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Wetland Biology

Activity: Population and Distribution Status of Chrysemys picta, the painted turtle, along Republican River Corridor, Nebraska

An educational activity designed and written by Deborah M. DeMarey

The following exercise is a fictitious situation based on factual information. It has been designed to enhance skills in scientific data analysis and problem solving using advanced technological media combined with accumulated prior knowledge from a variety of sources.

The Situation

You are a naturalist interested in the biology of Chrysemys picta, the painted turtle. Your field research has been an intensive study over the past five years documenting the numbers of individuals in the population and the reproductive fitness of this species along the Republican River. The expertise you have acquired on the life cycle of this species will aid decision-makers in providing and implementing sound management practices for the continuation of the species.

The Task

Present a comprehensive report on the status of the painted turtle along the Republican River Region in south-central Nebraska from its original distribution prior to the construction of the dam to its present day distribution.

You are encouraged to use air photos, satellite imagery and any other data set available to you. Many on-line links to this scenario have been included to direct you to additional sources of information. There is a vast amount of information contained within these paths. Some of the links will further direct you to related topics. As you proceed through your investigation, keep track of addresses and site locations / paths that you have found particularly helpful. Often it becomes difficult to re-track a path if you have forgotten the sequence of links.

Hint: Read and re-read the task of this investigation often. It will keep you focused. Break the task into smaller components. Formulate questions for each of these sections and seek avenues where you may expect to find solutions. Remember, there are an infinite number of ways to address this issue. You need to present documentation that definitively supports your approach.

Here are some questions to keep in mind as you begin your work. They are by no means all-inclusive; they are merely intended to help guide you to other avenues of investigation.

  • How might you estimate changes in suitable habitat for the survivability of the painted turtle?

First you will need to know what is suitable habitat for Chrysemys picta.

Then you will need to know the parameters of that habitat. Meaning that if we view suitable habitat as the mid-point on a gradient of habitat extending from unsuitable to uncontrolled population growth where on the continuum does the site in question fall?

  • Where will you find necessary information?
  • Who will know?
  • Does anyone know?
  • Where will you look to find out?

You will need to know the specific habitat requirements of the species through all stages of its life cycle.

  • Where does it live?
  • How large is its range?
  • What ecological conditions are necessary for successful reproduction?
  • Is it a migratory species or does it dwell in the area year round?
  • If it is a year round resident are its habitat requirements different in the winter than in the summer?
  • Do juveniles require different habitat conditions than adults?
  • Will the breeding cycle require ecological conditions specific only to that stage of development?
  • What ecological pressures are endured by this species throughout its life cycle?
  • What does it eat during all stages of its life cycle?
  • How great is the predation pressure on this species in general and at this particular site?
  • How will you estimate the extent of habitat currently and historically?

Hint: Use the aerial photographs to compare area. Based on your knowledge and investigation of lifecycle requirements, delineate areas of suitable habitat.

Air photo interpretation is a skill that among other things, compares the sizes of unknown features to objects of known sizes in the same image. One can be used as a reliable measure to estimate the relative size of another. When you look at the air photos, try to identify something that has a standard size. You may spot a standard single car garage, or a regulation basketball court. Maybe there is a boxcar from a freight train.

These and many others are examples of objects that are of recognizable standard size and shape. Mathematically, compute habitat loss or gain based on the differences (if any) measured. This technique is referred to as temporal and spatial variation. In short, it is the measurable difference (variation) in habitat change (space) over time ('temporal' refers to time).

  • What other considerations have to be investigated before your report will be complete?
  • How will you obtain these data keeping in mind that the presentation will be due shortly and that there is not enough time or money for you to conduct an on-site evaluation?

Hint: Construct a presentation based on the accumulated knowledge of others combined with the specifics of your research site. Why re-invent the wheel? If it has been done, use what others have learned to increase your knowledge base and build upon it. Formulate your original thoughts based on the accumulated scientific data available to you. By applying an already tested method to a new site, the researcher may avoid making some of the same errors, will lend further credibility to the original study and will further the advancement of science through successful replication.

The narration that follows includes some basic turtle data you know to be true about the species of concern.

Painted turtles, living the majority of their lives in the muddy bottoms of ponds, lakes and the adjacent wetlands, begin their yearly breeding cycle in late spring and early summer. During this time, their habitat requirements are expanded to include sandy, gravel areas in nearly full sun away from the pond. The female turtles, having mated in the pond begin to migrate to suitable nesting areas in the uplands. Migration is a nightly occurrence that begins a little after dusk and continues to just after sunset. Not all female turtles are ready to nest at the same time so the migratory pattern is stochastic with times throughout the breeding season that approximate a mass migration. The entire process spans a period of six to eight weeks.

Weather conditions, soil moisture, ambient temperature, and vegetative cover are among the many factors influencing the timing of the migration. If any of these factors is not conducive for successful nesting, migration and the completion of the breeding cycle does not occur. Some individuals will travel as far as a quarter mile in search of appropriate nesting areas. Once an area has been selected, the female will begin to scratch the ground, sniff it and move on. This process continues for an hour or more depending on the individual. When at last the nesting site has been selected, the female begins to dig a hole using her hind feet and claws.

As one leg tires, the other takes over. If she is interrupted or startled during this time, she most often will abandon the site, return to the pond and attempt to nest in a different location the following evening. Upon completion of the excavation of a nest, the female deposits one egg at a time into the nest. She positions it carefully and covers each egg with a thin dusting of sandy soil before laying the next. There may be as few as 1 egg in a nest and as many as 17; the averages range between 7 and 13 eggs per clutch.

When the clutch has been laid, the female carefully fills in the hole, firms the site with the weight of her body, camouflages the area by scratching gravel, sticks, and other plant debris loosely over the surface, and returns to the pond. Incubation for Chrysemys picta is approximately 11 weeks. Predation pressure on turtle nests is intense.

Snakes, raccoons, coyotes, and skunks feed heavily on turtle eggs. The number of turtles that survive to hatchling stage is approximately 25 percent. But that is only part of the survival equation. The hatchlings, now about the size of a nickel, must make their way successfully to the pond. Guided by the moonlight reflected off the surface of the water, the hatchlings scramble towards the pond. Some are preyed upon by the same animals that predate the nests. Others are eaten by birds or perish due to exhaustion, dehydration, starvation, or the inability to right themselves if they topple over. Those that reach the pond face predation pressure from large fish, shore birds, water snakes, and snapping turtles. As harsh as these events may seem, they are the natural occurrences and the population controls of a healthy ecosystem and food web. Introduce the human impact variables of habitat encroachment, fragmentation, urbanization, chemical pollution, and light saturation, and the equation is dramatically altered. Continuation of the species is dependent upon the species' ability to successfully reproduce. As you can see there are many variables that need to be carefully considered and evaluated if alterations to a natural system are proposed.