Recovery of a Headwater Stream from an Insecticide-Induced Community Disturbance

Abstract

Faunal assemblages, leaf litter processing rates, and seston export were followed in two streams two years after one was altered with an insecticide. Two years after treatment ended, the macroinvertebrate trophic structure of the treated stream (TS) community had recovered and was similar to that of the reference stream (RS); however, major differences in taxonomic composition persisted. Some taxa, e.g., Diplectrona metaqui and Pseudostenophylax, which were rare or absent in TS before treatment became locally abundant during recolonization. Other taxa (e.g., Peltoperla, Pycnopsyche, Diplectrona modesta, and Fattigia), which were known to be abundant in TS before treatment, remained absent or rare in TS and common in RS. Recovery of shredder functional group biomass in TS was accompanied by restoration of leaf litter processing rates, seston concentrations, and export. These had all decreased significantly in TS, relative to RS, during the treatment period. Despite the recovery of the shredder functional group biomass, the taxa dominating this group remained strikingly different from RS and pretreatment conditions in TS. Biomasses of some shredders (e.g., Lepidostoma spp. and Tipula) were twice as high in TS as in RS, while others (e.g., Peltoperla, Fattigia, and Pycnopsyche) were low or absent in TS. Although taxonomic differences still existed between TS and RS, trophic structure was similar between streams. Restoration of shredder biomass, leaf litter processing, and organic seston concentrations in TS supports the concept that stream ecosystems are resilient to disturbance. Thus recovery of trophic function may occur in a fashion not recognizable solely by taxonomic criteria.