MECHANISMS DETERMINING THE RELATIVE ABUNDANCE OF BRINE FLIES (DIPTERA: EPHYDRIDAE) IN YELLOWSTONE THERMAL SPRING EFFLUENTS

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Abstract
Of the three large ephydrid flies of Yellowstone’s thermal springs, Ephydra thermophila numerically dominates acidic springs because only it can tolerate their water. Alkaline spring effluents are shared by Ephydra bruesi and Paracoenia turbida. E. bruesi is excluded from alkaline high productivity springs, probably because its larval maturation time is long, relative to the short life of patches of larval habitat. Consequently, such springs support only P. turbida. Proportions of the two species in alkaline low-productivity springs are apparently determined by the proportions of available larval food occurring in still versus flowing water. P. turbida is superior in still water, and E. bruesi wins in flowing water. The analysis leads to predictions (i) that E. bruesi should be relatively abundant in low-productivity springs with pulsating flows, and (ii) that E. bruesi adults should prefer ovipositing in low-productivity effluents, while P. turbida adults should be less particular.