Characterizing Bighorn Sheep Foraging Sites Using the Modified Robel Pole in the Southern Black Hills, South Dakota

Abstract

Evaluating foraging behavior of bighorn sheep (Ovis canadensis) and filling information gaps for their habitat requirements is important for population level management in the southern Black Hills of South Dakota. Our objectives were to: (1) evaluate the overall summer foraging area post lambing use during July and August for standing herbage with the modified Robel pole (1.27 cm bands) with visual obstruction readings (VOR) related to clipped herbage at ground level; (2) calibrate the Robel pole visual obstruction (bands) with clipped vegetation; and (3) develop guidelines for monitoring the landscape of the bighorn sheep foraging areas. The study area is located in a ponderosa pine (Pinus ponderosa ) savanna with few shrubs and dominated with native grasses. Each transect had 10 visual obstruction (bands) stations spaced 10 meters apart with 4 visual obstruction readings at each station. At 4 stations, total vegetation was clipped at ground level within a 0.25-m2  circular hoop. Clipped standing herbage ranged from 418 kg/ha to 3731 kg/ha with a mean of 1519 kg/ha. VOR measurements ranged from 0.2 cm to 14.9 cm with a 3.9 cm mean. Calibration of the modified Roble pole (visual obstruction of bands) with transect means using linear regression reliability predicted average clipped standing herbage (dry weights) within the bighorn sheep foraging area. The relationship was significant (R2  = 0.65; F1, 27  = 50.75, P  < 0.01). Cluster analysis (ISODATA) applied to the pole readings (VOR) and herbage resulted in 3 categories: short, intermediate and tall. We recommend 14 Robel pole transects (100 m in length) for VOR measurements within key foraging areas for future monitoring of herbaceous biomass for bighorn sheep. Foraging sites were in areas with little overstory tree canopy, close to rocky escape terrain, and where abundant grasses and forbs had little woody debris. The modified Robel pole provides a simple, reliable and cost effective alternative to clipping vegetation and obtaining dry weights.