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

Abstract

Management generally targets the most tractable life stage to rescue declining populations; however, that stage may not have the largest influence on recovery. Freshwater turtles are declining globally and early stages are frequently targeted for management, although the effectiveness of these actions on population growth are relatively unknown because of incomplete demographic data. We estimated the hatchling yearly survival rate for a freshwater turtle in the field using in situ enclosures to collect missing demographic information. We used these data to develop demographic models to calculate growth rate for a hypothetical, declining population of wood turtles (Glyptemys insculpta) in Wisconsin, USA, 2014–2019. We modeled growth for populations across a range of scenarios from no management to combinations of nest protection and head‐starting at varying levels of effort. Nest protection alone did not increase population growth rate, while head‐starting alone increased population growth by 0.07, with the largest increase in growth rate, 0.11, resulting from combinations of both approaches. No combination of nest protection and head‐starting, without an increase in adult survival rate from the observed 0.88 to ≥0.95, led to population stabilization or increase. Populations of freshwater turtles, like the wood turtle, will likely only recover with a multi‐faceted approach that targets multiple life stages simultaneously.

Headstarting turtles to larger body sizes for multiple years increases survivorship but with diminishing returns

“…We confirmed overwinter survival of 4 direct‐release hatchlings (0yHS 4/21, 19%); to our knowledge, this is the first published report of overwintering survivorship of artificially incubated hatchling wood turtles (but see Bougie et al 2022 who used overwintering enclosures). Additionally, we confirmed that one of these 0yHS hatchlings survived 2 winters after release.…”

“…Thus, our headstarted wood turtles have higher survivorship than non‐headstarted counterparts but lower survivorship than those reported for headstarted turtles of the same and closely related species. Our conservative estimate of hatchling survivorship (0yHS 2/30, 7%; Table 3) is lower than modeled hatchling survival values by Berkeland (2020; 10–14%) and known‐fate hatchling survivorship in overwintering enclosures by Bougie et al (2022; 23%). Our lower estimate may be an artifact of using recapture data or true overall lower survivorship at our study site.…”

Demography and Viability of a Central Maine Population of Wood Turtles (Glyptemys insculpta)

“…The differences in the estimated λ values from Program MARK for our adults and subadults (1.14, 95% CI = 1.05–1.24), along with our baseline PVA value for the entire population, indicate that this population is likely slowly declining and that, despite a stable and slowly growing adult population, this will remain the case without sufficient recruitment and higher juvenile survival. This conclusion is supported through recent modeling work by Bougie et al (2022). Those authors found that head-starting and nest protection alone did not lead to population stability and that improving adult survival was important as well.…”

“…Our results indicate that focusing on early life stages is important to improve both our model estimates and the trajectory of our population. It should be noted, however, that this management strategy should accompany those based on conventional wisdom that improving or maintaining high adult survival is essential to population stability (Mullin et al 2020; Bougie et al 2022). Our PVA demonstrated that this population is likely declining and, at its current trajectory, has a 100% chance of experiencing extirpation within the next 150 yrs due to a lack of recruitment into the population.…”

Bigger is better: age class‐specific survival rates in long‐lived turtles increase with size

“…Recent studies of freshwater turtles have concluded that caging nests to protect against predation, or head‐starting hatchlings to augment recruitment, may be valuable conservation tools for declining turtle populations (Schindler et al 2017, Spencer et al 2017, Carstairs et al 2019, Campbell et al 2020, Thompson et al 2020). Models for some species suggest that head‐starting to increase hatchling survival could be successful at increasing population size; however, unless survival rates of other life‐stages are also increased, the population would decline immediately after head‐starting initiatives cease (Spencer et al 2017, Bougie et al 2022). Head‐starting can be labor‐intensive, expensive, and may produce similarly low or even lower survival rates compared to juveniles produced naturally (Tetzlaff et al 2019, Golba et al 2022).…”