The Microlepidoptera Fauna of Santa Cruz Island is Less Depauperate than that of Butterflies and Larger Moths

Surveys of Lepidoptera on Santa Cruz Island, California, indicate that the fauna of small to minute leaf mining moths from six families are proportionately better represented relative to the nearby mainland fauna, than are larger microlepidopterans (Tortricoidea and phycitine Pyralidae) or macrolepidopterans (Papilionoidea, Hesperioidea, Arctiidae and ennomine Geometridae). Collectively, 71% of the central coast leaf mining species occur on Santa Cruz Island; whereas in the other 5 groups surveyed 30-62% of the mainland fauna are resident. Species with small adults or with larvae that feed internally occur in higher proportion than do large and external feeding Lepidoptera. In three well-sampled host plant genera (Ceanothus, Quercus and Salix) used by both leaf miners and butterflies, 87% of the mainland leaf miner species are recorded for Santa Cruz Island, whereas only 35% of the butterflies are present. We postulate that this disharmony in species richness is due to the smaller areas required to maintain effective populations of leaf mining moths, which enhances the survival rates of these minute insects in small patches of host plant.