Quantification of the effect of lagomorph herbivory on grazing resources in a semiarid rangeland

Abstract Lagomorphs are selective herbivores that can strongly influence rangeland condition if their population densities increase through predator release. However, the effect size of lagomorph herbivory has not previously been quantified for the semiarid rangelands of the southwestern United States, where their predators are suppressed. We used a differential exclosure experiment replicated across 20 sites in a public rangeland in southern Utah to quantify the effect of lagomorph herbivory (jackrabbits and cottontails) for comparison with ungulate herbivory (cattle and bison). Across our 11‐year study, we measured aboveground net primary productivity (ANPP; 3 × 1‐year data sets) and long‐term standing crop (SC; 1 × 8‐year data set). We also measured nutrient concentrations in grass and soil samples in plots with complete exclosure (lagomorphs and ungulates out), partial exclosure (lagomorphs in and ungulates out), and no exclosure (open control plots) for 8 years. From the grass data, we calculated the effect size of lagomorph grazing as a 26.6% reduction in ANPP and an 11.3% reduction in SC, compared to ungulate grazing as a 24.9% reduction in grass ANPP and a 72.2% reduction in SC. We found lagomorphs had no effect on forb ANPP but actually increased forb SC, which is consistent with previous evidence that lagomorphs facilitate noxious weeds on these rangelands. There were no significant effects of either lagomorph or ungulate herbivory on plant and soil nutrient levels. Using the effect sizes of lagomorph herbivory on grass ANPP and SC, applied to the animal unit months (AUMs) prescribed for that rangeland, we calculated that lagomorphs currently impose an annual economic cost of between US$30.16 km −2 (SC) and US$70.64 km −2 (ANPP) in the Henry Mountains region of southern Utah, USA. We recommend that managers consider the lagomorph effect on rangeland resources in relation to the costs and benefits of conserving their predators, which are currently suppressed indirectly through various anthropogenic effects (raptors) and directly by agency‐funded control programs (coyotes).