Competition and facilitation between a native and domestic herbivore: Tradeoffs between forage quantity and quality

Authors: Augustine, David; Springer, Timothy

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2013
Publication Date: 6/1/2013
Citation: Augustine, D.J., Springer, T.L. 2013. Competition and facilitation between a native and domestic herbivore: Tradeoffs between forage quantity and quality. Ecological Applications. 23(4):850-863.

Interpretive Summary: In the Great Plains of North America, black-tailed prairie dogs are often thought to compete with cattle for forage. However, prairie dogs are also important for the conservation of some plants and animals that are native to the Great Plains. We examined how prairie dogs affect the amount and quality of forage growing on their colonies in Colorado and South Dakota. We also compare our findings with data previously collected from a group of colonies in Montana. At two sites (one in Colorado and one in Montana) that were studied during a period of below-average rainfall, forage both on and off colonies was so short and sparse that cattle were likely to be limited by the rate at which they could consume forage, rather than the rate at which they could digest it. Under these conditions, prairie dogs are likely to compete with cattle by reducing their weight gains. For two groups of prairie dog colonies studied during a period of above-average rainfall (one in Colorado and one in South Dakota), forage both on and off colonies was tall and dense enough for cattle to be limited by digestion rather than the rate of forage consumption. At the Colorado site, our findings predict that prairie dogs would increase cattle weight gains regardless of prairie dog abundance. At the South Dakota site, our results predict that prairie dog could either increase or suppress cattle weight gains depending on the size on the colony. Overall, our findings show prairie dogs can both enhance and suppress the weight gains of cattle depending on local weather conditions and the proportion of a pasture that is occupied by prairie dogs.

Technical Abstract: Potential competition between native and domestic herbivores for forage is a major consideration influencing the management and conservation of native herbivores in rangeland ecosystems. In grasslands of the North American Great Plains, black-tailed prairie dogs (Cynomys ludocivianus) are widely viewed as competitors with cattle, but are also important for biodiversity conservation due to their role as a keystone species influencing a wide range of native flora and fauna. We examined spatiotemporal variation in prairie dog effects on growing-season forage quality and quantity based on measurements from three colony complexes in Colorado and South Dakota and from a previous study of a fourth complex in Montana. At two complexes experiencing below-average precipitation, forage availability both on and off colonies was so low (12 – 54 g•m-2) that daily forage intake rates of cattle were likely constrained by instantaneous intake rates and daily foraging time. Under these dry conditions, prairie dogs (1) substantially reduced forage availability, thus further limiting cattle daily intake rates, and (2) had either no or a small positive effect on forage digestibility. Under these conditions, prairie dogs are likely to compete with cattle in direct proportion to their abundance. For two complexes experiencing above-average precipitation, forage quantity on and off colonies (77 – 208 g•m-2) was sufficient for daily forage intake rates of cattle to be limited by digestion rather than instantaneous forage intake rates. At one complex where prairie dogs enhanced forage digestibility and [N] while having no effect on forage quantity, prairie dogs are predicted to facilitate cattle weight gains regardless of prairie dog abundance. At the second complex where prairie dogs enhanced digestibility and [N] but reduced forage quantity, effects on cattle can vary from competition to facilitation depending on prairie dog abundance. Our findings show that the high degree of spatiotemporal variation in vegetation dynamics characteristic of semiarid grasslands is paralleled by variability in the magnitude of competition between native and domestic grazers. Competitive interactions that may be visibly evident during dry periods can be partially or wholly offset by facilitation during periods when forage quantity does not limit the daily intake rate of cattle and forage digestibility is enhanced on prairie dog colonies.