Age Structure Alteration in a Caddisfly Population after Habitat Loss and Recovery

Monthly collections (1975-1976) from a sericostomatid caddisfly population, Gumaga nigricula (McL.), in a northern California (USA) spring seepage indicated that age structure was numerically dominated by early larval instars (55-82% each month) but pupae (< 1-6%) were also present year-round. Severe drought in 1977 resulted in cessation of the normally permanent spring flow and total habitat loss occurred for over three months; no G. nigricula apparently survived. Two years after habitat recovery (1979), a single cohort population with a clear temporal succession in age class dominance from early to late larval instars was evident. Temporallyrestricted aerial recolonization of G. nigricula adults from a non-drought affected habitat most likely produced the shift to a single cohort population which has persisted through two generations (i.e. 2 yr).