Blanding's Turtle Demography and Population Viability

Abstract

In anticipation of US federal status classification (warranted, warranted but precluded, not warranted), scheduled for 2023, we provide population viability analysis of the Blanding’s turtle Emydoidea blandingii , a long-lived, late-maturing, semi-aquatic species of conservation concern throughout its range. We present demographic data from long-term study of a population in northeastern Illinois and use these data as the basis for viability and sensitivity analyses focused on parameter uncertainty and geographic parameter variation. We use population viability analysis to identify population sizes necessary to provide population resiliency to stochastic disturbance events and catastrophes and demonstrate how alternative definitions of ‘foreseeable future’ might affect status decisions. Demographic parameters within our focal population resulted in optimistic population projections (probability of extinction = 0% over 100 years) but results were less optimistic when catastrophes or uncertainty in parameter estimates were incorporated (probability of extinction = 3% and 16%, respectively). Uncertainty in estimates of age-specific mortality had the biggest impact on population viability analysis outcomes but uncertainty in other parameters (age of first reproduction, environmental variation in age-specific mortality, % females reproducing, clutch size) also contributed. Blanding’s turtle demography varies geographically and incorporating this variation resulted in both mortality- and fecundity-related parameters affecting population viability analysis outcomes. Possibly, compensatory variation among demographic parameters allows for persistence across a wide range of parameter values. We found that extinction risk decreased and retention of genetic diversity increased rapidly with increasing initial population size. In the absence of catastrophes, demographic conservation goals could be met with a smaller initial population size than could genetic conservation goals; ≥20-50 adults were necessary for extinction risk <5% whereas ≥50-110 adults were necessary to retain >95% of existing genetic diversity over 100 yrs. These thresholds shifted upward when catastrophes were included; ≥50-200 adults were necessary for extinction risk <5% and ≥110 to more than 200 adults were necessary to retain >95% of existing genetic diversity over 100 yrs. Impediments to Blanding’s turtle conservation include an incomplete understanding of geographic covariation among demographic parameters, the large amount of effort necessary to estimate and monitor abundance, and uncertainty regarding the impacts of increasingly frequent extreme weather events.

Spatial and thermal ecology of juvenile head‐started Blanding's turtles Emydoidea blandingii

“…temperature and precipitation) for the active season in addition to the brumation period. Lastly, while population augmentation may continue to be a factor in the recovery of Blanding's turtles and similar species, managers still need to account for predator presence/abundance in release site areas, as consistent (Urbanek et al 2016) or punctuated (Gasbarrini et al 2021) predation events may be a hindrance to long‐term population viability (King et al 2021, Golba et al 2022). Overall, we think that the site of release was adequate for necessary resources for juveniles to survive during the active season and brumation period.…”

“…Similar to many other long‐lived vertebrates, Blanding's turtles display delayed sexual maturation, slow growth, low hatchling/juvenile survivorship and slow reproductive rates in natural populations (Congdon and Van Loben Sels 1993, Congdon et al 1993, 2001). This suite of life‐history characteristics, their need for a large amount of heterogenous land, the extensive anthropogenic development of prairie‐wetland ecosystems, and increased populations of predators (Gehrt et al 2010) explains the current conservation status and long‐term vulnerability of Blanding's turtles (King et al 2021). There have been increased efforts to mitigate these population declines through head‐starting reintroduction programs, in which eggs collected during oviposition are incubated, and hatchlings are reared ex situ for one to two years before being released into their maternal populations as juveniles (Buhlmann et al 2015, Thompson et al 2020).…”

Juvenile proportion as a predictor of freshwater turtle population change

“…If sufficient data on demographic variables such as fecundity, maturation age, and survivorship at different life‐history stages are available, modelling methods such as matrix population models and population viability analysis can be used to forecast future demographic trends (e.g., Feng et al, 2019; Folt et al, 2016; Shoemaker et al, 2013), predict impacts of future threats (e.g., Fernández‐Chácon et al, 2011; Levy et al, 2021; Manzo et al, 2021), or project outcomes of alternative management strategies (e.g., Crawford et al, 2018; Knoerr et al, 2022; Reed et al, 2009). However, the performance of population models for turtles and tortoises may be constrained by factors such as the use of survivorship estimates from different species or locations or a flawed determination of initial population size (King et al, 2021). And for many of the world's species and populations, appropriate demographic data are entirely lacking (e.g., Martins & Souza, 2009; Price et al, 2021; Seateun et al, 2019).…”

“…However, the reliability with which the proportion of juveniles foreshadows future demographic change is uncertain. For example, a scarcity of juveniles may simply reflect low detectability or intermittency of recruitment (Congdon et al, 1993; King et al, 2021).…”

Longevity Estimates of Ornate Box Turtles (Terrapene ornata) in Iowa

“…We checked buckets daily from mid-May to early July, at which time we removed the fences and buckets. Turtle-finding dogs were also employed for 4 consecutive days at both Greencastle and Mallard Pools in 2019, 2021, and 2022(Boers et al 2017; D.F.H. resumed visual-encounter surveys throughout the area in 2020-2022.…”