JWM study: Turtles appear resilient to minor human development

Turtles are known to be especially sensitive to human pressures on ecosystems, but certain species may be resilient to minor human development, according to new research. Biologists have been studying ornate box turtles (Terrapene ornate) in the sand prairie region of northwest Illinois for 20 years. The new study looked at the long-term mark-recapture dataset to evaluate the effects of habitat modification on survival rates of the turtle, which is listed as threatened in the state.

“Animals with small home ranges may be less likely to move if development occurs,” said Sarah Mitchell, a researcher at Iowa State University and the first author of the study published in the Journal of Wildlife Management. “Turtles have specific habitat requirements for a variety of necessary activities, including feeding, nesting and hibernating. Locations that offer the proper habitat for all activities can be hard to locate, and roads, rivers and other obstacles may inhibit movements.”

A researcher holds a juvenile ornate box turtle. ©Brooke Bodensteiner

A researcher holds a juvenile ornate box turtle.
©Brooke Bodensteiner

Mitchell and her team performed seasonal surveys from 1996-2015 at three sites near the Upper Mississippi River National Wildlife and Fish Refuge in Illinois that had different amounts of human development. The disturbed study site, composed of two adjacent locations, contained cottages, driveways and mowed grass. The undisturbed site, or refuge site, was undeveloped except for a bike path, says Mitchell. All sites were accessible by the public.

Using a mark-recapture method, the researchers observed 102 ornate box turtles — including males, females and juveniles — at the refuge site, compared to 47 turtles at the disturbed sites. The most significant difference in captures was between males and females. Females were more likely to be caught than males at both sites. At the refuge site, the annual survival rate for females was 31 percent higher than females at the disturbed site. Males also reflected this trend, but the trend was not statistically significant, says Mitchell.

Mitchell says ornate box turtle populations rely on the survival of adults for growth and stability. Adults have very few predators, which makes survivorship very high in wild populations. Further, the turtle’s ability to reproduce for 15 to 20 years offsets the low survival rate of offspring, says Mitchell.

The researchers were surprised to find that ornate box turtles were resilient to small amounts of human development in the refuge site, but a larger sample size is necessary to observe small effects. Further monitoring over a longer period is necessary to observe minor population changes and enhance estimates of survival rates of the turtles, says Mitchell. The additional research also will help managers make more informed decisions regarding the impacts of human development on the threatened turtles’ survival.

“We’re looking at survival rates for a very cryptic and long-lived species,” Mitchell said. “To get good results we need to have a very long study because recapture rates would be so low, so we need many years to get an estimate.”

Sean McNealy is an undergraduate at the University of Missouri majoring in science and agriculture journalism and fisheries and wildlife sciences. He will receive his BS in December 2016.

Header Image: An adult male ornate box turtle. The species is listed at threatened in Illinois. ©Fredric Janzen