To learn more about my current research projects, please visit the Georgia Sea Turtle Center research webpage.
Below I have provided summaries of some recent and ongoing projects that are representative of my research interests. For a comprehensive list of the research I’ve been involved in, please see my publications page.
Restoration of Bird and Reptile Assemblages in Degraded and Fire-suppressed Longleaf Pine Forests
Longleaf Pine forests once spanned throughout the coastal plain of the southeastern United States but have been reduced to a fraction of their historic extent, making this unique ecosystem one of the most endangered on the continent. Longleaf Pine forests that aren’t developed or converted to agricultural fields were often managed without fire; however, periodic burning is essential for this forest to maintain its natural structure and associated species. Interest is growing in the restoration of remaining Longleaf Pine forests but there is little information regarding how to achieve this restoration over the long-term, particularly when restoration of wildlife assemblages is a goal. To fill this information gap, my collaborators (notably Craig Guyer at Auburn University and Lora Smith at Ichauway) and I studied Longleaf Pine forests on Eglin Air Force Base that were once fire-suppressed but have been managed with fire (and various other related techniques) over the past 15 years.
We found that lizard abundances in degraded habitats bounced back quickly following forest restoration and this effect was maintained over the course of the study (Steen et al. 2013. Restoration Ecology 21:457-463). On a larger scale, populations of habitat-specialist birds like Red-cockaded Woodpeckers and Bachmann’s Sparrows were restored after reintroducing burning to the forest although the entire bird assemblage required over ten years of this burning to get back to what we expect was historically present (Steen et al. 2013. Ecological Applications 23:134-147). We found almost identical trends when examining the reptile assemblage (Steen et al. 2013. Ecological Applications 23:148-158). Overall, we demonstrated that wildlife in degraded Longleaf Pine forests can be restored with fire, but this fire must be applied regularly over a long period of time. I am interested in expanding this work to identify the mechanisms behind the response of wildlife populations to restoration by determining how their movement patterns and birth and death rates change following forest management.
Interactions Between Snakes Influence How Different Species Persist Together
My work has provided quantitative evidence that dynamic and ongoing interspecific interactions may influence the composition of snake assemblages, upending the paradigm that snake co-occurrence is due largely to resource partitioning that arose over evolutionary time. Specifically, my collaborators (notably Chris McClure) and I have described how the presence of one snake, such as the Eastern Diamond-backed Rattlesnake, reduces the probability that similar snakes (i.e., Timber Rattlesnakes) are present above and beyond any habitat preferences (Steen et al. 2007. Journal of Wildlife Management 71:759-764), a trend we attribute to competition over resources (Steen et al. 2014. Journal of Animal Ecology 83:286-295). Speaking of competition, we have also described how Black Racers are smaller in areas where the larger Coachwhip is present; this shift in body size may result from competition over prey (Steen et al. 2013. Journal of Zoology 289:86-93). Most recently, we explored how the abundance of the snake-eating (and mysteriously declining) Kingsnake may keep in check the abundance of one of their favorite prey items, the Copperhead (Steen et al. 2014. Herpetologica 70:69-76). The relationship between these species may explain why many Copperheads are now being observed in areas where Kingsnakes have disappeared.
Sampling Secretive Wildlife and Analyzing the Data: Limitations at Every Step
Many species of wildlife are difficult to survey with much confidence, that is, it’s often hard to know how effective a researcher may be at sampling wildlife when only a few individual animals are encountered. When we only detect a few individuals of a given species, it could mean that the species is truly rare or that they are common but we have a poor ability to find them. Recent statistical advances are not effective when we don’t catch enough animals to determine if the required assumptions are met (Steen et al. 2012. Reptile Biodiversity: Standard Methods for Inventory and Monitoring) and this has important implications for our ability to learn about secretive wildlife (Steen 2010. Herpetological Conservation and Biology 5:183-188). Separately, species richness estimators are used to estimate the number of species that are undetected at a site; in turn this number is used to determine how wildlife assemblages respond to habitat change. But, we shouldn’t expect all species to respond to change similarly (Steen et al. 2010. Open Environmental Sciences 4:24-30). My collaborators and I recently developed computer simulations demonstrating how common methods of estimating species richness may not be effective when comparing vertebrate assemblages because sample sizes are too low to generate reliable results. Our findings have important implications for how we determine how wildlife assemblages respond to habitat change.
From Preferred Hiding Spots to Suitable Landscapes: Multi-scale Habitat Selection of Wildlife
To generate effective conservation plans that consider the landscapes that are necessary for wildlife, we need to identify the landscape-level habitat features that influence why a species is present in an area but also the habitats used by individual animals within that area. Previously, this multi-layered information was unavailable for snakes in the southeastern United States, despite this group containing rare and imperiled species. Using a database of snake captures from across the southeastern United States, my collaborators and I described the natural history of thirteen poorly understood species of snakes and identified the habitat that should be present in a landscape to ensure it is suitable for their populations (Steen et al 2012. Ecological Applications 22:1084-1097). Separate research projects have used radio-telemetry or incidental observations to identify the habitats used by individual Kingsnakes (Steen et al. 2010. Copeia 2010:227-231) and Timber and Eastern Diamond-backed Rattlesnakes (Steen et al. 2007. Journal of Wildlife Management 71:759-764). In a similar study, my collaborators and I recently combined both landscape and within-site data to determine features of the landscape that determine whether the federally-threatened Red Hills Salamander is present as well as microhabitat characteristics that influenced the distribution of their burrows (Steen et al. 2014. Journal of Wildlife Management: in press). The hierarchal and comprehensive framework outlined above is needed to quantify multi-scale habitat selection for a wide variety of wildlife.
Effects of a Coal Fly Ash Spill on Freshwater Turtle Ecotoxicology and Reproduction
On 22 December 2008, 1.1 billion gallons of coal fly-ash slurry were released into the Emory River when a retaining wall ruptured at a coal-burning plant in Kingston, Tennessee. Working with collaborators at Virginia Tech, I coordinated a large-scale (~9,000 turtle captures) field and lab-based study determining whether turtles in the vicinity of the fly-ash spill had elevated concentrations of contaminants including arsenic, mercury, and selenium and also whether female turtles were passing these contaminants to their offspring. Further, we are studying whether elevated contaminant concentrations are associated with negative reproductive effects, such as smaller or fewer eggs in a clutch and decreased hatching success. Finally, we are researching whether hatchling turtles suffer any negative developmental effects as well as determining whether certain species are more vulnerable to accumulation of the contaminants associated with the fly-ash spill. More information about this project can be found here.
Road Mortality and Freshwater Turtle Conservation
We’ve all seen turtles on the road that have been run over by cars. But are there any population-level impacts of this individual mortality? James Gibbs and I (as well as numerous collaborators) determined that female turtles were vulnerable to road mortality (Steen et al. 2006. Animal Conservation 9:269-273) because they go on terrestrial nesting migrations, often crossing roads. Unfortunately, these mature female turtles are the individuals that are most important for ensuring the population can sustain itself. We conducted a field study on wild freshwater turtle populations and found that populations in wetlands surrounded by many roads had few females (Steen and Gibbs 2004. Conservation Biology 18:1143-1148), an effect we attributed to the chronic mortality of females in these habitats. Because the extent of roads has been increasing in North America over the last century, we predicted and confirmed that turtle populations have become increasingly male-biased over this same time period (Gibbs and Steen 2005. Conservation Biology 19:552-556).
Overall, we became convinced that females turtles were most vulnerable to road mortality, their chronic mortality is resulting in male-biased populations and finally, that this effect was increasing over time. To manage and conserve freshwater turtle populations, it is therefore necessary to determine how far nesting females were traveling overland. We obtained this information by consulting turtle researchers and the published literature; the result (Steen et al. 2012. Biological Conservation 150:121-128) is an extensive compilation of freshwater turtle nesting spatial ecology. Now, if land managers and policy makers wish to protect female freshwater turtles on their nesting migrations, they know what they need to do. I hope to further this research by determining whether artificial nesting areas could be effective at replicating the environmental conditions of natural nests as well as at persuading turtles to nest in specific areas away from pre-existing roads.
Reintroducing a Top Predator to Degraded Ecosystems
North America’s largest native snake, the Indigo Snake (Drymarchon couperi) is a federally threatened species that was extirpated from Alabama sometime in the mid-20th century, largely due to habitat loss and fragmentation. Since 2009 and coinciding with large-scale habitat restoration, approximately 125 Indigo Snakes have been reintroduced to Conecuh National Forest, with another 200 slated to be released over the next five years. . In 2017 we began reintroducing Indigo Snakes to Apalachicola Bluffs and Ravines Preserve in the Florida panhandle. Overall, in addition to establishing new populations of Indigo Snakes and aiding in their federal recovery, we hoped to learn more about the ecological role as well as their spatial ecology and survival.
Invasive Tegus in the Southeastern United States
An invasive species in southern Florida currently of great concern is the Argentine Black and White Tegu (Tupinambis merianae),in part because these big lizards (up to 4 ft -1.2m- and over 10 lbs -4.5 kg-) are big predators of native species. A pressing question is whether these invasive lizards, currently restricted to southern Florida, will be able to thriver further north. To determine whether this species is physiologically suited to survive in a temperate climate an
d in cooperation with the United States Geological Survey, we are holding tegus in outdoor enclosures at Auburn University. We conducted surgeries to implant the tegus with radio-transmitters, so that in the case of an escape we can quickly recapture the animal, and we also implanted small temperature recording devices that will enable us to monitor the animal’s temperature and figure out if they are able to adapt to this relatively temperate climate. If the animals survive the winter, we will then play matchmaker and see if the animals can successfully reproduce. Our study will reveal much about the thermoregulatory behavior of tegus while also informing policy makers about the potential for this species to successfully invade Alabama and Georgia.
We are also examining the population demography of tegus captured in the wild in Florida to better understand their population structure and generate insights regarding the efficacy of current removal programs.
Check out a short video describing our tegu work: