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Non-additive effects of mixed-species leaf-litter on a benthic stream community in the Pacific Northwest Kolodziejczyk, Renata

Abstract

Organic matter inputs from three riparian tree species (red alder, western redcedar, and western hemlock) were investigated for food resource value to stream macroinvertebrates and fungi. Leaflitter species were tested both individually and in mixed-species combinations (7 treatments) using two sets of stream mesocosms. Within the initial 3 weeks, ergosterol content (an index of fungal biomass) showed little variation between litter types. Differentiation started around 6 weeks and progressed over the 12 week duration of the experiment to where fungal accrual in hemlock litter was significantly higher that that of alder or cedar. In one set of mesocosms, macroinvertebrate biomass was significantly higher in alder-containing treatments, particularly for the shredderdetritivore species, while chironomids such as Chaetocladius and Heterotanytarsus were significantly more abundant in conifer-only treatments. No increase in shredder abundance or biomass coincided with the increase in ergosterol content, even when hemlock litter attained a peak of 810 pig ergosterol/g detritus. Shredder-detritivores were the group most affected by mixed-leaf species treatments. Data from single-litter channels were used to generate predicted abundance and biomass of macroinvertebrates for mixed-litter channels. Comparisons of predicted versus observed values in mixed-litter treatments found significantly higher 'non-additive effects' in abundance and biomass for alder-cedar and in biomass for alder-hemlock treatments. These effects were due primarily to the shredders Zapada and Lepidostoma, which achieved a higher abundance and biomass than predicted. Individual Zapada were also larger in alder-cedar and alder-hemlock treatments. A potential mechanism for this non-additive effect may be associated with faster accrual of fungal biomass on the alder leaf-litter when in mixed alder-conifer litter combinations compared to single-litter treatments. Results differed between mesocosms, and in one set of mesocosms with lower overall macroinvertebrate densities, no distinct pattern was evident. These results suggest a fungal mediated improvement in resource quality when in mixed-litter combinations.

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