Prarie dog (Cynomys ludovicianus) influence on forage quantity and quality in a grazed grassland-shrubland ecotone

Authors: CONNELL, LAUREN - University Of Wyoming; Porensky, Lauren; SCASTA, J. DEREK - University Of Wyoming

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2018
Publication Date: 2/26/2019
Citation: Connell, L., Porensky, L.M., Scasta, J. 2019. Prarie dog (Cynomys ludovicianus) influence on forage quantity and quality in a grazed grassland-shrubland ecotone. Rangeland Ecology and Management. 72:360-373. https://doi.org/10.1016/j.rama.2018.10.004.
DOI: https://doi.org/10.1016/j.rama.2018.10.004

Interpretive Summary: Black-tailed prairie dogs (Cynomys ludovicianus) have high dietary overlap with livestock, which can lead to conflicts between agriculture and conservation. Research suggests prairie dogs can enhance forage quality, but it remains unclear what drives this pattern, or how quality enhancements might vary throughout the growing season and across different plant communities. In a northern mixed-grass prairie, we collected forage samples on prairie dog colonies and at sites without prairie dogs during June, July, and August 2016 - 2017 for forage quality, and August 2015 - 2017 for herbaceous biomass. To isolate mechanisms affecting forage quality, we collected samples of all herbaceous species as well as samples of a palatable cool-season grass, western wheatgrass (Pascopyrum smithii). Across years and plant sample types, crude protein, phosphorus, and fat were 12-44% greater and neutral detergent fiber was 6-10% lower on prairie dog colonies than at sites without prairie dogs. Different sample types revealed that quality enhancements were driven by both shifts in species composition and also shifts in the growth stage of plants on colonies. Across years, aboveground herbaceous biomass did not differ significantly between prairie dog colonies and sites without prairie dogs (on-colony: 832 ± 139 lbs/ac, off-colony: 876 ± 104 lbs/ac). However, the effects of prairie dogs on herbaceous biomass were highly dependent on spring precipitation. In years with dry springs, herbaceous biomass was lower on colonies than sites without prairie dogs, but this pattern was reversed in years with wet springs. For producers in this ecosystem, our data suggest that prairie dog colonies are associated with the risk of major forage loss in drought years. Across years, enhanced forage quality and high wet-year production may help to offset this risk for agricultural producers.

Technical Abstract: Black-tailed prairie dogs (Cynomys ludovicianus) have high dietary overlap with livestock, which can cause forage-centric conflicts between agriculture and conservation. Research suggests prairie dogs can enhance forage quality, but it remains unclear how the strength of trade-offs between quality and quantity varies throughout the growing season, or the degree to which increased forage quality is caused by altered species composition versus altered plant physiology. To assess the effects of prairie dog herbivory on forage in a northern mixed-grass prairie, we collected samples on prairie dog colonies and at sites without prairie dogs during June, July, and August 2016 - 2017 for forage quality, and August 2015 - 2017 for herbaceous biomass. To isolate mechanisms affecting forage quality, we collected both composite samples of all herbaceous species and samples of western wheatgrass (Pascopyrum smithii [Rydb.] Á. Löve). Across years and plant sample types, crude protein, phosphorus, and fat were 12-44% greater and neutral detergent fiber was 6-10% lower on prairie dog colonies than at sites without prairie dogs. The effects of prairie dogs on forage quality persisted throughout the season for western wheatgrass samples (all treatment*time p-values = 0.4). Across years, aboveground herbaceous biomass did not differ significantly between prairie dog colonies and sites without prairie dogs (on-colony: 933 ± 156 kg/ha, off-colony: 982 ± 117 kg/ha). The effects of prairie dogs on herbaceous biomass were significantly influenced by spring precipitation. In years with dry springs, herbaceous biomass was lower on colonies than sites without prairie dogs. However, this pattern was reversed in years with wet springs. Our results demonstrate season-long enhanced forage quality on prairie dog colonies, which is due to both compositional and phenological shifts associated with prairie dog herbivory. Across years, enhanced forage quality may help to offset reductions in forage quantity for agricultural producers.