OOS 46-2 - Cascading effects of herbivores on vegetation heterogeneity, grassland bird conservation, and livestock production in a rangeland social-ecological system

Friday, August 11, 2017: 8:20 AM
Portland Blrm 253, Oregon Convention Center
David J. Augustine, Rangeland Resources Research Unit, USDA-ARS, Fort Collins, CO, Justin D. Derner, USDA-ARS, Rangeland Resources Research Unit, Cheyenne, WY, Kristin P. Davis, Colorado State University and Cameron L. Aldridge, Fort Collins Science Center, U.S. Geological Survey, Fort Collins, CO

The western Great Plains of North America have a long evolutionary history of intense grazing by a diverse herbivore assemblage including colonial rodents and large ungulates. Today, the region continues to function as a grazing ecosystem where the intensity and spatiotemporal pattern of herbivory are largely driven by the economics of livestock production. Livestock managers and scientists have developed grazing strategies that sustain plant productivity and desirable species composition, but concerns exist about cascading effects on faunal diversity. We report on two landscape-scale experiments assessing the influence of varying grazing regimes on vegetation heterogeneity, livestock production and grassland bird conservation in eastern Colorado. In the first, we evaluate approaches to increase grazing intensity in a manner that generates habitat for disturbance-dependent birds, focusing on the difference between increased grazing intensity by cattle alone versus the combined grazing effects of black-tailed prairie dogs (Cynomys ludovicianus) and cattle. In the second, we compare a pulsed, rotational cattle grazing regime with season-long grazing in terms of effects on birds that breed in tall, dense vegetation.


Intense cattle grazing for 6–10 years generated surprisingly small changes in vegetation cover due to the tolerance of the dominant shortgrass to periodic defoliation. In contrast, grazing by prairie dogs, which can defoliate grasses to crown height, substantially increased bare soil exposure and created nesting habitat for the mountain plover (Charadrius montanus). Mountain plovers were most abundant on large (>80 ha) colonies (7.9 – 14.4 birds km-2), less abundant on smaller colonies (2.7 – 5.3 birds km-2), and absent from grassland grazed intensively by cattle only. Conversely, grasshopper sparrows (Ammodramus savannarum), which nest in grassland with near-continuous canopy cover, were rare in treatments with either prairie dog or cattle grazing in the previous year (net changes of -0.8 to 0.2 sparrows plot-1) but increased substantially in pastures rested from grazing >1 year (+1.3 sparrows plot-1; P = 0.02). Cattle at moderate stocking gained weight at a similar rate in pastures with and without prairie dogs (0.92 vs 0.95 kg/head/day averaged over 7 years), but gains were reduced 13% by prairie dogs in the driest year (0.65 vs. 0.75 kg/hd/day; P = 0.03). The rotational grazing regime that included year-long rested pastures reduced cattle weight gains by 16% relative to season-long grazing. Thus, management of colonial rodents and large herbivores has cascading effects on vegetation structure and faunal diversity, with tradeoffs for livestock managers in this social-ecological system.