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Long-term resource limitation reduces insect detritivore growth in a headwater stream

We measured larval growth rates of 2 dominant stream detritivore groups to assess the mechanism underlying declines in invertebrate production following exclusion of terrestrial litter inputs to a forested headwater stream. Larval Tallaperla spp. (Plecoptera:Peltoperlidae) were chosen as representative shredders and non-Tanypodinae Chironomidae (Diptera) were selected as representative collector-gatherers. Larval growth rates were measured in the treatment stream and in 2 undisturbed reference streams using in situ growth chambers. Estimates of daily growth rates were derived from change in mean length of larvae over incubation periods. Initial larval length was a significant predictor of growth in each stream for both taxonomic groups (r2 = 0.43–0.72, p < 0.05). Comparison of significant regression lines showed that size-specific growth of both Tallaperla spp. and chironomids was significantly reduced in the litter exclusion stream (ANCOVA, p < 0.05). Lower chironomid growth rates in the treatment stream than in the control streams indicate that production estimates based on the instantaneous growth method are actually lower than previously reported for the site. Mortality of Tallaperla spp. was also significantly higher in the treatment stream than in control streams (ANOVA, p < 0.05). Reduced growth of these representative taxa apparently results from reduced quantity of organic matter food resources. These results show that reduced growth is partially responsible for observed declines in detritivore production in the litter-exclusion stream.