Food web efficiency in desert streams

Production of animal biomass and the number of trophic levels supported by an ecosystem depend in part on rates of primary production, disturbance, predator-prey interactions, and the efficiency of energy flow through food webs. Of these factors, food web efficiency has been among the most difficult to quantify empirically. Thus, both the drivers and consequences of variation in food web efficiency remain largely unstudied in field settings. We estimated food web efficiency in nine desert streams spanning gradients of flash flood recurrence, resource availability, and trophic structure. Food web efficiency was estimated as fish community production relative to gross primary production at an annual timescale, based on quarterly observations of fish biomass and stream metabolism. Gross primary production was greatest in streams characterized by flashier flow regimes and greater relative light, temperature, and nitrogen availability. Fish production ranged from 0.02 to 0.50 g C m(-2) yr(-1), food web efficiency ranged from 9.5 x 10(-5) to 1.8 x 10(-2), and both properties decreased with flashier flow regime, light, temperature, and nitrogen availability, but were not associated with food chain length. These results, combined with opposite effects of environmental variation on primary vs. fish production, indicated that the effects of disturbance regime (i.e., scouring floods), light, and temperature on fish production were not strongly mediated by bottom-up controls. Estimates of food web efficiency under ambient disturbance and resource regimes suggest that a decoupling of energy flow from primary producers to upper trophic levels may prevail in hydrologically dynamic desert stream ecosystems.