Hatch Success and Recruitment Patterns of the Bog Turtle

Abstract

Researchers suggest that several bog turtle (Glyptemys muhlenbergii) populations in North Carolina, USA, are in decline and have few remaining individuals and low annual survival probability. Most populations are dominated by older adults with few juveniles encountered; however, the proportion of juveniles encountered in 2 populations is higher. It is unknown why the juvenile:adult ratio varies among populations. We conducted a nest monitoring study in 2016 and 2017 to test the hypothesis that sites with fewer juvenile encounters would be where nest predation was highest. We documented the fate of 272 eggs from 83 nests encountered across 7 sites in North Carolina. On average 28% of eggs hatched across all sites over both years, but we observed large variation in hatch success among sites. Predation by mesopredators and small mammals was the primary cause of nest failure. The probability of nest predation decreased with greater emergent vegetation density and increased with greater distance to the edge of the wetland. Cooler temperatures, which prolonged incubation and thus increased predation risk, may also hinder recruitment at higher elevation sites. Our observations are consistent with the hypothesis that nest predation would be highest at sites with fewer juvenile encounters. Managers concerned about low bog turtle recruitment rates should consider the role of nest predation and the potential benefits of management that increases hatch rates. © 2020 The Wildlife Society.

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

“…Of the five bog turtle populations modeled, we found only two that exhibited stability or growth when incorporating site‐specific vital rates (Sites A and B). As evidenced by capture data, recruitment has been continuous for at least a decade at sites A and B, and successful recruitment likely results from lower nest predation, moderate incubation temperatures (Knoerr et al ., 2021), and high juvenile survival (Tutterow et al ., 2017). Our results for Sites A and B demonstrate how population stability can be maintained even when adult survival declines (Fig.…”

“…Data collected in 1996–2017 indicated that juveniles were highly represented in both populations (observed juvenile fractions > 0.4) and juvenile survival (0.81 and 0.77, respectively) was an average of 38% higher in these populations than in Sites D and E (values derived from Tutterow et al ., 2017 and more recently collected data; see ‘Materials and methods’ section). Sites A and B experienced successful recruitment in each of the last 10 years, determined by counting growth rings over the 2016–2017 field seasons, and egg survival rates were high in 2016 and 2017 (62 of 120 eggs survived to yield a 0.52 pooled average egg hatch rate; Knoerr, Graeter & Barrett (2021)).…”

“…Similar to Daigle & Jutras (2005), we believe that the lower proportion of juveniles encountered likely reflects less recruitment in these populations. Juvenile survival is estimated to be 0.49 at site D and is not empirically available for Site H. Egg survival was <0.01 (1 of 106 eggs hatched) at Site D in 2016–2017 and 0.15 (4 of 26 eggs) at Site H in 2017, with losses primarily due to predation by skunks at Site D and small mammals at Site H (Knoerr et al ., 2021).…”

“…Four of the five sites from this study were monitored for egg survival. Eighty‐three bog turtle nests were found in 2016–2017 through radio‐telemetry, thread spooling, and nest searching (Knoerr et al ., 2021). The number of eggs in each nest was recorded at first discovery, generally (>80%) within 12 h of nesting, providing an estimation of clutch size.…”

“…We generated site-specific clutch sizes (2.96-3.77 eggs) for sites where we detected at least eight nests. Clutch Animal Conservation 25 (2022) 53-64 ª 2021 The Zoological Society of London frequency (0.85) was derived by palpating all captured adult female turtles in June 2016 and 2017 in the four intensively studied populations to determine if females were gravid (Knoerr et al, 2021). Because encounter data are slightly female-biased (Tutterow et al, 2017), we estimated annual adult survival for each NC bog turtle population individually and found no evidence of sex-specific differences in survival.…”