INVERTEBRATE RESISTANCE AND RESILIENCE TO INTERMITTENCY IN A DESERT STREAM

Invertebrate responses to water loss were investigated during drying, dry and rewetting phases in Sycamore Creek, an intermittent Sonoran De sert stream. Some taxa were resistant to drying because they could tol erate rapidly changing water quality and/or move upstream to avoid str anding. Community shifts occurred at one site when it became isolated from up- and downstream reaches; taxa such as beetles, hemipterans, an d the snail Physella virgata became dominant. No community changes wer e detected at a second site which remained connected with upstream rea ches by surface flow. Mortality after water loss was severe as few ind ividuals survived longer than 10 days. Low resistance during the dry p hase was associated with rapid moisture loss from sediments. Invertebr ate persistence in intermittent reaches was due to recolonization afte r rewetting; however, density increases after floods which reestablish ed flow at dry sites were lower than reported values for perennial sit es in Sycamore Creek. Slower rates of recovery may reflect the composi tion, reduced size and remoteness of macroinvertebrate colonist pools. Nonetheless, invertebrate persistence in desert streams where both fl ooding and drying are frequent is due largely to their ability to rapi dly recolonize disturbed sites.