By 2003, the Bay checkerspot butterfly could no longer be found at Edgewood Natural Preserve. Elsewhere on the peninsula, local populations had gone extinct as well. Scientists discovered a common thread: urban smog and automobile emissions.
This air pollution is a slow-release nitrogen fertilizer: it enriches nitrogen-poor serpentine soils, allowing European grasses to crowd out the native plants. The prime suspect at Edgewood: Highway 280.
Each day, more than 100,000 vehicles speed past, spewing nitrogen oxides (NOx). Catalytic converters introduced in the 1990s inadvertently introduced a more potent fertilizer, ammonia (NH3) — worsening the problem.
The butterfly’s favorite habitat — serpentine grasslands — had been converted to fields of non-native annual grasses, thanks to the enrichment of the soil. These grasses were crowding out the native plants that the butterflies and caterpillars required. Italian ryegrass seemed to be one of the worst offenders.
Scientists confirmed that the closer you got to the freeway, where Edgewood’s nitrogen pollution was measured at the highest levels, the thicker the ryegrass. The farther from the freeway, the more Bay checkerspot host and nectar plants.