Population models reveal the importance of early life‐stages for population stability of an imperiled turtle species

Abstract

Demographic models are useful for projecting population trends, identifying life stages, most important to population dynamics, and investigating the demographic effects of potential management scenarios. We incorporated site-specific population parameters into stage-based matrix models to estimate population growth and to assess potential management scenarios for five intensively sampled (>15 years) populations of federally threatened bog turtles (Glyptemys muhlenbergii) in North Carolina, USA. Only two of the five populations modeled were stable or growing under estimated vital rates. Long-term sampling of bog turtle populations in NC suggests the declining populations in this study share several demographic characteristics with other populations in the region. Elasticity analysis revealed small changes in adult survival rates have the largest effect on population growth rate. These models also highlight that increased survival of egg and juvenile stages can sometimes buffer higher adult mortality and emigration, and reduced survival at multiple life stages can induce population-level decline. Our results indicate that management scenarios targeting increased recruitment (especially a head-start scenario) provide increased population growth among all populations, and allowed two of three declining populations to reach stability under current estimated vital rates. Population growth rates will be higher when population augmentation coincides with habitat restoration efforts that increase survival and site fidelity at all life stages. These models emphasize the importance of considering site-specific dynamics when evaluating conservation interventions for an imperiled long-lived species.

Not all management is equal: a comparison of methods to increase wood turtle population viability

“…Increasing nest survival was not enough to reverse or prevent population decline for spotted turtles ( Clemmys guttata ; Enneson and Litzgus 2008), eastern long‐necked turtles ( Chelodina longicollis ; Spencer et al 2017), or wood turtles ( Glyptemys insculpta ; Mullin et al 2020). Conversely, head‐starting showed mixed results, with some populations exhibiting positive growth and others still declining (Enneson and Litzgus 2008, Mullin et al 2020, Knoerr et al 2021). While both nest protection and head‐starting are commonly used management actions, the overall effect of combining nest protection and head‐starting efforts on population growth is rarely quantified.…”

“…Our objectives were to determine if the theoretical population was declining, identify which life stages were most responsible for population decline, and finally evaluate the effectiveness of management actions on population growth rate. Based on studies of other populations of freshwater turtles, we hypothesized that management actions such as nest protection, head‐starting, and those that target adult survivorship would positively affect growth rate of wood turtles in Wisconsin (Heppell et al 1996, Heppell and Crowder 1996, Enneson and Litzgus 2008, Spencer et al 2017, Knoerr et al 2021). Given that older stages are often the most important for population growth in freshwater turtles (Congdon 1993, Heppell et al 1996, Mullin et al 2020, Knoerr et al 2021), we predicted that models including only nest protection would have the smallest impact on population growth, followed by models with only head‐starting, models that included nest protection and head‐starting, and finally models that also increased adult survival in conjunction with the other 2 management actions.…”

Lost reproductive value reveals a high burden of juvenile road mortality in a long‐lived species

“…The estimated stable-stage distributions predicted high frequencies of early stages (eggs and small juveniles) relative to adults, which is typical of turtle population models (Enneson & Litzgus, 2008;Knoerr et al, 2021;Salice et al, 2014). The stage distribution of nonnesting snapping turtles observed on roads in APP differed from expected distributions under each of the three juvenile survival scenarios (χ 2 = 119.4, 224.3, 78.5, all p ≤ 0.001).…”

“…Some mitigation strategies, such as constructed nesting areas or time‐ and season‐specific interventions (Beaudry et al, 2010; Crawford et al, 2018), target specific demographic classes, which, in our study system, probably represented less than half of the road mortality burden. Even for long‐lived species dependent on high adult survival, interventions targeting juveniles may provide the greatest conservation benefit when adult‐focused interventions achieve diminishing returns (Bennett et al, 2017; Crawford et al, 2014; Cunnington & Brooks, 1996; Knoerr et al, 2021; Sergio et al, 2021). Similar to prioritizing areas with the most conservation value (Boyle et al, 2017; Laurance et al, 2014), life‐history stage‐specific mitigation planning informed by demographic vulnerabilities standardized by RV can help to optimize the value of conservation resources.…”

Noninvasive camera data and spatial capture–recapture models reveal strong temporal variation in fawn survival

“…For many long‐lived wildlife species, population growth rates are strongly influenced by variation in neonate and juvenile survival and recruitment (Ciuti et al, 2015; Knoerr et al, 2022; Robinson, Desimone, et al, 2014; Sæther et al, 2013). Estimating survival and recruitment of neonates often requires costly, labor‐intensive, and invasive techniques (Duquette et al, 2014; Mills et al, 2008).…”