Brice Krohnand Ryan Windhorst, Department of Biology, University of Nebraska Kearney, Kearney, Nebraska 68849-1140, USA
Mentor: Dr. Charles Bicak, Professor, Department of Biology, University of Nebraska at Kearney, Nebraska 68849-1140, USA. Email: bicakc@unk.edu)
To Cite This Paper:
Krohn, B. and R. Windhorst. 1997. Burrow spacing and plant diversity in prairie dog habitats. Senior Research Thesis, Department of Biology, University of Nebraska at Kearney, Kearney, Nebraska.
Abstract: In 1997 burrow spacing and plant diversity of Cynomys ludovicianus was studied north of Harlan County Dam (N 40.750, W 99.160 ), Harlan County, Nebraska, and north of Cottonmill Park (N 40.090, W 99.220 ), Buffalo County, Nebraska. One 60-yard diameter transect at each site allowed averages in burrow spacing and plant diversity counts from eight subplots per transect. The studies compared a small habitat versus a large habitat. Variabilities in location, soil type, and water availability showed a difference between the two prairie dog habitats.
The black-tailed prairie dog (
Cynomys ludovicianus) originally inhabited prairies from southern Canada to Mexico and from the eastern foothills of the Rocky Mountains to the tall grass prairie (Hall 1981). Prairie dogs can inhabit areas from only a few square meters to an unlimited amount of land. A single prairie dog colony occupied about 64,750 square kilometers in Texas (Merriam 1902). Currently, where their populations are relatively uncontrolled by humans, prairie dog colonies range in size from tens to hundreds of hectares (Knowles 1986). However, the size of a single colony may fluctuate dramatically during its decades of habitation, and colonies also may be abandoned or become extinct (Knowles 1986, O’Meilia et al. 1982). Prairie dogs have been conditionally viewed as competitors with cattle for range land resources; eradication programs have reduced populations to less than two percent of those several decades ago (Summers and Linder 1978). The degree of competition between cattle and prairie dogs is not known. Merriam (1902) calculated that forage consumption of one cow was equivalent to that of 257 prairie dogs. It is believed that some prairie dogs could destroy as much as eighty percent of forage through feeding and clipping vegetation (Taylor and Lotfield 1924). These ideas based on the size range and vegetation surrounding the prairie dogs’ habitat have encouraged studies to prove that mound spacing is not affected by habitat size and that plants are not more diverse from one habitat to another.
The purpose of this study was to prove the following null hypotheses: (1) there is no significant difference between burrow spacing versus the size of the prairie dog habitat at alpha = 0.05, (2) there is no significant difference between plant diversity versus the size of the prairie dog habitat at alpha = 0.05.
MATERIALS AND METHODS
The study consisted of two prairie dog habitats in central Nebraska. The first study was conducted on a transect north of Harlan County Dam, Harlan County, Nebraska, in a small open prairie surrounded by trees and a small creek. A second transect was studied northwest of Cottonmill Park, Buffalo County, Nebraska, in a large prairie grassland with trees located north and southwest of the transect. The habitat also had a creek lying within its borders. Global Positioning System (GPS) was used to locate the centers of the two transects used in the study. The first procedure consisted of measuring out the transects to make a sixty yard circle by finding a center point and marking a thirty foot radius with orange flagged stakes. A measuring wheel was used to determine distances. The transects were broken into four quadrants which allowed creation of x,y coordinates for each burrow found in the habitat. This allowed plotting of points onto a spreadsheet. The spreadsheet allowed conversion of numbers, enabling each burrow to be plotted onto a map showing the transect area. Two different procedures were performed to give accurate averages of distance between burrows.
The first procedure started in the northwest quadrant, connecting the points all the way around the transects until reaching the starting point. The average burrow spacing was found and the procedure was run on both transects for comparison (Fig. 1).
Fig. 1. Burrows from two different prairie dog towns in central Nebraska in fall 1997.
The second procedure started with the addition of an equation to the spreadsheet, distance equals the square root of (x2-x1)2 + (y2-y1)2, computing distances between burrows. The last burrow of each quadrant was compared to the first burrow of the next quadrant continuing through the entire transect. This allowed the use of the equation down the entire spreadsheet. The distances showed average burrow spacing results for each transect. Completion of these studies resulted in determining plant diversity for each transect. A 1.0-m2 grid, divided and numbered into 16 equal squares, allowed specific ground areas to be counted and recorded for lab study. Each quadrant had two sites where plants and grasses were retrieved, making a total of eight sites per transect. The 1.0-m2 grid was set at predetermined x and y coordinates before laying out the transects.
Each sample taken was placed in brown paper sacks and labeled with its correct transect location and quadrant number for further testing in the laboratory. Two plant/grass identification books identified all of the plants and grasses found in each sample (Stubbendieck et al., 1994; Whitson et al., 1992). Two different studies were ran, one to make sure enough samples were taken and the other to show plant diversities between transects. Shannon’s principle allowed the first test to be completed by listing the plant species, the number of new species and cumulative number of new species. The zero in the eighth test proved enough tests had been run for each study. The plant diversity index was derived by using the Shannon-Weaver Diversity Index Computation (Table 1). The program allowed entering of each plant species discovered. The transects were labeled, the number of individuals in each taxa were entered in the computation, omitting any line that had a zero, allowing a true diversity number to compare the transects.
Table 1. Plant species, number of individuals occurring in transects and diversity difference significance.
RESULTS AND DISCUSSION
The procedures incorporated 98 burrows in the Harlan transect and 86 burrows in the Cottonmill transect. The first study had an interesting outcome: burrow spacing in Harlan and Cottonmill transects were 5.46 yards vs. 5.45 yards, respectively. The study resulted in no significant difference (
P > 0.05) between transects.
The second method of study also resulted in acceptance of the null hypothesis. The burrow spacing proved to be 7.75 yards vs. 6.13 yards. Numbers in the spreadsheet ranged from 1 to 55, influencing the outcome. Rearrangement of numbers in the spreadsheet could change the outcome, but could lead to a biased study. No other studies were found to incorporate information and findings into the procedures. Many more studies need to be done in central Nebraska to find an overall relationship between prairie dog habitats and burrow spacing. Studies could include several transects within one habitat, habitats under the same stress and an equation that would compare one burrow to all burrows within the same transect continuing throughout the entire transect.
Plant diversity studies concluded the null hypothesis was rejected. Differences between habitat location (Harlan transect being in a flood plain, while Cottonmill transect is located in loess hills), showed differences of 0.79 and 0.60 respectively. This proved to be a statistically significant difference even though 7 out of 12 species were found in both transects.
The comparisons of the mound spacing was significant between small and large habitats, although the tests were subjected to a highly different conditions. These conditions included a different arrangement of burrows on the spreadsheet which under other arrangements could lead to different P-values other than the concluded P = .0678 obtained in this study. Plant diversities did prove to be greatly influenced by the size and/or location of the habitats. Differences in the soils, water content, and location of the habitats are all factors of influence that proved the null hypothesis wrong.
LITERATURE CITED
- Hall, E. R. 1981. The mammals of North America. 2nd ed. John Wiley and Sons, NY.
- Knowles, C. J. 1986. Some relationships of black-tailed prairie dogs to livestock grazing. Great Basin Naturalist 46:198-203.
- Merriam, C. J. 1902. The prairie dog of the great plains. USDA Yearbook 1901:257-270.
- O’Meilia, M. E., F. C. Knopf, and J. C. Lewis. 1982. Some consequences of competition between prairie dogs and beef cattle. Journal of Range Management 35:580-585.
- Stubbendieck, J., G. Y. Friisoe, and M. R. Bolick. 1994. Weeds of Nebraska and the Great Plains. Nebraska Department of Agriculture, Bureau of Plant Industry. Lincoln, Nebraska. 589 pp.
- Summers, C. A., and R. L. Linder. 1978. Food habits of the black-tailed prairie dog in western South Dakota. Journal of Range Management 31:134-136.
- Taylor, W. P., and J. V. G. Loftfiel. 1924. Damage to range grasses by the Zuni prairie dog. U.S. Department of Agriculture Bulletin 1227. 15 pp.
- Whitson, T., L. Burrill, S. Dewey, D. Cudney, B. Nelson, R. Lee, and R. Parker. 1992. Weeds of the West. Western Society of Weed Science.
Krohn, B. and R. Windhorst. 1997. Burrow spacing and plant diversity in prairie dog habitats. Senior Research Thesis, Department of Biology, University of Nebraska at Kearney, Kearney, Nebraska.