Grazing by black-tailed prairie dogs Cynomys ludovicianus and North American bison Bison bison produces and maintains a series of characteristic vegetation types around prairie dog colonies. This combined grazing may be important in terms of conservation of these plant communities. This study assessed the individual and combined influences of these two herbivores in maintaining vegetation characteristics of a prairie dog colony in a mixed-grass prairie at Wind Cave National Park, South Dakota, USA.
The study site (inhabited by prairie dogs for about 27 years) was dominated by buffalo grass Buchloe dactyloides, with numerous other graminoids and herbs present. In December 1984, four treatments were initiated: i) grazing by prairie dogs and bison, ii) bison exclusion, iii) prairie dog exclusion, iv) exclusion of both.
In each of two blocks (approx. 40 m apart), two 50 x 50 m fenced exclosures were erected, and two unfenced 50 x 50 m areas marked out. Prairie dogs were removed from one exclosure and one adjacent unfenced area in each block (and as required throughout the experiment).
From May to September 1985 and 1986, above-ground biomass was measured monthly (plants clipped in 10, 0.25 m² frames in each treatment of each block). Canopy cover by species was estimated (in 15, 0.1 m² frames) and used to assess species diversity, equitability and dominance. Graminoid and forb shoot nitrogen concentration was determined monthly from samples; mammal grazing had been shown to increase shoot nitrogen in an earlier study.
In 1985, no treatment had a significant effect on plant biomass characteristics. In 1986, prairie dog and bison exclusion each significantly affected biomass (average over the growing season): prairie dog exclusion - increases in above-ground total, current year’s, and graminoid biomass (36%, 36% and 43% respectively); similarly bison exclusion, increases of 32%, 37% and 50%. The biomass increase where both praire dogs and bison were excluded was similar to the sum of that from exclusion individually.
In both years, plant species diversity, equitability and dominance were similar in all treatments, although forb abundance decreased slightly and graminoids increased slightly in1986. Results may have been influenced by precipitation and grazing intensity. For example, in 1986 precipitation was above average, standing crop increased and bison use was more intensive.
Average graminoid leaf nitrogen concentration declined slightly (but significantly) after prairie dog removal (1.5 to 1.4%) in 1985, and after bison exclusion (1.6 to 1.5%) in 1986. In forbs it did not change significantly in response to treatment (across treatments averaging 2.1% in 1985, 1.8% in 1986).
Results suggest that grazing by prairie dogs and bison (and other ungulates) were of about equal importance in maintaining the vegetative characteristics of the prairie dog colony.
If using or referring to this published study, please read and quote the original paper, this can be viewed at: https://www.uair.arizona.edu/holdings/journal/issue?r=http://jrm.library.arizona.edu/Volume44/Number2/