitigate the
effects of large carnivores and hunting mortality.
In this study, we used position data from 39 GPS-collared female moose
in south-central Scandinavia to investigate seasonal calf survival
during summer and autumn/winter. We examined the survival probability of
calves in relation to the presence of wolves, wolf predation risk, bear
density, human harvest and associated hunting risk, habitat
productivity, clearcuts and young forests (hereafter referred to as
young forests) and snow depth. During summer, we hypothesized that calf
survival would be higher in areas with greater productivity and a higher
proportion of young forests (forage opportunity hypotheses ),
while it would be lower in the presence of wolves and with increasing
bear density (predation hypotheses ). During autumn-winter, we
predicted that calf survival would decrease with increasing harvest and
hunting risk and would be lower in the presence of wolves and with
higher wolf predation risk (predation–hunting hypotheses ). We
also hypothesized that calf survival would be lower in areas with
greater snow depth in the presence of wolves compared to areas without
wolves (predation*snow hypothesis ). Additionally, we predicted
calf survival to be lower in areas with a higher proportion of young
forests, which are known to be risky habitats for moose during winter
(Gervasi et al., 2013; Ausilio et al., 2023) (habitat
hypothesis ).
Recent studies have shown that migration can influence neonatal ungulate
mortality by reducing the risk of predation for females and their calves
(White et al., 2014; Berg et al., 2019). Migratory behaviour allows
individuals to better utilize delayed green-up at higher altitudes and
experience less intraspecific competition by leaving winter
concentration areas (Van Moorter et al., 2021). Thus, we tested the
hypothesis that calves with migrating mothers would have higher survival
rates compared to those with stationary mothers, during both summer and
winter (migration hypothesis ). Lastly, we predicted that calf
survival would vary across years due to annual climatic variations that
may affect moose forage availability and thermoregulation (climate
hypothesis ).
By investigating these factors and their influence on moose calf
survival, we aim to contribute to an increased understanding of the
underlying factors shaping calf survival, which ultimately affect moose
population demography and dynamics.