Do macroconsumers affect insect responses to a natural stream phosphorus gradient?

Abstract

We assessed the role of macroconsumers (e.g. fishes and shrimps) in affecting the response of insect assemblages to a natural phosphorus gradient formed by six streams with different phosphorus levels (range 12–350 μg/l). We hypothesized that insect responses to the phosphorus gradient would be strongest in the absence of macroconsumers. Within each stream, macroconsumers were allowed access to, or were excluded from, leaf packs using electric `fences'. Macroconsumers did not have significant effects on insects, but there was a significant phosphorus effect. Insect assemblages in high-phosphorus streams had 3–12-fold greater biomass and 3–11-fold greater abundance than assemblages in low-phosphorus streams. We also found that insect responses to phosphorus were more variable when assessed on the natural benthic substrate (e.g. mixed detritus) than in standardized leaf packs. In both substrates, the relationship between insects and phosphorus was not linear: abundance and biomass increased with phosphorus concentration to an asymptote. This suggests that insects were responding to a food resource gradient (e.g. fungi and bacteria). The Michaelis–Menten model provided a good fit for the relationship between insects and phosphorus concentrations, with half-saturation constants ranging from 12 to 60 μg SRP/l. The asymptotic relationship observed between phosphorus and insects suggests that phosphorus saturation occurred above a threshold of ca. 100 μg SRP/l. Our results provide support for the hypothesis that detritus-based food webs are mainly controlled by bottom-up forces.