Introduction
Understanding the variability and elasticity of vital rate parameters is important to better understand population dynamics for ungulate species (Tuljapurkar & Caswell, 1997). Although adult female survival generally has the greatest effect on population growth rates for ungulate populations, adult survival tends to be stable; conversely, offspring survival has less impact on population growth but is more variable (Gaillard, Festa-Bianchet & Yoccoz, 1998; Gaillard et al., 2000; Raithel, Kauffman & Pletscher, 2007; Chitwood et al., 2015). Manipulating offspring survival to achieve various management and conservation goals is therefore a viable method to increasing population size (Crouse, Crowder & Caswell, 1987; Coluccy et al., 2008; Johnson et al., 2010). However, better understanding what specific variables affect offspring survival will likely increase effectiveness of management and conservation efforts.
Intrinsic variables such as birth mass (Cook et al., 2004; Lomas & Bender, 2007; Shuman et al., 2017) and capture age (Grovenburg et al., 2014) as well as extrinsic variables including weather (Ginnett & Young, 2000; Warbington et al., 2017; Michel et al., 2018) and landscape composition and configuration (Gulsby et al., 2017; Gingery et al., 2018; Michel et al., 2018) affect ungulate offspring survival. Other factors such as maternal body condition may also affect offspring survival as ungulate mothers in better body condition are more likely to produce larger, healthier offspring with greater chances of survival than those in poor body condition (Carstensen et al., 2009; Duquette et al., 2015; Shallow et al., 2015). Additionally, predation is generally the largest natural cause of offspring mortality for several ungulate species (white-tailed deer, Odocoileus virginianus , Grovenburg et al., 2011; Chitwood et al., 2015; elk, Cervus canadensis , Griffin et al., 2011; Brodie et al., 2013; moose, Alces americanus , Keech et al., 2011, Severud et al., 2019; pronghorn, Antilocarpa americana , Jacques et al., 2007, 2015); however, the number of predators an ungulate population is exposed to does not necessarily equate to increased mortality (Kautz et al., 2019). Consequently, factors affecting offspring survival can be area specific (Grovenburg et al., 2011); therefore, understanding which factors most influence offspring survival for a given ungulate population is warranted.
Although there are several ecological variables that affect ungulate offspring survival, field methodology can also affect derived survival estimates for a population. Derived survival estimates tend to be greater for opportunistically captured neonates compared to those captured via vaginal implant transmitters (VITs) due to increased left truncation in the opportunistically captured datasets (black-tailed deer, O. hemionus sitkensis , Gilbert et al., 2014; white-tailed deer, Chitwood et al., 2017; Dion et al., 2020). This variation can affect management and conservation efforts when survival estimates are needed to model population growth rates and abundance as these metrics are often used to determine the number of individuals that can be sustainably harvested from a population. Model selection and interpretation of the effects of ecological covariates on ungulate neonate survival can also vary by capture method (Gilbert et al., 2014). Therefore, assessing how capture method affects both derived survival estimates and interpretation of the relationship between ecological covariates and survival will further understanding of the population dynamics for a given species within an ecosystem.
Our objective was to assess variation in survival estimates and subsequently assess potential variation in model selection and ecological covariate interpretation related to capture method for white-tailed deer neonates captured from three study areas in North Dakota and South Dakota, USA. We also assessed how intrinsic (capture age, birth mass, sex) and extrinsic (percent canopy cover, precipitation, distance to road, distance to water) ecological covariates affected neonate survival through 3-months (neonates) and 6-months (juveniles) of age. We predicted neonates captured via VITs would display decreased survival compared to opportunistically captured neonates (Gilbert et al., 2014; Chitwood et al., 2017; Dion et al., 2020). We also predicted survival would increase with increased birth mass (Cook et al., 2004; Lomas & Bender, 2007; Shuman et al., 2017), age (Nelson & Woolf, 1987; Rohm, Nielsen & Wolf, 2007; Grovenburg et al., 2011), canopy cover (Rohm, Nielsen & Wolf, 2007; Sternhagen, 2015), and increased distance from the nearest road (Rost & Bailey, 1979; Stankowich, 2008). We predicted that survival would decrease with increased precipitation (Warbington et al., 2017, Dion et al., 2020) and increased distance from water (Adams & Hayes, 2008; Long et al., 2009; Ditchkoff, 2011). Finally, we predicted survival would be lower for females compared to males (Shuman et al., 2017).