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.