BY SANJNA PARULEKAR AND RASHMI GUTTAL.

The dark Italian rye grass, a short weed brought by the Spanish more than 200 years ago, stands prominently among the light-colored native plants on Tulare Hill in South San Jose.

Scientists believe that the grass, fed by nitrogen from the city's polluted air, has choked both the Goldfield and Purple Owl's Clover, the flowering wild plants favored as a food and home of the endangered Bay Checkerspot Butterfly.