Turtle Research at the INHS Great Rivers Field Station

In North American aquatic ecosystems, turtles often make up a large portion of the biomass. Even so, relatively few long-term research projects currently examine life history and ecology of aquatic turtle populations. Programs conducted in Michigan (E.H. George Reserve) and South Carolina (Savannah River Ecology Laboratory) demonstrate the need for long-term studies of turtles.

Beginning in 1992, researchers at the INHS Great Rivers Field Station began to examine turtle demographics, reproductive ecology, and life history strategies. Primarily, studies focused on the red-eared slider (Trachemys scripta elegans) because that species is the dominant riverine turtle in Pool 26 of the Mississippi River, where this study was done. Nonetheless, data were gathered on eight other less commonly encountered species as well.

Demographic studies: Technicians of the fisheries component of the Long-Term Resource Monitoring Program collect large numbers of aquatic turtles in nets set to track fisheries resources in Pool 26. Size, sex, and specific identity data on these turtles are collected in the field. Moreover, many of these turtles are brought back to the laboratory where more detailed measurements can be made. After being individually marked, they are returned to their capture site. Recaptures allow population demographics for pool-wide backwater use to be determined.

Reproductive ecology: Beginning in 1994, turtle studies were expanded to include a component on reproductive output. Studies are conducted by collecting nesting females at Stump Lake in Jersey County and in Swan Lake and Pohlman Slough in Calhoun County. These females lay their eggs in the laboratory where the eggs are then weighed, counted, and incubated. Data collected allow comparisons of reproductive output among years and between sites.

We have found that reproductive output varies among years, with years following extensive flooding tending to coincide with lowered reproductive output. We have also found differences between sites in reproductive effort. At sites with extensive aquatic vegetation (an important food resource for aquatic turtles), the turtles tend to lay more but smaller eggs than do turtles from sites with little or no aquatic vegetation. The evolutionary implications of these findings are now under experimental examination.

Life history strategies: The eggs that female turtles lay hatch in our laboratory after incubation. Many of these hatchlings are then released at the site of their female parents. Some, however, are used in nondestructive experiments designed to examine life history strategies. Our main focus is to identify variables that influence hatchling survivorship. Thus, we have performed experimental releases each year beginning in 1995. Our findings suggest that larger hatchlings are more likely to survive than smaller ones. We also found that predation by birds is a major source of mortality in migrating hatchling turtles. Other experiments have examined responses to subfreezing temperatures, competition, and variable incubation environments.

Our efforts have resulted in research publications in general interest journals such as Ecology, Journal of Evolution Biology, and The American Midland Naturalist. Moreover, our findings have appeared in international herpetological journals such as Journal of Herpetology, Herpetologica, and Copeia. Overall, our findings have appeared in more than 50 publications in less than 10 years. Importantly, our program has also afforded graduate and undergraduate students opportunities to complete research projects. Student projects not only provide experience in field biology but also advance our understanding of chelonian biology.

John K. Tucker, Center for Aquatic Ecology

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