by
Eric Ribbens
Department of Biological Sciences
Western Illinois University
Welcome to your new career as a plant population ecologist! As an ecologist, you are interested in how plants interact within a population, with other species, and with their surrounding environment. You have recently accepted a position at a university in central Minnesota and have been looking for an interesting plant species in the area to study. There is a county park being developed on 105 hectares of abandoned granite quarry land a few miles away from the university, and in a conversation you had recently with George, the Stearns County Parks Director, he mentioned that one of the plant species found in the parkland is Opuntia fragilis, the brittle prickly pear, which in his opinion should be on the Minnesota state list of endangered and threatened plant species.
Photo 1: Quarry Park, Stearns County, Minnesota.
Photo 2: Prickly pears growing in moss on a rock outcrop.
You asked George to show you the population, and after walking through a scrubby red oak forest growing around piles of granite slag and water-filled quarries, you found an exposed granite outcrop about 20 meters long and 15 meters wide with several clusters of prickly pear growing along one edge of the outcrop. Immediately you fell in love with them. They are tiny, each joint about five centimeters long, bristling with spines, and sprawling over a mat of brownish moss growing in a crack in the granite. If you look at the illustration above, each rounded structure is a joint, often called a cladode or a pad; the yellower end structures are the new growth. By the way, botanists call these pads cladodes; they are really stem segments. Cacti leaves are modified into spines. If you look at the picture of prickly pears shown here you can see how the entire plant sprawls over moss. You quickly realized that this species might be an ideal organism to study—it is relatively easy to find and identify, quite rare, the population is completely isolated, with nice discrete boundaries, George would love to have you investigate the ecology of Opuntia fragilis at the park, and you can work on your suntan while you study them.
Now you are sitting in your office, your head bursting with ideas and questions. Where should you begin? You’ve already spent some time in the library and have some basic information. Particularly valuable was a book by Lyman Benson (1982), The Cacti of the United States and Canada. In this book, Benson refers to brittle prickly pear several times. The book has a range map (see Figure 1) showing that Opuntia fragilis is widely scattered across 25 states and five Canadian provinces. However, in most of those regions the populations are widely scattered and the plant is actually quite uncommon.
You learn that Opuntia fragilis was one of the first four species of cacti collected from the U.S. interior, and was first collected by Thomas Nuttall on the shores of the Missouri River. Opuntia fragilis occurs up to latitude 58 degrees in northern Alberta and British Columbia, where winter temps can be -40 degrees C. Benson proposed that its prostrate growth form and thickened pads are both adaptations to a cold climate. He reasoned that these traits were adaptive to cold, because the low growth form means it is quickly covered by protective snow, and thick pads provide a reduced surface-to-volume ratio. Both of these traits would reduce freeze damage.
Benson also mentioned that the dry fruits are capable of epizoochorous transportation (i.e., they get stuck in the fur of animals). In his opinion the wide northern range of Opuntia fragilis was caused by the easy fragmentation of the pads. Benson wrote, “The plains buffalo occurred in enormous herds, and their great hairy bodies would have been ideal for transporting the small cactus joints. The places where buffalo lay down probably included many plants of Opuntia fragilis.” According to Jack E. Schmautz, U.S. Forest Service (personal communication, 1976), the cactus is common at the edges of “slick spots” formed on solodized solonetz soils in eastern Montana and western North Dakota. These areas “were probably buffalo wallows.” According to Larry W. Mitich (personal communication), the species flowers only sparingly in North Dakota. Ownbey and Morley (1991), in their book about the plants of Minnesota, stated that Opuntia fragilis is scattered across the state, with most populations growing along the Minnesota River, but with several disjunctions, including the population in Stearns County. In Minnesota, Ownbey and Morley (1991) show that it is found in your county, but not in any other county within 80 miles (top central dot, Figure 2).
 Figure 1: Approximate distribution of O. fragilis in North America. Redrawn after Benson 1982 and other information. |
 Figure 2: Approximate distribution of O. fragilis in Minnesota. Based on Ownbey and Morley, 1991. |
How should you begin your research? What questions do you want answered? What should you do to find the answers? Decide what your plans are and design your first summer of field work. Show your design to your instructor, and then proceed to Part II of the case.
Date Posted: 11/03/05 nas
Image Credits: Title illustration by M.E. Eaton in 1910 for The Cactaceae: Descriptions and Illustrations of Plants of the Cactus Family by Nathaniel Britton and J.N. Rose (1919, 1923, 4 vols. Washington: Carnegie Institution of Washington). All other photos and figures are by the author unless otherwise noted.
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