Cars, cows, and checkerspot butterflies: Nitrogen deposition and management of nutrient-poor grasslands for a threatened species

Nutrient-poor, serpentinitic soils in the San Francisco Bay area sustain a native grassland that supports many rare species, including the Bay checker spot butterfly (Euphydryas editha bayensis). Nitrogen (N) deposition from a ir pollution threatens biodiversity in these grasslands because N is the pr imary limiting nutrient for plant growth on serpentinitic soils. I investig ated the role of N deposition through surveys of butterfly and plant popula tions across different grazing regimes, by literature revives, and with est imates of N deposition in the region. Several populations of the butterfly in south San Jose crashed following the cessation of cattle grazing. Nearby populations under continued grazing did not suffer similar declines. The i mmediate cause of the population crashes was rapid invasion by introduced a nnual grasses that crowded out the larval host plants of the butterfly. Ung razed serpentinitic grasslands on the San Francisco Peninsula have largely resisted grass invasions for nearly four decades. Several lines of evidence indicate that dry N deposition from smog is responsible for the observed g rass invasion. Fertilization experiments have shown that soil N limits gras s invasion in serpentinitic soils. Estimated N deposition rates in south Sa n Jose grasslands are 10-15 kg N/ha/year; Peninsula sites have lower deposi tion, 4-6 kg N/ha/year. Grazing cattle select grasses over fobs, and grazin g leads to a net export of N as cattle are removed for slaughter. Although poorly managed cattle grazing can significantly disrupt native ecosystems, in this case moderate, well-managed grazing is essential for maintaining na tive biodiversity in the face of invasive species and exogenous inputs of N from nearby urban areas.