Direct and indirect food web regulation of microbial decomposers in headwater streams

Abstract

The direct and indirect regulation of primary productivity has been well established in autotrophic‐based ecosystems; however, less is known about the processes affecting decomposers in detrital‐based ecosystems. Because, small headwater, woodland streams are a dominate feature in most ecosystems and are tightly linked to terrestrial detritus, understanding decomposer‐mediated functions in these systems is critical for understanding carbon processes across the landscape. In this light, we conducted a microcosm and mesocosm experiment to test the direct and indirect food web effects on decomposers in small stream ecosystems. The results from the microcosm experiment supported an existing literature, demonstrating that nutrients directly stimulate decomposers and that microbivores directly reduce decomposers. Based on well‐founded food web theory in autotrophic systems, we predicted that fishes from different trophic‐functional guilds would indirectly stimulate decomposers by enhancing dissolved nutrients and by reducing microbivore densities. Our mesocosm experiment partially supported these predictions. Specifically, we found that fishes that consumed mostly terrestrial foods increased decomposers from the bottom–up by enhancing allochthonous nutrient loading into the stream ecosystems. Contrary to our predictions, however, predatory fishes that consume microbivores did not increase decomposers from the top–down. Rather, in streams with the predatory fish species, microbivores increased (rather than decreased) on leaf litter. This may have resulted from an experimental artifact associated with refuge provided by leaf packs. In conclusion, our data demonstrate that decomposers are regulated by similar direct and indirect processes important in autotrophic‐based ecosystems. This provides further evidence that food web processes can regulate leaf decomposition and flux of detrital carbon through ecosystems.

Positive effect of shredders on microbial biomass and decomposition in stream microcosms

“…In spite of this downstream loading, some studies have shown that nutrients released by grazers may have positive indirect effects on periphytic algae in situ (Rosemond, Mulholland & Elwood, 1993; Steinman, 1996; Liess & Hillebrand, 2004). In contrast, the effects of detritivore excretion on decomposers remain poorly understood, although some evidence indicates that detritivorous fish may exert a positive bottom‐up effect on decomposers by enhancing nutrient loading in the water column (Hargrave, Hamontree & Gary, 2010). We argue that, similarly, nutrients released by detritivores feeding within detritus should favour decomposers (fungi and bacteria) colonising that litter.…”

Invasive willows drive instream community structure

“…Consistency of predator abundance across riparian type suggests that the predators of this study are generalists, further implying that changes in the basal food resource are driving response and that these changes are dampened at higher trophic levels. We cannot rule out the possibility that changes to grazer composition between riparian types was also influenced by top‐down pressure to some degree, but since the change was specific to grazer/collector–gatherer ratios and impacts of benthic predators on herbivore density in streams is known to be weak (Shurin et al ., ; Hargrave, Hamontree & Gary, ), we believe our test streams were driven more strongly by bottom‐up interactions.…”

In‐stream structure alters density‐dependent fish effects in stream ecosystems

“…This nutrient enrichment may stimulate algae and microbial productivity/biomass from the bottom-up and increase density, biomass and production of primary consumers (grazing invertebrates; Gido & Matthews 2001). To the contrary, fishes may also reduce densities of primary consumers through direct predation, which may cascade and enhance benthic algae or microbes from the top-down (Power 1990;Hargrave et al 2010). Furthermore, direct physical interactions between large-bodied consumers and the stream ecosystem also can drive ecosystem properties and functions (Flecker 1996).…”