California Sea Grant College Program

Records indicate that entomologists have collected insects from the California Channel Islands since the late 1800s. Despite over 100 years of entomological collection on the islands, several aquatic insect groups remain poorly described. This study represents the first intensive collection of California Channel Island aquatic insects. Samples taken from seven Santa Cruz Island streams from 1990 to 1997 yielded 41 taxa previously undescnbed from the California Channel Islands and 47 new records for Santa Cruz Island. Compared to the nearby mainland. Santa Cruz Island streams support a depauperate fauna. Of the 161 total taxa (generic level) listed for Santa Cruz Island and the nearby mainland, only 97 occur on the island. The assemblage of taxa on Santa Cruz Island does not represent a random subset of the total. Aquatic flies (Diptera) and beetles (Coleoptera) are over­ represented on the island. while caddisflies (Trichoptera) and stoneflies (Plecoptera) are under-represented. This disharmonic island assemblage may result from differences in the dispersal and colonization abilities of aquatic insect taxa. In addition, the depauperate nature of riparian vegetation on Santa Cruz Island might exclude aquatic groups relying heavily upon allochthonous stream input.

The aquatic insect assemblages of Santa Cruz Island vary among streams, habitats within streams, and seasons. Contrary to theoretical expectations, streams with larger catchments do not support greater numbers of total taxa or taxa within individual orders. The stream with the smallest catchment, Black Point, supports the greatest number of taxa. In contrast to streams with larger catchments, Black Point rarely experiences winter scour. Therefore, differences in disturbance regime may contnbute to the observed variation in richness among island streams. A disturbance experiment did not support the hypothesis that disturbance frequency results in decreased taxonomic richness. However, this experiment did illustrate that the effects of experimental disturbance vary with substratum type and taxonomic group.