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Eric D. Salo, SDSU Wildlife and Fisheries, Brookings, SD; Kenneth F. Higgins, USGS/BRD-SD Cooperative Fish and Wildlife Research Unit, Brookings, SD; William T. Barker, Animal and Range Sciences, NDSU, Fargo, ND; and Kristel K. Bakker, Natural Resource Department DSU, Madison, SD.
Photography by Rick Bohn
Research suggests that grassland birds are the most rapidly declining group of endemic bird species in North America (Knopf 1994, Herkert 1995). According to the North American Breeding Bird Survey data from 1966 to 1993, 70 percent of the 29 species, which are considered characteristic of North American prairies, have experienced population declines (Fuhlendorf and Engle 2001). Habitat loss, alteration, and fragmentation associated with many land use practices are suggested as factors in these declines. In North Dakota, agricultural practices like crop and livestock production are the major land uses. Research has shown that cropland provides very few benefits for most species of breeding birds, while grazing lands, when managed properly, can provide benefits for both wildlife populations and the producer (Barker et al. 1990).
Historically, grazing was a natural process that occurred in most of the Northern Great Plains. Grassland birds are a diverse group of wildlife that was likely to have been influenced by this natural grazing. Knopf (1996) suggested that all of the bird species endemic to prairies evolved within a grazed grassland mosaic with a gradient from idle areas to excessively disturbed areas. Furthermore, research has shown that some species seem to benefit from the effects of grazing while other species show negative, or uncertain responses to grazing.
The purpose of this study was to determine the effects of four levels of grazing intensity and the temporal effects of grazing systems on nongame birds. To do this we repeated two previous studies that evaluated the effects of grazing systems in south central North Dakota: one in Conservation Reserve Program (CRP) grasslands (Kennedy 1994) and one in native mixed-grass prairie (Messmer 1990).
All grazing treatments were on CRP tracts or mixed-grass prairie lands managed by the North Dakota State University Central Grasslands Research Extension Center (CGREC). The CGREC is located about 14.4 kilometers (9 miles) northwest of Streeter, North Dakota, on the border of Stutsman and Kidder counties. Native prairie idle areas from previous studies were subjected to management changes so new areas within close proximity to CGREC were selected to compare nongame bird and vegetation data with each of the native prairie grazing treatments. These areas were idle for three years or more and are located on waterfowl production areas managed by the U.S. Fish and Wildlife Service’s Kulm Wetland Management District and Long Lake National Wildlife Refuge.
Conservation Reserve Program Grazing Systems
The CRP demonstration project is located in Stutsman County about 3.2 kilometers (2 miles) west of Streeter and 8 kilometers (5 miles) southeast of the CGREC. Vegetation planted in the CRP plots in 1985 included a mixture of tall wheatgrass (Agropyron elongatum), intermediate wheatgrass (A. intermedium), smooth bromegrass (Bromus inermis), yellow sweet clover (Melilotus officinalis), white sweet clover (M. alba), and alfalfa (Medicago sativa). Treatments in the CRP study area include a three-cell rotation system, a season-long pasture, and an idle area. Cattle are released onto these treatments around mid May and are removed in mid September.
Native Prairie Grazing Systems
The native mixed-grass prairie grazing treatments studied by Messmer (1990) included: season long, short duration, and twice-over rotation grazing systems. The grazing season typically began about mid May each year and ended around late October or early November. Only the twice-over rotation grazing system is managed similar to how it was managed during previous studies. Since then, the short duration and season long study sites have experienced some minor changes in management regime, which is typical of privately owned rangelands. Producers might often change management practices to account for weather conditions and fluctuating herd sizes. Even though management was slightly modified, these sites still give an indication of the temporal effects of grazing systems.
In 1989, the CGREC started conducting research to determine how grazing intensity affected livestock and forage production (Patton et al. 2001). For the grazing intensity study the CGREC set up four grazing treatments including light, moderate, heavy, and extreme treatments. Each treatment is replicated three times in pastures of about 12 hectares (30 acres) in size. Each year pastures are stocked so 35, 50, 65, and 80 percent of the forage produced in an average year is removed in the light, moderate, heavy, and extreme treatments, respectively (Patton et al. 2001). Stocking dates for the grazing intensity treatments are from mid to late May, with release and removal dates adjusted yearly to keep utilized forage within the desired percentage for each treatment.
Bird surveys were conducted three times each year along permanent, systematically placed belt transects with a fixed length of 100 meters (109.4 yards) to 200 meters (218.7 yards) and a width of 100 meters (109.4 yards). In order to minimize the potential for edge effects, transects were placed 30 meters (32.8 yards) or greater from wetlands and pasture borders. Surveys were conducted from around sunrise until 3 to 4 hours after sunrise and only on days when wind speeds were less than 20 km/hr (12.4 mi/hr) and there was little or no precipitation. Survey transects were walked at an approximate speed of 1.0 km/hr (0.62 mi/hr) to 1.5 km/hr (0.93 mi/hr) and birds present in the belt transect were recorded using visual and auditory cues. After each bird survey transect was completed, a 15 minute “walk-about” around the treatment area was conducted to document all nongame bird species present in the treatment area, including the belt transect area.
Vegetation data were collected at 25 stations, spaced 8 meters (26.2 feet) apart, parallel to the permanent bird survey transect. Vegetation structure was characterized by measurements taken to the nearest quarter decimeter, on a modified Robel pole (Robel et al. 1970). Visual Obstruction Readings (VOR) were taken at the point of 100 % visual obstruction of the pole, at a distance of 4 meters (13.1 feet) and a height of 1 meter (3.3 feet), and recorded from each of the four cardinal directions around the pole. Litter depth was also measured and recorded in millimeters.
The most abundant species surveyed in native prairie study sites during 2001-2002 were the Clay-colored Sparrow (Spizella pallida), Grasshopper Sparrow (Ammodramus savannarum), Chestnut-collared longspur (Calcaris ornatus), and Savannah Sparrow (Passerculus sandwichensis), which comprised 37.5, 18.2, 9.5, and 7.0 percent of the nongame birds surveyed, respectively. In the CRP study area, the most abundant species included the Savannah Sparrow, Bobolink (Dolichonyx oryzivorus), Grasshopper Sparrow, and Clay-colored Sparrow, which comprised 29.1, 27.8, 21.2, and 13.9 percent of the birds surveyed, respectively. Species richness was higher for all study areas in native prairie than in CRP treatment fields and was also higher during all of the previous studies than the current study (Table 1).
The Effects of Grazing Systems in CRP
Species richness was lower for birds surveyed in the CRP rotational, season long, and idle fields during the current study period compared to previous studies (Table 1). For both study periods, species richness was higher in the rotational grazing treatment than in the idle and season long treatments (Table 1). Average breeding bird densities were also lower in the season long and rotation treatments, while they increased slightly in the idle treatment (Table 2). The Savannah Sparrow, Bobolink, and Grasshopper Sparrow have remained the most common species in the CRP study areas between the two study periods; however, the rank order in species percent composition has changed. The most common species in the rotation, season long, and idle treatments during the previous studies, was the Grasshopper Sparrow (Table 3). During the current study, the Savannah Sparrow was the most common species in the season long and idle treatments, while the Bobolink was the most common in the rotational treatment (Table 3). Between the previous and current study periods, mean VOR values were lower in the rotation and season long treatments, but higher in the idle treatment (Table 4).
The Effects of Grazing Systems in Native Mixed-Grass Prairie
Species richness values for birds surveyed in the native prairie grazing systems also decreased for all treatments from the previous to current study periods (Table 1). As with the CRP study area, species richness was higher in the twice-over rotation treatment than in the other treatments (Table 1). Decreases in average bird density in the native prairie grazing systems study area were also recorded for all grazing regimes (Table 2). The Grasshopper Sparrow remained the most common species in the twice-over rotation grazing system during both study periods (Table 5). However, the Chestnut-collared Longspur went from one of the least abundant in twice-over rotation to the second most abundant species in percent composition (Table 5). The Clay-colored Sparrow was the dominant species in the season long, short duration, and idle treatments for both the previous and current study periods (Table 5). Percent composition of Clay-colored Sparrows also increased in these treatments, with percent composition doubling in both the season long and short duration treatment areas (Table 5). Average VOR values were lower on the twice-over rotation and idle treatments for the current study periods than during previous study periods, while season long and short duration have remained about the same (Table 4).
The Effects of Grazing Intensity
Species richness in the grazing intensity study area was greatest in the extreme treatment and lowest in the low and idle treatments (Table 1). However, breeding bird density was highest in the low grazing intensity and lowest in the extreme grazing intensity (Table 2). Clay-colored and Grasshopper Sparrows made up the highest percent composition of breeding birds in the idle, low, and moderate grazing treatments (Table 6). However, the percent of Chestnut-collared Longspurs and various shorebirds increased with grazing intensity (Table 6). As one would probably expect, average values for litter depth and VOR decreased with increased grazing intensity (Table 4).
Much of the native grasslands left today are located on private lands that are used solely for livestock production. Based on preliminary analysis, our results indicate that whether in CRP or native mixed-grass prairie, different grazing regimes affect most grassland bird species differently. As reported in other studies throughout the Great Plains, we found that some species will increase or decrease in abundance because of the effects of grazing on grassland habitats. Our data indicates that these responses are not necessarily detected immediately, but some are more dependent on the length of time that these grazing regimes are in operation. A final report of findings for this study will be presented in the form of a Master of Science thesis.
Funding for this project was provided by the Renewable Resource Extension Act (RREA) funds via South Dakota State University with Larry Tidemann’s assistance. Special thanks goes to Dennis Whitted, Jay Volk, and other staff from the NDSU Animal and Range Science Department for the many hours they spent in the field collecting vegetation data. We would also like to thank Paul Nyren, Anne Nyren, Bob Patton, and the rest of the staff at Central Grasslands Research Extension Center for their hospitality and assistance through the 2001 and 2002 field seasons, as well as Dr. Timmothy Wittig from the SDSU Mathematics and Statistics Department for consulting us during this project. We would also like to extend our appreciation to Ed Meendering and Natoma Buskness from the U.S. Fish and Wildlife Service Kulm Wetland Management District and Long Lake National Wildlife Refuge for assistance in finding Waterfowl Production Areas to use during this project. The South Dakota Cooperative Fish and Wildlife Research Unit and the Wildlife and Fisheries Sciences Department at South Dakota State University, Brookings, SD, provided additional support for this project
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