Ground-Dwelling Arthropod Community Response to Livestock Grazing: Implications for Avian Conservation.

Terrestrial arthropods are a critical component of rangeland ecosystems that convert primary production into resources for higher trophic levels. During spring and summer, select arthropod taxa are the primary food of breeding prairie birds, of which many are imperiled in North America. Livestock grazing is globally the most widespread rangeland use and can affect arthropod communities directly or indirectly through herbivory. To examine effects of management on arthropod community structure and avian food availability, we studied ground-dwelling arthropods on grazed and ungrazed sagebrush rangelands of central Montana. From 2012 to 2015, samples were taken from lands managed as part of a rest-rotation grazing program and from idle lands where livestock grazing has been absent for over a decade. Bird-food arthropods were twice as prevalent in managed pastures despite the doubling of overall activity-density of arthropods in idle pastures. Activity-density on idled lands was largely driven by a tripling of detritivores and a doubling in predators. Predator community structure was simplified on idled lands, where Lycosid spiders increased by fivefold. In contrast, managed lands supported a more diverse assemblage of ground-dwelling arthropods, which may be particularly beneficial for birds in these landscapes if, for example, diversity promotes temporal stability in this critical food resource. Our results suggest that periodic disturbance may enhance arthropod diversity, and that birds may benefit from livestock grazing with periodic rest or deferment.

A degree-day model of sheep grazing influence on alfalfa weevil and crop characteristics.

Domestic sheep (Ovis spp.) grazing is emerging as an integrated pest management tactic for alfalfa weevil, Hypera postica (Gyllenhal), management and a degree-day model is needed as a decision and support tool. In response to this need, grazing exclosures with unique degree-days and stocking rates were established at weekly intervals in a central Montana alfalfa field during 2008 and 2009. Analyses indicate that increased stocking rates and grazing degree-days were associated with decreased crop levels of weevil larvae. Larval data collected from grazing treatments were regressed against on-site and near-site temperatures that produced the same accuracy. The near-site model was chosen to encourage producer acceptance. The regression slope differed from zero, had an r2 of 0.83, and a root mean square error of 0.2. Crop data were collected to achieve optimal weevil management with forage quality and yield. Differences were recorded in crude protein, acid and neutral detergent fibers, total digestible nutrients, and mean stage by weight. Stem heights differed with higher stocking rates and degree-days recording the shortest alfalfa canopy height at harvest. The degree-day model was validated at four sites during 2010 with a mean square prediction error of 0.74. The recommendation from this research is to stock alfalfa fields in the spring before 63 DD with rates between 251 and 583 sheep days per hectare (d/ha). Sheep should be allowed to graze to a minimum of 106 and maximum of 150 DD before removal. This model gives field entomologists a new method for implementing grazing in an integrated pest management program.