Effects of resource manipulation on selected primary and secondary consumers in two detritus-based southern Appalachian streams
Johnson, Brent Robert
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This research was conducted in the context of a larger experiment aimed at assessing the effects of long-term litter exclusion in an Appalachian headwater stream ecosystem. The exclusion experiment provided an opportunity to examine effects of resource reduction on consumers. Mark-recapture and dietary studies were conducted to determine the effects of litter exclusion on the larval salamander Eurycea wilderae, a vertebrate stream predator. Litter exclusion resulted in significantly reduced salamander density, individual growth, biomass, and production. Reduced density in the treatment stream likely results from drift of hatchlings, whereas a dietary shift may be responsible for reduced growth. Salamanders in the treatment stream had fewer prey per gut and switched from primarily consuming copepods to relying more on non-tanypodine midges. There was no difference in salamander movement among streams. Results from a downstream recovery reach were often intermediate for measured parameters, indicating residual effects from upstream treatment. In-situ chambers were used to measure effects of litter exclusion on growth of two dominant insect detritivore groups. Larval Tallaperla spp. were chosen as representative shredders and non-Tanypodinae Chironomidae were selected as representative collector-gatherers. Comparison of significant regression lines showed growth of both taxonomic groups were significantly lower in the litter exclusion stream. Tallaperla spp. mortality was also higher in the treatment stream. These results show that reduced growth is partially responsible for declines in detritivore production in the treatment stream and that chironomid production is actually lower than previously reported. Nutrients were added to another headwater stream to measure effects of enrichment in a detritus-based ecosystem. A mark-recapture study was conducted on larval E. wilderae to measure effects of enrichment on growth of a representative vertebrate predator. Compared to pre-treatment, growth rates were significantly higher in both reference and treatment streams during the treatment period, indicating interannual growth variation. Growth rates in the nutrient addition stream, however, were significantly higher than the reference stream during treatment. Larvae prey heavily on copepods and chironomids that would respond quickly to enrichment. These preliminary results indicate that enrichment in detrital systems may indirectly affect higher trophic levels in ways similar to living-plant based systems.