This study determined the difference in disappearance of vegetation between cattle and prairie dogs on pastures with prairie dog towns. During the summers of 2002 and 2003, data were collected on pastures in south-central South Dakota. Forage removed by prairie dogs on the on-town sites was three times as great as forage removed by cattle on the on-town sites for the June and July sampling periods. Cattle removed two times more forage on off-town sites than on on-town sites. Stocking rates on pastures with prairie dog towns should be adjusted to account for forage disappearance due to prairie dogs.
The black-tailed prairie dog (Cynomys ludovicianus) is a native rodent found throughout the shortgrass and mixed-grass prairies of North America. The presence of prairie dog towns and their effect on forage availability for large ungulates, such as cattle, has been very controversial. Ranchers contend that prairie dogs severely reduce forage available to their livestock, increase weeds, and increase erosion, while other groups claim that a variety of studies suggest that prairie dogs have a beneficial or neutral effect on livestock (National Wildlife Federation, 1998).
It is known that prairie dogs clip vegetation in addition to what they eat in order to see predators. Laboratory studies reveal how much prairie dogs eat, yet little work has been done to quantify total disappearance of vegetation attributed to prairie dogs. The level of competition between cattle and prairie dogs is another area of concern. Cattle and prairie dogs have similar diets, which consist of approximately 87% graminoids (Uresk, 1984; Uresk, 1986) suggesting considerable overlap and competition between the two herbivores. Competition between cattle and prairie dogs may create a problem when stocking cattle on pastures supporting prairie dog towns. The objective of this study was to determine the difference in disappearance of vegetation between cattle and prairie dogs on pastures with prairie dog towns.
The objective of this study was to determine the difference in disappearance of vegetation between cattle and prairie dogs on pastures with prairie dog towns.
The project was carried out in south-central South Dakota on mixed-grass prairie rangelands. The climate of the region is continental and semiarid with hot summers and cold winters. Annual precipitation averages approximately 19 inches near Mission, SD (NOAA, 2000) and over half the yearly precipitation falls during the growing season. The soils in the study area are predominantly silts and loams (USDA-SCS, 1974). Vegetation is typical of the mixed-grass prairie with a variety of warm- and cool-season species. Common grass species in the area include western wheatgrass (Pascopyrum smithii [Rydb.] A. Love), little bluestem (Schizachyrium scoparium (Michx.) Nash), needleandthread (Stipa Comata Trin. & Rupr.), prairie sandreed (Calamovilfa longifolia [Hook.] Scribn.), blue grama (Bouteloua gracilis (H.B.K.) Lag. Ex Griffiths), and buffalograss (Buchloe dactyloides (Nutt.) Engelm.) (USDA-SCS, 1974).
The project involved three mixed-grass prairie pastures grazed by cattle with varying degrees of prairie dog town coverage (percent of pasture area). Data collection took place on the three pastures during the summers of 2002 and 2003. Vegetation samples were collected during two sampling periods in each year: late spring (June) and mid-summer (July) in an effort to adequately sample both the cool- and warm-season species common to the region. Two sites were selected on each study pasture. One site was the prairie dog town (on-town site). The second site (off-town site) was near the on-town site, but had no prairie dog activity. These off-town sites were chosen based on similarity of soils with their paired on-town site. Soils on all on-town and off-town sites were evaluated by a Natural Resources Conservation Service soil scientist.
It was important to this study that the soils were matched within each pair of on- and off-town sites, because sites with the same soils have the same potential for production of plant communities and biomass. Differences in species and/or production can then be more closely tied to differences in use. On-town sites in this study were dominated by annual grasses and forbs as well as by the shorter perennial grasses including blue grama, buffalograss, sedge species (grouped as shortgrasses), and red threeawn. Off-town sites were dominated by perennial forbs and grasses including the shortgrasses, needleandthread, and prairie sandreed.
Three types of plots were used in this study: 1) uncaged plots available for grazing by both prairie dogs and cattle (NONE EXCL), 2) plots fenced/caged so as to exclude cattle but allow use by prairie dogs (CATTLE EXCL), and 3) plots covered by cages which excluded both prairie dogs and cattle (ALL EXCL). All plots were 0.25 m2.
Forty NONE EXCL plots were established on each prairie dog town and 20 on each off-town site. These plots were “permanent”, that is, the same plots were used for all sampling periods. To avoid marking these plots with stakes or flags which would attract herbivores to them, we obtained a GPS location for each, marked the corners of each plot with metal stakes driven into the ground, and then used those coordinates and stakes to relocate each plot at each sample date.
Ten CATTLE EXCL plots were established on each prairie dog town. These were fenced by creating an exclosure with barbed wire and fence posts to ensure availability to prairie dogs (we were concerned prairie dogs might not go into exclusion cages, even though the wire mesh would have permitted their entry). Another 10 CATTLE EXCL plots were established on each off-town site. These were covered with an exclusion cage constructed with a large mesh wire over a metal frame.
Ten ALL EXCL plots were established on each prairie dog town; none were established on the off-town sites since these sites were not utilized by prairie dogs. The ALL EXCL plots were covered with an exclusion cage constructed of chicken wire mesh over a metal frame to prevent access by prairie dogs and cattle.
At the end of each 3-week sampling period (one in June of each year and one in July of each year), cages were removed and all plots were estimated for biomass by species. Estimates for aboveground biomass by species were obtained using two methods. For all plots in 2002 and the NONE EXCL plots in 2003, double sampling methods were used appropriate for each species, including the use of cover, reference units (Andrew et al., 1979, 1981), and plant volume (Johnson et al., 1988) as estimators. In 2003, plots underneath the movable cages were clipped and sorted by species. Biomass estimations were made for individual species with the exception of two groups which were lumped together: blue grama, buffalograss, and sedge species were grouped together as shortgrasses and downy brome and Japanese brome were classified as annual brome.
Differences between ALL EXCL plots and the CATTLE EXCL plots on towns gave an estimate of forage disappearance (destruction + consumption) due to prairie dogs. Differences between ALL EXCL plots and NONE EXCL plots gave an estimate of total disappearance by all herbivores. By subtracting disappearance due to prairie dogs from total disappearance, disappearance due to large herbivores was estimated for on-town sites. For off-town sites, differences between CATTLE EXCL plots and NONE EXCL plots provided an estimate of forage disappearance due to cattle in areas not grazed by prairie dogs.
The experimental design for this study was a complete randomized block design (pastures are blocks) with three factors: year, season, and grazing treatment (prairie dogs on-town, cattle on-town, and cattle off-town). The dependent variable was forage disappearance. Analysis of variance was used to test the effects of grazing treatment, year, and year x grazing treatment on forage disappearance. Year and year x grazing treatment effects were analyzed with repeated measures using PROC MIXED with AR(1) covariance model structure. Treatment mean differences were separated using the LSMEANS statement with the PDIFF option in PROC MIXED.
Our study clearly demonstrates that prairie dogs are highly competitive with cattle, reducing the amount of forage available for livestock consumption throughout the growing season. Forage removed by prairie dogs (460 lb/acre) on the on-town sites was nearly three times as great (P<0.10) as forage removed by cattle (160 lb/acre)on the on-town sites during the June sampling period. Surprisingly, the apparently greater amount of forage removed by cattle on the off-town sites (330 lb/acre), compared with forage removed by cattle on the on-town sites in June was not statistically different, likely due to large variation during that sampling period. Additionally, prairie dogs on the on-town sites removed a similar amount of forage as cattle did on the off-town sites. During the July sampling period, forage removed by prairie dogs (320 lb/acre) on the on-town sites was approximately three times greater (P<0.05) than forage removed by cattle (110 lb/acre) on the on-town sites. Forage removed by cattle on the off-town sites was more than two times greater than forage removed by cattle on the on-town sites. This may indicate that the forage on a prairie dog town is less accessible and/or desirable to cattle than forage on similar off-town sites. Prairie dogs also removed a similar amount of forage on on-town sites compared to cattle on off-town sites. The combined periods of June and July showed that prairie dogs (760 lb/acre) removed nearly three times more forage than cattle (230 lb/acre)did on prairie dog towns, indicating a significant level of competition for forage between cattle and prairie dogs. Forage removed by cattle (520 lb/acre) on the off-town sites was more than two times greater (P<0.10) than forage they removed on the on-town sites. Finally, forage removed by prairie dogs on the on-town sites was significantly greater than forage removed by cattle on the off-town sites (P = 0.11). The greater forage disappearance during these two grazing periods due to prairie dogs versus forage disappearance due to cattle on on-town sites demonstrates that prairie dogs can significantly reduce the quantity of forage that is available to cattle. Total forage removed in the combined sampling periods on on-town sites (cattle + prairie dogs) was nearly twice as much as forage removed on the off-town sites (990 lb/acre vs 520 lb/acre). This level of forage removal on prairie dog towns greatly decreases the likelihood that the on-town plant community will shift species composition toward a more desirable plant community for livestock production. Less forage was removed by cattle on on-town sites than on off-town sites in this study. One explanation for this is that long-term prairie dog activity has shifted the species composition on on-town sites such that undesirable livestock forage species represent approximately 69% of the forage biomass. Red threeawn and annual brome made up 50% of the species composition on prairie dog towns, but these are not desirable livestock forage species due to their narrow window of grazing availability. However, on off-town sites, species desirable for livestock forage make up 84% of the forage biomass. A second explanation is that defensive clipping activity by prairie dogs during the growing season reduces the plant height of all grasses thereby limiting accessibility by cattle. For these reasons, livestock are not able to utilize as much forage on on-town sites and the total amount of forage available in the pasture is reduced. Literature Cited Andrew, M.H., I.R. Noble, R.T. Lange, and A.W. Johnson. 1979. A nondestructive method for estimating the weight of forage on shrubs. Austral. Rangel. J. 1:225-231. Andrew, M.H., I.R. Noble, R.T. Lange, and A.W. Johnson. 1981. The measurement of forage weight: three methods compared. Austral. Rangel. J. 3:74-82. Johnson, P.S., C.L. Johnson and N.E. West. 1988. Estimation of phytomass for ungrazed crested wheatgrass plants using allometric equations. J. Range Manage. 41:421-425. National Wildlife Federation. 1998. Petition for rule listing the black-tailed prairie dog (Cynom ys ludovicianus) as threatened throughout its range. NOAA. 2000. Climatological Data Annual Summary: South Dakota 2000. Vol. 105, No. 13. Uresk, D.W. 1984. Black-tailed prairie dog food habits and forage relationship in western South Dakota. J. Range Manage. 37:325-329. Uresk, D.W. 1986. Food habits of cattle on mixed-grass prairie on the northern Great Plains. Prairie Nat. 18:211-218. USDA-SCS. 1974. Soil Survey of Todd County, South Dakota. U.S. Dept. of Agric., Soil Conserv. Serv.
Educational & Outreach Activities
Stoltenberg, M.B, P.S. Johnson, L. Xu., and A. Smart. 2004. Effects of prairie dogs and cattle on vegetation disappearance on prairie dog towns in mixed-grass prairie. p. 193. In Abstracts: Rangelands in transition. Society for Range Management 57th Annual Meeting. Jan 24-30, Salt Lake City, UT.
Stoltenberg, M., P.S. Johnson, A.J. Smart, and Lan Xu. 2004. Contributions of prairie dogs and cattle to vegetation disappearance on prairie dog town in mixed-grass prairie. The South Dakota Center for Biocomplexity Studies Rushmore Regional Conference on Biocomplexity, August 11-12, 2004, at the Holiday Inn City Centre in Sioux Falls, South Dakota. Available Online at http://www.sdepscor.org/cbs/Abstracts.htm.
Stoltenberg, M.B, P.S. Johnson, A.J. Smart, and L. Xu. 2004. Effects of prairie dogs and cattle on vegetation disappearance on prairie dog towns in mixed-grass prairie. p. 94-98 In: Beef Cattle Research Report, South Dakota State University, Brookings, SD.
Stoltenberg, M.B. 2004. Effects of prairie dogs on plant community composition and vegetation disappearance in mixed-grass prairie. M.S. Thesis. South Dakota State University. Brookings, SD.
Our study clearly indicates that prairie dog towns reduce livestock forage available to cattle. In the case of this study, prairie dog town acres provided only half the forage to livestock as did the same sites without prairie dogs. This reduction likely involves both a shift in composition toward less desirable plant species and a reduction in accessibility due to clipping.
Traditional carrying capacity calculations are based on vegetation seral stage, which is primarily influenced by species composition. While such calculations would indicate a reduction in carrying capacity for acres of prairie dog town, the result would be an overestimate. Further adjustments must be made to account for the forage being removed by prairie dogs. Prairie dogs on cattle pastures should be treated in a similar fashion to other herbivores such as elk or insects that compete with cattle for forage. Without this adjustment, the risk of abusing the pasture, both on- and off-town, by overgrazing exists.
Not accomplished yet.
Areas needing additional study
There is a need to assess economics and strategies such as light stocking to reduce the effects of prairie dog town expansion.