from Part III - Patterns and Processes
Published online by Cambridge University Press: 05 May 2015
Background
Microbes (Bacteria, Archaea, and Fungi) are the most diverse and numerous organisms on earth. They are essential components of every ecosystem and occupy almost every trophic position and niche space (i.e., as heterotrophs, photoheterotrophs, chemoautotrophs, and photoautotrophs; see DeLong et al., 2014). Saprotrophic or heterotrophic species are unique in their capacity to break down and mineralize organic matter (OM) that forms the basis of the carbon, nutrient, and energy cycles. As such, they are the biochemical engineers responsible for the mobilization and mineralization of non-living OM in terrestrial and aquatic ecosystems, and they contribute extensively to the respiratory exchanges of carbon (mainly CO2 and CH4) between the biosphere and the atmosphere. The overwhelming dominance of microbes in OM decomposition has led to extensive exploration into the top-down and bottom-up processes regulating their biomass and diversity, and their consequences for biogeochemical cycling. These heterotrophic microbes form the basis of the decomposer food web, supporting a wide diversity of detritivorous invertebrates and protists, and in turn, a wide range of vertebrate and invertebrate predators.
Unlike autotrophic food webs, where living plants are the primary trophic level, detrital food webs are based on the production of non-living organic material that cannot grow in response to reduced consumption rates, but can accumulate when introduced from adjacent ecosystems. In this sense, the detritus-based food web (often referred to as the “brown” food web) is a “donor-controlled” system, relying on allochthonous inputs from the autotrophic (plant-based) sub-system. As such, these systems can never be exclusively top-down controlled, because microbial predation cannot directly affect the production of OM. Instead, top-down and bottom-up processes are on a continuum of relative importance, and ultimately, the effects of predation propagate as increased or decreased rates of OM decomposition, and thus OM accumulation. In essence, as microbial activity increases, decomposition rates (i.e., the initial loss of OM) generally increase, but when invertebrate predators (henceforth referred to as “grazers”) exert top-down control of microbial communities, their ability to decompose OM can be altered.
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