Background and Purpose: With an exponentially growing populations comes infrastructure, accompanied by roads. The direct and indirect effects of roads have been and will continue to be an important conservation concern for species that utilize a network of connected habitats. Most all taxa are affected by roads, however, herpetofauna, especially freshwater turtle species are disproportionately affected by additive adult mortality.  Being an already vulnerable species group with high juvenile mortality and high adult and subadult survivorship, anthropomorphic stressors, such as road mortality, are concerning. Roads not only act as a barrier of dispersal—effecting gene flow and changing animal behavior, but they are also the leading causes of death for some species. For turtle species, roads disproportionally effect nesting females who travel large distances across a landscape gradient—leading to a shift in sex ratios. Species like the Spotted Turtle who use a combination of wetlands and upland habitat across the landscape are more at jeopardy than wetland species that facilitate life processes in a single wetland. 

Study Species: Spotted Turtles are a small, freshwater wetland species that inhabit a variety of wetland types including vernal pools, bogs, marshes, roadside ditches, and small streams. With an expansive range from southern Canada to northern Florida, Spotted Turtles are found in many environments across the Atlantic Coast. They are declining across their range, however, due to a variety of environmental stressors including poaching, habitat fragmentation, wetland loss, road mortality, pollution, illegal trade, and many more. Spotted Turtles have a short active season, being in the water from March through June, and then aestivating and nesting during the summer months. They have large home ranges, especially nesting females—using the upland habitat bordering their wetlands as areas for nesting, short migration between wetlands, and aestivating. With an increase in urbanization, these upland habitats are being highly fragmented by roads—leading to high rates of road mortality. Although there is much said in the literature about the impacts of roads on turtles, there have been few examining the impact of road mortality on freshwater turtle populations. 

Methods and Results:  The study site was located in central Maryland with a paved, two-lane road intersecting it. The average number of vehicles driving on the road was 2017 to 2087 vehicles per days. North of the road were four ephemeral wetlands totaling 6.94 hectares in area. To the south were three permanent and one ephemeral wetland. Spotted Turtles were caught by hand and collapsible mesh minnow traps. All turtles were marked and measured. Both sides of the road (including the northern and southern wetland complexes) were treated as separate populations. PVA’s or population viability analyses were used to simulate Spotted Turtle population trajectories under baseline conditions with added road mortality.  

After running the models, both the northern and southern populations showed negative growth rates. Without road mortality, extinction risk is about 20-24% in 150 years, however, with road mortality, extinction risk is greater than 90%. Every slight increase in adult mortality increases the rate of extinction. While road mortality itself was only around a 2% loss annually, it is catastrophic for these small, isolated Spotted Turtle populations. 

Importance: There are large data gaps into what we know about Spotted Turtles, specifically in more robust populations where they are more abundant. This study is one of the first studies to attempt to quantify the direct effects that roads have on freshwater turtle viability. Although Spotted Turtles suffer from a variety of environmental stressors as mentioned before, road mortality can dramatically reduce population viability.  Many of the isolated areas Spotted Turtles inhabit are now being affected by urban sprawl and increasing infrastructure. These once remote upland area are now surrounded by neighborhoods, and many of these wetland complexes have been segmented by roads. There have been other studies looking at road mortality on freshwater turtle populations but depending on the species and the current status and structure of their populations, even large starting populations may have high extinction probabilities. 

Future Work: In North Carolina, road mortality appears to be especially high in areas like the inner/outer banks where Spotted Turtle populations are more robust. In my research, I have noticed that rural areas with multiple wetland complexes seem to result in high road mortality rates in Spotted Turtles. Although these areas are not city centers, and you may not even consider them urban, the infrastructure necessary to support growing populations is there. Studies like these should be replicated across Spotted Turtles’ range. Given that these turtles inhabit such a broad variety of ecosystems, the replication of this study in more southern populations is pivotal. Slight differences in behavior across populations may yield different results related to road mortality. In North Carolina, our populations are more active during the Spring months, and towards the end of May females will begin traveling to nest. For some wetland species, like the Spotted Salamander, that make great migrations once a year to reproduce, people have collaborated with local departments of transportation to aid in these mass movements during the winter months. Although it may not be as realistic for Spotted Turtles, there are ways to mitigate road mortality. Because Spotted Turtles are considered a species of concern in some places across the United States and are listed as endangered through the IUCN, you would have to be careful not to reveal the exact locations of these populations due to poaching. Progress is currently being made to increase the detection of Spotted Turtles across their range through a Competitive State Wildlife Grant funded by USFWS. By increasing the amount of data we have on Spotted Turtles in general, we can make informed conservation decisions regarding road mortality and other stressors threatening their status. 

Reference: Howell, H. J., & Seigel, R. A. (2019). The effects of road mortality on small, isolated turtle populations. Journal of Herpetology, 53(1), 39–46. https://doi.org/10.1670/18-022