Developing Pronghorn Carrying Capacity Estimates for Trans-Pecos, Texas

Jacob C. Locke, Carlos E. Gonzalez, Justin T. French, Louis A. Harveson, and Shawn S. Gray (TPWD)

Texas Parks and Wildlife Department has engaged in pronghorn (Antilocapra americana) restoration efforts since 2010 to bolster the declining Trans-Pecos population. These efforts focused on restocking ranges that historically supported large numbers of pronghorn. However, population targets remain vague in the absence of an estimate of the population size that available habitat can support. Because of this, we estimated pronghorn carrying capacity in two restoration areas: the Marfa Northwest management unit (~216,190.23 ac) and the Marathon Basin (~64,660.07 ac).

We conducted vegetation sampling during the cool-dry and warm-dry seasons of 2019 and 2020. These seasons represented key times when available forage may limit population performance. We restricted sampling to forb species, as these account for approximately 85% of annual pronghorn diets. We also restricted sampling to grassland habitats within the two areas, reducing our study areas to 187,081.01 ac (Marfa Northwest) and 34.204.33 ac (Marathon Basin). We randomly sampled the two areas using 125 (Marfa Northwest) and 50 (Marathon Basin) plots. We collected all forbs in each plot to estimate forb biomass and forage carrying capacity based on a daily intake rate of 3.60 lb/day per pronghorn.

We hypothesized our carrying capacity estimates would exceed the current population numbers of 145 (Marfa Northwest) and 479 (Marathon Basin), with Marfa Northwest being further from carrying capacity compared to the Marathon Basin. We estimated carrying capacity to be 1,231 (Marfa Northwest) and 734 (Marathon Basin) individuals in the cool-dry season of 2019, based on a 25% harvest efficiency. Estimates in the warm-dry season of 2019 were 1,640 (Marfa Northwest) and 623 (Marathon Basin) individuals, also based on a 25% harvest efficiency. The results of this study will provide information needed to set measurable management targets for continued restoration efforts.

Funding sources: Texas Parks and Wildlife Department, Texas Parks and Wildlife Foundation, San Antonio Livestock Exposition, and the Borderlands Research Institute.