The Californian urban butterfly fauna is dependent on alien plants

Using the unusually well‐documented butterfly fauna of Davis, Yolo County, California, it is shown that the mainly native species commonly observed in gardens breed mostly or entirely on alien plants, especially naturalized weeds. Over 40% of the fauna has no known native hosts in the urban–suburban environment. Were certain alien weeds to be eradicated or their abundance greatly reduced, the urban‐suburban butterfly fauna would disappear. This might be regarded as an unfortunate, and perhaps intolerable, side‐effect of such programs.