Population Extinction of a Checkerspot Butterfly Driven by Vehicular Ammonia Emissions

A population of the threatened Bay checkerspot butterfly crashed from 3500 butterflies in 1997 to likely extinction in 2003. Non-native annual grass crowded out larval host-plants over much of the habitat, primarily driven by nitrogen deposition from tailpipe emissions of NH₃ from 100,000 vehicles per day on a roadway bisecting the habitat. NH₃ levels (measured with passive monitors) are elevated adjacent to the roadway, but are near background levels 400 m away. Grass cover was higher closer to and downwind of the road, and hostplant cover was inversely related to grass cover. Results from a first-order model show that N-deposition levels adjacent to the roadway are similar (10 kg-N ha-1 yr-1) to levels downwind of the heavily urbanized Santa Clara Valley (where grass invasions have led to the extinction of large populations). This local butterfly extinction is unexpected fallout of the adoption of three-way catalytic converters in 1990s. The only known occurrence of an endangered plant, Pentachaeta bellidiflora, exists west of the freeway and may be at long term risk. The serpentine grassland ecosystem provides a model system for understanding local and regional impacts of NH₃ emissions on biodiversity.

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