Consumer presence and resource diversity independently induce stability of ecosystem function in a Piedmont stream

With the acceleration of species loss across multiple ecosystems, the mechanisms explaining subsequent changes in ecosystem processes are under continuing investigation. In detritus‐based ecosystems, such as soils, small streams, and wetlands, one consequence of tree species loss is the shift in the species composition of leaf litter resources to consumers. Given substantial variation in resource quality among senesced leaf species, organic matter processing rates are known to change with litter species loss as both microbial and invertebrate consumers respond to loss of resource diversity. While the effects on processing rates are now well documented, the implications for such species loss on the stability of organic matter processing have not been explicitly tested. In a field experiment, leaf litter diversity was manipulated as single‐ and mixed‐species treatments in a full‐factorial design with the presence/absence of a functionally important leaf‐shredding consumer, the caddisfly Pycnopsyche gentilis . It was hypothesized that in the absence of the consumer, loss of leaf litter species would result in higher variability (i.e., lower stability) in organic matter processing rates, owing to the portfolio effect commonly observed in plant communities. However, compensatory feeding by the consumer should offset the effect of leaf litter species loss. The results showed higher variation in litter processing among single‐species leaf treatments compared to diverse mixtures. When P. gentilis had access, variation among single‐species litter treatments was significantly reduced (i.e., stability increased), and was statistically indistinguishable from high diversity litter treatments. In small streams, which comprise >70% of stream miles in river drainages and often rely on allochthonous resources from riparian vegetation, how loss of stream‐side forest species influences stability of in‐stream organic matter processing can be independent of important detritivorous consumers.