SPRING PERMANENCE, TRICHOPTERA SPECIES RICHNESS, AND THE ROLE OF DROUGHT

A continuing six-year drought has verified a model for caddisfly (Tric hoptera) species richness and spring permanence in Sierra Nevada, Cali fornia, cold spring sources. An intensive study of 21 cold springs was conducted from 1982-1987, prior to extended drought, in three second- order stream basins, elevation 1963 m to 2408 m. Invertebrates were sa mpled with continuous emergence traps, bottom samplers, and hand colle cting. Physical/chemical variables were measured as part of the early study and were remeasured in September 1990 and 1992, after four and s ix years of drought. Species analysis of Trichoptera showed that sprin gs were highly individualistic (average Jaccard similarity = 23%). Spe cies richness ranged from 2 to 18 per spring, 9 species were restricte d to constant-temperature spring sources, and 36 caddisfly species wer e collected overall. Species richness was positively correlated with c alcium (r(2) = 0.452) which was intercorrelated with magnesium, specif ic conductance, alkalinity, and pH. High concentrations of dissolved i ons were a function of deeper springs and an indication of more perman ent springs. Prior to drought, caddisfly richness showed a positive re lationship with discharge (r(2) = 0.366); however, after six years of drought, a stronger relationship emerged: as long-term fluctuation in discharge increased, caddisfly richness decreased (r(2) = 0.527). Some springs low in caddisfly species dried during the drought, while spri ngs high in species persisted. Calcium was a proxy for spring permanen ce. Springs rich in Trichoptera were also rich in other invertebrates. Turbellaria and Plecoptera were absent or low, prior to drought, in f our springs that are now dry or nearly so. Relict and endemic species were found in constant and persistent springs. Logging, road building, livestock grazing, water development, groundwater pumping, wildfire, and other impacts can affect water volume, riparian vegetation, timing of flow, chemical concentrations, solar radiation, and temperature re gimes making springs uninhabitable to species restricted to them. Spri ngs may contain disproportionately large numbers of unusual species fo r their small size and have been ignored too long by aquatic ecologist s and land managers.