Comparing Sustainability of Grazing in the Nebraska Sandhills: Which Regime is Best for Cattle and Wildlife
Initial results examining the sustainability of three grazing systems suggest that the systems studied exhibit system-specific benefits for the grassland bird community in the Sandhills. Therefore, management recommendations that complement wildlife populations in addition to being economically viable to the producer may vary depending on specific wildlife management and individual ranch goals. Managing rangelands in part for wildlife habitat may result in economic return from fee hunting, small-scale tourism-based industries such as wildlife watching, in addition to long-term production from the land.
Our objective is to determine the potential of long, medium, and short duration grazing systems to provide for sustainable grassland wildlife populations. By using birds as bioindicators, determine how grassland wildlife communities respond to different habitats provided by grazing systems in the Nebraska Sandhills. Measuring the following variables facilitated this objective:
1)bird community species richness
2)grassland bird abundance and productivity
2002: Initial work began by securing permission and establishing relationships with private ranchers in the Sandhills region. The Sandhills TaskForce, a group of private landowners and conservation agency representatives dedicated to the sustainable management of the grassland and wetland resources of the region helped facilitate this process. Treatments of long, medium, and short duration grazing systems were loosely defined according to length of grazing duration; 3+ months, approximately 1 month, and 2-5 days, respectively. Eight ranchers were selected from those who agreed to participate in the study and whose operations fit within our treatment definitions and geographic restrictions. Surveys were conducted on three replicates of each grazing system for a total of nine pastures.
A total of 30 species were recorded. Bird species richness was greatest (29 species) on long duration pastures. Medium and short duration pastures had species counts of 19 and 13 species, respectively. Bird density was greatest (12.33 birds/hectare) on short duration pastures, but not significantly so. A total of 58 nests were monitored. Species most frequently found nesting in the study pastures were grasshopper sparrows (11), Western meadowlarks (10), lark sparrows (9), and mourning doves (9). Passerine nests comprised 67% of the nests found. Of the 39 passerine nests monitored, 28% were successful (fledged > 1 young). Daily nest success by system was highest (98%) on medium duration pastures, but not significantly so. Long and short duration pastures both had approximately 93% DNS rates. Pooled nest success was 94%. Common nighthawks and lark sparrows exhibited the highest DNS, 99% and 98% respectively. Two nests (3.4%) were apparently unsuccessful as a result of cattle damage.
Visual obstruction readings (VOR), used to determine height and density ranged from an average of 0.44 dm on long duration pasture to an average of 0.34 dm on short duration pastures. One measurement, recorded to the nearest 0.5 d m was recorded from the Robel pole to obtain VOR. The lowest average litter depth value among systems was 0.05 cm on long duration pastures. Short duration pastures had the deepest average litter depth readings (0.11 cm). Litter depth measurements were taken to the nearest cm. All vegetation measurements were taken approximately during the early, middle, late portions of the field season (June 4-July 22).
2003: Based on our pilot season, we adjusted our data collection methods. Study pastures were classified according to ecological site and range condition. Additional VOR readings allowed us to further analyze data on multi-landscape scale. Bird survey methods remained unchanged.
Early in the year, grazing information was collected for the study pastures to determine AUMs for the 2002 grazing season and reconfirm landowner participation in the study. Working with the same producers, we secured permission for 3 additional pastures, which increased our treatment replication to four, for a total of 12 study pastures. A field report summarizing the data collection efforts from the summer of 2003 was distributed to participating ranchers and Sandhills Taskforce members.
A total of 53 species were recorded during the field season. Long, medium, and short duration pastures had species counts of 39, 41, and 38, respectively. Overall bird density was significantly greater on medium duration pastures (15.6 birds/hectare). A total of 87 nests were monitored. Grasshopper sparrows (26), lark sparrows (20), Western meadowlarks (15), and mourning doves (11) were the species found most frequently nesting on the study pastures. Occurrences of lark buntings (sightings and reproductive efforts) recorded in 2002 were not detected in 2003. Seventy-one percent of the nests found were passerine with raw nest survival of 39%. Overall, daily nest success on short duration pastures was significantly higher (93.0%) compared to long (90.4%) and medium (92.5%) duration pastures. Pooled nest success was 92%. Out of all the nesting species, common nighthawks (96%) and Western meadowlarks (92%) had the highest DNS. As in 2002, only two nests (2.3%) were apparently destroyed as a result of cattle activity.
Average VOR readings, in decreasing order, were found on short (0.34 dm), long (0.31 dm), and medium (0.29 dm) duration pastures. A VOR reading was defined as the average of four readings taken from the cardinal directions measured to the nearest 0.25 dm. Three VOR readings were taken at each sampling unit along the transect. This will permit calculation and analysis of the vegetation structure heterogeneity at both large- (pasture) and small- (2 m radius) scales. Methods loosely follow that of Wiens (1974). Average litter depth was deepest on medium duration pastures (0.45 cm). Long duration pastures were somewhat similar with an average litter depth of 0.41 cm. Short duration pastures had an average of 0.31 cm of litter depth. In 2003, two additional litter depth measurements were taken at each sampling unit. All litter depth measurements were taken to the nearest 0.5 cm. Vegetation sampling occurred once during June 16-26 and again during July 29- Aug 5.
Initial evaluation of our multiple landscape scales showed a positive relationship between small- and large-scale heterogeneity. Long and medium duration pastures become more heterogeneous, while short duration pastures become more homogeneous over time. Preliminary analysis using 2002 bird data suggests that species richness exhibits a positive relationship with large-scale heterogeneity on grazed pastures, while DNS exhibited a negative relationship with large-scale heterogeneity.
Results from 2004 will help to solidify project conclusions and management recommendations as well as differentiate drought vs. treatment effects. After study completion and thorough data analysis, gleaned information will be distributed to ranchers through the Sandhills TaskForce.
Wiens, J. A. 1974. Habitat heterogeneity and avian community structure in North American grasslands. The American Midland Naturalist 91:195-213.
Impacts and Contributions/Outcomes
This research is helping to fill an information void regarding the relationship between grazing and grassland birds, particularly in the Sandhills, the largest continuous tract of grassland south of Canada. It is helping to provide the basis for making informed range management decisions. Although economic benefit is not quantifiable at this stage, agencies such as the Sandhills TaskForce will use this information to help allocate their project monies more efficiently to ranch management improvement projects. These projects financially assist the private producer while helping to sustain the grassland environment. Wildlife managers also may also use information from this study when using grazing as a wildlife habitat management tool.