Traversing the eastern Georgian Bay rock barrens in search of nesting turtles: Does temperature and moisture have an affect on hatch success?

My first experience working with turtles was the summer of 2017, where I volunteered for Grundy Lake Provincial Park’s turtle monitoring program. I was immediately fascinated by how turtles were able to venture on land and choose a nest site amidst the busyness of a fully operating campground. After being fortunate enough to watching a turtle nest for the first time, I was hooked. I knew turtle work was for me! That fall, I approached Dr. Chantel Markle about volunteer opportunities after hearing her speak about her research as a guest lecturer in my second-year ecology class. Later, my volunteer position blossomed into a research assistant opportunity with the McMaster Ecohydrology Lab the summer of 2018. Starting that April, I embarked on my first field season, ready to spend the summer researching turtles and their habitats within the eastern Georgian Bay region.

Unfortunately, Ontario’s turtle populations are threatened by a variety of factors, including road mortality and habitat loss (Gibbons et al. 2000). Habitat changes, such as the loss of wetlands and uplands to make way for roads or infrastructure can be extremely detrimental to turtle populations because turtles rely on these habitats to persist (Gibbons et al. 2000). In the eastern Georgian Bay region, female turtles leave the wetland from late May to early July, and venture on land to find a spot to lay their eggs. Females are known to travel long distances, traversing habitats like forests and small temporary wetlands to reach nest sites (Ernst and Lovich 2009; Edge et al. 2010; Markle and Chow-Fraser 2014).

Natural nesting habitats in open rocky landscapes tend to be shallow soil deposits covered by lichen and mosses (Litzgus and Brooks 1998; 2000; Beaudry et al. 2010; Markle and Chow-Fraser 2014). If natural nesting habitat is limited, female turtles may turn to road shoulders or other anthropogenic areas to lay eggs, which not only puts turtles at an increased risk of road mortality, but also puts the eggs at high risk of being consumed by predators (Christens and Bider 1987; Temple 1987).

Female turtles must explore the open rocky outcrops in search of a nest site. However, this can be challenging because turtles must select a site with enough soil to contain their eggs. The selected nest site must also balance moisture and temperature conditions. The female’s decision on where to lay her eggs is extremely important because the nest site itself influences the likelihood that the eggs will hatch (Kolbe and Janzen 2002; Morjan 2003; Hughes and Brooks 2006; Mitchell and Janzen 2019). That said, the unique soil temperature and moisture conditions in turtle nesting habitat, and their effects on hatch success, are not well understood across the rocky landscape. This hinders our ability to restore and create nesting habitat that accurately resembles habitats naturally found in the rock barrens landscape.

With the goal of understanding natural turtle nesting habitat across the rock barrens, the McMaster Ecohydrology Lab monitored turtle nests, measured various nest site properties, and compared the nest sites selected by female to other non-nest sites nearby to develop the first holistic look at turtle nesting habitat in a rock-barrens landscape. We also conducted a landscape-scale survey of a larger area (660 ha) to estimate potential nesting habitat availability to turtles.

If you haven’t taken part in a turtle nesting survey, you may be wondering how we locate nesting turtles. First things first: bug gear. Coincidentally, turtle nesting season overlaps with peak bug season in the eastern Georgian Bay region. The “trifecta” of bugs as we call it (black flies, mosquitos, and deer flies) are out in full force; to avoid itching all over, it’s best to cover up!

Next, we quietly explore the area — if we are lucky, like finding a needle in a haystack, we catch sight of a turtle. We monitor the turtle from a distance, making sure not to disturb her. Once the female finishes nesting, we install a cage above the nest to protect it from predators like raccoons, skunks, and foxes. Protective cages are very important, because they can drastically reduce the rate of nest depredation, which often approaches 100% (Riley and Litzgus 2013). Protecting nests means more hatchlings will be recruited into the population.

We return to the nest site within 12 hours of the female laying her eggs, complete additional measurements and install monitoring equipment. This involves carefully measuring, weighing, and then returning the eggs back to their original position within the nest cavity, as well as completing a vegetation survey and installing temperature and moisture probes.

Over the two summers of fieldwork (2018 and 2019), we characterized nesting habitat by measuring soil temperature and moisture (every 15 minutes) and taking note of the types of vegetation and degree of shading across twelve turtle nests.

The shape and slope of rock under soil deposits (and turtle nests), known as bedrock morphology, is quite variable. Three morphologies can be found in our landscape: (1) crevice morphologies that have soil in bedrock cracks, (2) ledge sites that have soil up against the side of the bedrock, and (3) flat sites that have soil accumulated on gently sloping bedrock. Morphology is important, because direct contact of the shallow soil with the underlying granite is unique to a rock-barrens landscape, which seems to be important for maintaining incubation temperatures (Markle et al. 2021) and could be especially critical for hatching success near the species northern range limit.

We found that natural nesting sites for various turtle species across the eastern Georgian bay rock barrens have minimal shading by trees and other vegetation and soil depths that range from about 7.5–22 cm. Female turtles selected sites that allowed water to drain quickly and were predominantly covered by lichen instead of moss and grasses. In this relatively undisturbed landscape with minimal residential and industrial development, areas with deeper soils and an open canopy (i.e., the type of habitat that turtles need to nest), are limited to less than 3% of the study area!

Our research revealed that rock barrens nesting habitat has unique temperature and moisture dynamics that are closely intertwined with landscape properties, including bedrock morphology and soil type. Yet, disturbances such as the development of roads, infrastructure, and the impacts of climate change, continue to impact and threaten limited nesting habitat (Gibbons 2000; Markle et al. 2021). Therefore, it is extremely important to strengthen protections of these critical open rock barren habitats and develop strategies to restore and create nesting habitat that most accurately replicates natural conditions to support turtle populations. Knowing this, our research group is testing a habitat creation design that replicates natural nesting habitat found on a rock barrens landscape.

You can find the full article at:

Markle, C. E., Sandler, N. A., Freeman, H. C., & Waddington, J. M. (2021). Multi-scale assessment of rock barrens turtle nesting habitat: Effects of moisture and temperature on hatch success. Ichthyology & Herpetology, 109(2). https://doi.org/10.1643/h2020125 

References

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Christens, E., & Bider, J. R. (1986). Reproductive ecology of the painted turtle (Chrysemys picta marginata) in southwestern Quebec. Canadian Journal of Zoology, 64(4), 914–920. https://doi.org/10.1139/z86-138 

Gibbons, J. W., Scott, D. E., Ryan, T. J., Buhlmann, K. A., Tuberville, T. D., Metts, B. S., … Winne, C. T. (2000). The Global Decline of Reptiles, Déjà Vu Amphibians. BioScience, 50(8), 653.

Hughes, E. J., & Brooks, R. J. (2006). The good mother: Does nest-site selection constitute parental investment in turtles? Canadian Journal of Zoology, 84(11), 1545–1554. https://doi.org/10.1139/z06-148 

Edge, C. B., Steinberg, B. D., Brooks, R. J., & Litzgus, J. D. (2010). Habitat selection by Blanding’s turtles (Emydoidea blandingii) in a relatively pristine landscape. Écoscience, 17(1), 90–99. https://doi.org/10.2980/17-1-3317

Ernst C. H., and Lovich J. E. 2009. Turtles of the United States and Canada, 2nd ed. Johns Hopkins University Press.

Kolbe, J. J., & Janzen, F. J. (2002). Impact of nest-site selection on nest success and nest temperature in natural and disturbed habitats. Ecology, 83(1), 269–281. https://doi.org/10.1890/0012-9658(2002)083[0269:ionsso]2.0.co;2 

Litzgus, J. D., and Brooks, R. J. (1998). Reproduction in a northern population of Clemmys guttata. Journal of Herpetology,32(2), 252-259. doi:10.2307/1565305

Litzgus, J. D., and Brooks, R. J. (2000). Habitat and Temperature Selection of Clemmys guttata in a Northern Population. Journal of Herpetology, 34(2), 178. doi: 10.2307/1565413.

Markle, C. E., and Chow-Fraser, P. (2014). Habitat Selection by the Blanding’s Turtle (Emydoidea blandingii) on a Protected Island in Georgian Bay, Lake Huron. Chelonian Conservation and Biology, 13(2), 216–226. doi: 10.2744/ccb-1075.1

Markle, C. E., Sandler, N. A., Freeman, H. C., & Waddington, J. M. (2021). Multi-scale assessment of rock barrens turtle nesting habitat: Effects of moisture and temperature on hatch success. Ichthyology & Herpetology, 109(2). https://doi.org/10.1643/h2020125 

Mitchell, T. S., & Janzen, F. J. (2019). Substrate influences turtle nest temperature, incubation period, and offspring sex ratio in the field. Herpetologica, 75(1), 57. https://doi.org/10.1655/d-18-00001 

Morjan, C. L. (2003). Variation in nesting patterns affecting nest temperatures in two populations of painted turtles (Chrysemys picta) with temperature-dependent sex determination. Behavioral Ecology and Sociobiology, 53(4), 254–261. https://doi.org/10.1007/s00265-002-0570-3 

Riley, J. L., & Litzgus, J. D. (2013). Evaluation of predator-exclusion cages used in turtle conservation: Cost analysis and effects on nest environment and proxies of Hatchling Fitness.  Wildlife Research, 40(6), 499. https://doi.org/10.1071/wr13090 

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