Final Report for LNC07-280
Short term outcomes include: 1) knowledge of the rate of vegetation change following removal of prairie dogs, the impacts of grazing strategies on those changes, and the economic costs and benefits, 2) 200 ranchers and the scientific community will be informed of post-prairie dog vegetation recovery rates, and 3) 12 producers will measure forage change after prairie dog control. Intermediate outcomes include: 1) improved forage production for 12 cooperating ranches and 2) adoption of appropriate post-prairie dog control grazing management strategies by 25 regional ranchers. Thousands of acres of prairie dogs are undergoing control measures in the western region of the North Central Region, however there is scarce information on the rate of improvement of vegetation to be expected, appropriate grazing management strategies, and overall economic consequences. The focus of this project was on vegetation change on prairie dog towns where control measures occurred in fall/winter 2007-08 on 6 cooperator pastures, where 3 were grazed in summer and 3 grazed in fall/winter 2008-2011. Exclosures were established on the former prairie dog town in each pasture to examine the non-grazing option. Vegetation change over 4 years for each grazing strategy were evaluated. An additional 7 pastures were added to the study in 2011. Three of the pastures had active prairie dog towns, and the remaining 4 had been without prairie dogs for approximately 9 years (2 pastures) and 25 years (2 pastures). Differences in 2011 vegetation composition were evaluated between active prairie dog towns, short-term prairie dog-free (4 years grazed summer, winter, or not grazed) towns, moderate-term prairie dog-free (9 years) towns, and long-term prairie dog-free (25 years) towns. Results of the study have been presented at field days held on cooperator sites, at research station field days, and at scientific meetings. The results of this study have also been presented, by request, at a variety of meetings with ranchers, agency personnel, and scientists. Journal publications, Extension publications, and SDSU Beef Reports are being prepared for publication.
The black-tailed prairie dog (Cynomys ludovicianus Ord) is a native rodent found throughout the short- and mixed-grass prairies of North America. It is clearly valuable as a component of these grassland ecosystems, providing habitat to a number of plant and animal species. It is also viewed as a destructive pest when its populations are large. Prairie dogs clip vegetation short within their towns to enhance their ability to see predators (King 1955). This results in a shift of the plant community to a low seral stage, which, in the mixed-grass prairie, is represented by species of shorter stature, an increase in annual forbs and grasses, and an increase in species with greater grazing tolerance and/or lower palatability. As a result, less forage is available to large ungulates, such as bison and cattle (Koford 1958, Coppock et al. 1983, Bonham and Lerwick 1976, Archer et al 1987, Stoltenberg et al. 2005b). Prairie dogs were determined to be a candidate for threatened species (under the Endangered Species Act) by the U.S. Fish and Wildlife Service in 2000 (USDI USFWS 2000) but were removed from that status in August 2004. A state management plan that ensures maintenance of an acceptable minimum of prairie dog acres in South Dakota was approved by the South Dakota legislature, thereby allowing excess acres in the state to be available for prairie dog control. The South Dakota Game, Fish and Parks Department initiated prairie dog control measures for the state and for federal agencies in fall 2004 with the goal of providing ranchers with relief from losses of livestock forage due to prairie dogs that had migrated from federal onto private lands during a federal moratorium on their control. Additional control measures were implemented by the state in fall 2005. Where control measures have occurred, the expectation is that rangelands which are now characterized by extensive bare ground, annual forbs and grasses, and few desirable perennial grasses due to prairie dog occupation (Stoltenberg 2004, Stoltenberg et al. 2005a) will return to a more stable perennial, grass-dominated composition with improved ground cover for protection from erosion and greater production of livestock forage. The potential benefits for ranchers are great. Stoltenberg (2004) and Stoltenberg et al. (2005b) demonstrated that cattle were able to obtain only half as much forage/ha on ecological sites with prairie dogs compared to the same kinds of sites without prairie dogs, and Beutler et al. (2005) estimated the loss to producers at $10.40/ac of prairie dogs. Very little is known, however, about the rate at which desirable species will return following prairie dog control and the consequences of management strategies on that rate. Although ranchers have a number of management options that they may employ following prairie dog control, including seeding, rest, fire, and grazing, the biological effectiveness and economic consequences of those options are unknown. We know of only one study that has attempted to address the question of the impact of cattle grazing on plant community recovery following prairie dog control in mixed-grass prairie (Uresk 1985). That study concluded that prairie dog control did not increase production of individual plant species over a 4-year time period, regardless of whether or not the site was grazed by cattle. The design of that study, however, especially the lack of information regarding the similarity of soils across treatments within a site, limits the strength of that conclusion. Clearly, additional work is needed to evaluate the response of rangelands following prairie dog control regarding ground cover, species changes, and forage production, and to determine the impacts of post-control treatments on vegetation change. It will also be important to evaluate the economic consequences of prairie dog control so that ranchers can make informed choices regarding the management of rangelands with prairie dogs.
While South Dakota maintains the largest acreage of prairie dogs in the United States, they are also abundant in the western portions of other North Central Region states (North Dakota, Nebraska, and Kansas). Recent drought in the region and a moratorium on control measures resulted in rapid expansion of prairie dogs onto private rangelands, significant economic costs to ranchers, and a reduction in ranchers’ ability to maintain sustainable enterprises. NCR-SARE recently funded research that provided critical information on the impacts of prairie dogs on ranches in the region (NCR-SARE project #GNC 03-021, resulting in Stoltenberg 2004 and Stoltenberg et al. 2005a, 2005b). Beutler et al. (2005) used those results to determine the cost of prairie dogs to ranching operations. We have a unique opportunity, due to ongoing prairie dog control efforts by the state of South Dakota and other entities, to provide ranchers with additional critical information on the response of rangelands to prairie dog control. Many projects involving range management and cattle have been funded by NCR SARE, but only the Stoltenberg project (NCR-SARE project #GNC 03-021) has dealt specifically with interactions between prairie dogs and cattle. The importance of prairie dogs to the sustainability of agriculture in the western states of the North Central Region (ND, SD, NE, KS) makes this project very important to NCR SARE.
The goal of this project was to evaluate plant community and forage production changes on prairie dog towns following prairie dog control, and to determine the effect of grazing on those changes. Specific objectives of this project were:
1. Evaluate the impact of grazing (none, growing season, non-growing season) on changes in plant community on former prairie dog towns, including
a. Total biomass production
b. Changes in biomass and species richness for perennial and annual species
2. Evaluate differences in plant communities on active towns and towns from which prairie dogs were removed for 4 years and greater.
This project was conducted in southwestern South Dakota. The climate of the region is continental and semiarid with hot summers and cold winters. Annual precipitation varies with location, and ranges from 35 cm to 53 cm per year, with approximately two thirds of the precipitation occurring from April through September (U.S. Department of Commerce 2003). This study was conducted on native rangeland pastures on sites with current or former prairie dog towns located on Clayey Ecological Sites. Pastures utilized in the study include both private and federal (U.S. Forest Service National Grasslands) rangelands. The study was conducted in two phases. In the first, prairie dog towns that were controlled (prairie dogs removed) in fall 2007/winter 2008 were used to evaluate vegetation change over a 4-year period for 3 post-control grazing strategies: non-grazed (Control), growing season grazed (GS), and non-growing season grazed (NGS). GS and NGS treatments were established on 3 pastures each; study sites were established on the former prairie dog town on each pasture. Within each study site, 3 sub-sites were identified (providing subsampling locations), each of which included a grazed area and an exclosure to provide the paired Control treatment. All grazing occurred with cooperator livestock at moderate stocking rates (based on Natural Resources Conservation Service (NRCS) recommendations). In the second phase, active prairie dog towns (n=2), towns controlled for approximately 9 years (n=2), and towns controlled for 25+ years (n=3) were located. Three sub-sites were established on each town/former town for data collection. Phase 2 was established to evaluate longer-term changes in plant communities following prairie dog removal.
This study was not involved in prairie dog control efforts, but utilized sites where control measures had already been implemented. No seeding, chemical or mechanical treatments were applied to study pastures.
Phase 1: Ten plots (0.25m2) were established in summer 2008 at each grazed sub-site on all pastures. Ten plots (0.25m2) were also established in each exclosure. All plots in the grazed area and in the exclosures were sampled non-destructively in summer 2008 to establish initial conditions and in summers of 2009, and 2010 to evaluate changes over time. Plots were sampled destructively in 2011 to provide a final estimate of vegetation characteristics following 4 years of prairie dog exclusion. Data collected include a complete species list, percent bare ground and litter, and the cover, biomass, and height of each species in each plot. Non-destructive estimation of biomass included the use of reference units (Andrew et al. 1979, 1981), cover, and volume (Johnson et al. 1988), as appropriate for each species.
Phase 2: Ten plots (0.25m2) were established in summer 2011 at each sub-site on pastures with active prairie dog towns, and on pastures with towns on which prairie dogs had been controlled for 9 years or 25+ years. All plots were sampled destructively in summer 2011 to provide an estimate of vegetation characteristics. Data collected include a complete species list, percent bare ground and litter, and the cover, biomass, and height of each species in each plot.
Species frequency and biomass were averaged within the grazed and exclosure plots for each subsite. Because the grazed area and exclosure for each subsite were purposely paired, data were analyzed using paired t tests, providing 9 replicate differences for GS and 9 for NGS sites. Analyses included comparisons of total species as well as plant functional groups (e.g. native species, introduced species, introduced annual grasses, etc.). Significance was determined for P ? 0.1.
At first glance, one might assume that protection from livestock grazing following removal of prairie dogs should result in greater plant species richness by allowing species to re-inhabit areas in response to substantial reductions in grazing by both wildlife and livestock. However our data indicate that, 4 years post-prairie dog removal, the total number of species resident on grazed treatments was greater than on their associated non-grazed Controls for both summer grazed (GS) and winter grazed (NGS) treatments (Table 1). This difference in species richness is explained almost entirely by a greater number of native species on grazed sites, as there were no differences between grazed and control for introduced species for either the GS or NGS treatments. These results suggest that grazing may be useful for, or at least not harmful to, improving the likelihood that native species will re-inhabit heavily disturbed sites. The difference in native species richness between grazed and control treatments is largely explained by a greater number of native annual species, both grasses and forbs, on grazed sites. Interestingly, there were no differences between grazed and non-grazed treatments for introduced (non-native) species.
It is expected that, over time, removal of prairie dogs should result in greater native and perennial species richness as native perennial grasses and forbs re-establish on those previously degraded sites. This was observed in this study for the early (4 year) transition from active prairie dog towns to prairie dog-free plant communities (Table 2). The data, however, suggest that sites with longer protection from prairie dogs may experience an overall drop in species richness after the first 4 years. This is consistent with research from other ecosystems, in which greatest species diversity was observed shortly after disturbance was released, and declined as the plant community progressed to an intermediate seral stage. In forested ecosystems, diversity then increases somewhat as the climax plant community becomes dominant on the site (e.g. Bormann and Likens, 1985; Sprugel, 1976).
Richness of native perennial species was fairly stable across treatments, compared to much greater fluctuations in native annual species (Table 2). Native annual species appear to inhabit disturbed sites early in succession and dwindle in occurrence as higher seral species begin to dominate. This same pattern does not appear to be true for introduced annual species, however. Richness of introduced species, and in particular introduced annual species, was fairly stable across the treatments (Table 2). Certainly these data do not represent a true time series, however they do suggest that introduced annuals present at the time of disturbance are likely to remain on the site (though perhaps at reduced cover or biomass) for a very long time. Soil seed bank reserves likely maintain a considerable introduced annual component.
Care should be taken when interpreting the data presented here for the active, 9 Year, and 25 + Year sites. The comparability of initial plant community conditions with the GS and NGS sites is unknown. Comparability of climatic conditions and grazing impacts for the GS, NGS, 9 Year, and 25+ Year sites during the first 4 years post-prairie dog are also unknown. Ideally, the GS and NGS sites should continue to be followed over time to evaluate actual changes in plant community factors, including species richness.
As might be expected, current year (2011) biomass in the non-grazed controls was overall greater than in the grazed areas for summer grazed pastures (see Total Species, Table 3). Grazing was occurring on the pastures during sampling, so a reduction in standing biomass is to be expected. We did not, however, expect the biomass for non-grazed controls to be greater than the grazed areas of winter grazed pastures. These data suggest that livestock grazing (both growing season and non-growing season) reduced the productive capacity of the sites compared to 4 years of rest. Reduced biomass, and, presumably, reduced production, was evident on grazed sites for nearly all of the native functional groups for both summer and winter grazed pastures compared to non-grazed controls (Table 3). There were, however, no differences between grazed and non-grazed sites for any of the introduced species functional groups (Table 3). This suggests that production of introduced species was unaffected by grazing treatment. Introduced species were dominated by annuals, and it is likely that production of introduced annuals is much more sensitive to precipitation fluctuations than grazing.
Standing biomass for sites ranging from active prairie dog towns and through 4 years (both GS and NGS), 9 years, and 25+ years of protection from prairie dogs demonstrates the expected increase in biomass after removal of prairie dogs from a site (Table 4). Active towns supported about half the forage of sites protected for 4 years, and even less compared to sites protected for 9 or more years. The biomass for the 9 Year sites is greater than for all others in this study. It is likely, however, that this is related to site differences rather than demonstrating an actual reduction in biomass as sites are protected for > 9 years.
The vast majority of biomass on all sites was produced by native species (Table 4). Interestingly, introduced species biomass, representing a fairly minor component of total biomass, was relatively similar across all treatments. This relates well to the species richness data (Table 2), in which introduced species richness is fairly stable across all treatments. Introduced annuals made up the bulk of the introduced biomass for all treatments except the active towns, where introduced perennials were the greatest component (Table 4).
The data from this study are being evaluated by a number of agencies and producers. We are in the process of presenting the results of this study at a variety of meetings to inform producers so that they are able to make decisions regarding how to manage pastures that have had prairie dogs reduced either through control measures or disease.
The economic analysis of the data from this study are ongoing. We expect that there will be a cost associated with the grazing of pastures following prairie dog control, however it appears that those losses are minor compared to the cost of idling pastures for 4 years.
Because dissemination of the results of this study is in its earliest stages, farmer adoption is minimal. We expect, however, that this study will provide many ranchers in the region with valuable insights into how to manage pastures with former prairie dog towns, and that adoption of our recommendations will be strong.
Educational & Outreach Activities
The results of this study are being disseminated through a variety of outlets, including reports, field days, and scientific publications. The publications are being developed currently, and will be submitted at the earliest date possible.
Areas needing additional study
Longer-term studies regarding vegetation change on rangelands in the North Central Region are sorely needed. Studies lasting only 3 to 4 years cannot address the true trajectories of change that occur as management changes on native grassland ecosystems. These ecosystems are crucial to the sustainability of agriculture in the region and provide countless goods, services, and benefits to ranchers as well as to those living in the towns and cities of the region.