Top-down interactions in southern Appalachian streams
Schofield, Katharine Ann
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Macroconsumers (fish, crayfish, shrimp) can influence numerous aspects of the stream environment, including sediment accumulation, algal and invertebrate assemblages, and leaf breakdown. The relative strength and outcome of these macroconsumer (or top-down) interactions can depend upon many biotic and abiotic factors. The main objective of this research was to examine temporal and spatial variability in the top-down effects of southern Appalachian stream macroconsumers. To do this, we conducted a series of exclusion experiments, using electricity to exclude fishes and crayfishes from benthic areas of streams.|To assess temporal shifts in macroconsumer impacts on leaf breakdown, exclusion experiments were conducted in summer and autumn using rhododendron leaf packs. Although rhododendron is typically considered low quality food, crayfish played a significant role in rhododendron leaf breakdown during both summer and autumn. Insect shredder and predator biomass did not differ between macroconsumer exclusion and control areas, indicating that crayfish directly accelerated rhododendron decay via shredding.|In the second set of experiments, macroconsumers and sediment were simultaneously manipulated to determine whether sedimentation reduced the relative strength of top-down effects. Because macroconsumer impacts can vary with substrate type, experiments were run with tiles and leaf packs. Small yet environmentally realistic increases in bedload transport and deposition (obtained via daily sediment addition) directly and indirectly affected algal and detrital-based benthic communities. Macroconsumers reduced total insect biomass on tiles, but this effect was eliminated with sediment addition.|To assess the influence of watershed development on top-down effects, macroconsumers were excluded at five sites representing a range of human watershed development. Macroconsumers influenced lower trophic levels at all five sites, despite cross-site physical, chemical, and biological differences. Although certain effects of watershed land use may tend to decrease the strength of top-down interactions (e.g., sedimentation), these reductions may be offset by other concurrent changes (e.g., increased nutrients).|Electric exclusion is a useful tool for assessing top-down effects, as it minimizes the artifacts associated with traditional cage experiments. However, the technique should be used wisely: the minimum voltages needed to exclude macroconsumers should be employed, and consistency of voltages both within and across sites should be maintained.