Synonyms

Decomposer food web food; Detritus cycle; Microbial loop

Definition

Food webs or portions thereof that are based on the decomposed particles of dead plants and animals, mediated by saprotrophic and scavenger organisms that break down organic matter into its constituent compounds.

Summary

While the basic photosynthetic production processes supporting all but extremophile-based food webs do not differ among most ecosystems, from an energy flow perspective (see Food Web/Trophic Dynamics ), the pathways whereby organic compounds reach metazoan consumer organisms can be both intricate and often confusing. This is especially the case in estuaries, wherein diverse living and detrital organic matter sources support mixed autotrophic and heterotrophic production (Figure 1). As opposed to direct herbivory (“grazing”) of living plants, food webs based on detritus involve the decay of photosynthetic products and even dead consumer organisms; however, whether or not detritus should be defined as including associated living decomposers and other microorganisms (Figure 2) has always been somewhat of a philosophical dispute (Darnell, 1967).

Detritus Food Webs, Figure 1
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Detritus food web. Estuarine food web energy flow indicating interactions among autotrophic (green: phytoplankton, marsh plants) and heterotrophic (brown: detritus) pathways of organic matter production, consumption (blue: consumers), inorganic resources (purple), transformations and storage (light gray) and energy (dark gray) (Modified from Day, J. W., Jr., B. C. Crump, W. M. Kemp and A. Yáñez-Arancibia (eds.) 2012. Estuarine Ecology, 2nd Edition, Hoboken, New Jersey: Wiley-Blackwell).

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Detritus Food Webs, Figure 2
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Illustration of the different forms of detritus and microbial microenvironments common to estuaries, including (left image; from Stocker and Seymour 2012 Microbiol. Mol. Biol. Rev. 76:792-812) “hot spots” of microbial activity in association with detritus, marine snow particles, and phytoplankton cells, and (right image; modified from the cover of Science, 5 February 2010; original image credits: R. Stocker, J. R. Seymour, G. Gorick) organic matter source, including zooplankton excretions (left), phytoplankton exudation (the “phycosphere”) (top; bottom right), phytoplankton lysis (top right), settling marine snow particles (center bottom), and copepod excretions (left).

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While detritus is the predominant food web source in some ecosystems, such as in soils, the occurrence and contribution of detritus to aquatic food webs have been more debatable. About the same time that Sir Alistair Hardy (1924) was describing a food web that supported Atlantic herring wholly by autotrophic production from algae, Summerhayes and Elton (1923) diagrammed a “nitrogen cycle” for Bear Island (Bjørnøya), Svalbard, that illustrated a more complex network also involving detritus from aquatic and terrestrial plants being decomposed by bacteria and protozoa before sustaining detritivores and ultimately higher-level consumers. With the discovery of “marine snow” (Alldredge and Silver, 1988) and the attendant “microbial loop” driven by dissolved organic matter (POM) (Pomeroy, 1974; Azam et al., 1983), even presumed autotrophically dominated ocean food webs were found to have highly integrated detritus pathways (Figure 2b). While detritus has long been considered to be a major driver of food web pathways in estuarine sediments (e.g., Newell and Field, 1983), it also became even more relevant to estuaries overall (Crump et al., 2012), especially with increased understanding of gravitational circulation processes that promote estuarine turbidity maxima as “biogeochemical reactors” (Baross et al., 1994; Savoye et al., 2012). What has become increasingly obvious from the more recent application of isotope and other biomarker sampling and experimentation in estuaries is that although detritus fuels and may even dominate many estuarine food webs, the extent to which it does varies considerably as a function of the type and region of estuary and the time frame (Odum, 1984; Peterson et al., 1985; Peterson and Howarth, 1987; Deegan and Garritt, 1997; Akin and Winemiller, 2006).

In many respects, estuaries have often been the nexus of the debate about the role of detritus food webs, touching on the core of many fundamental issues in ecological theory such as labile versus refractory organic matter sources (Mann, 1988); the importance of allochthonous, spatial subsidies (Polis et al., 1997); outwelling (Childers et al., 2000); compartmentalization (Raffaelli and Hall, 1992); community stability (Huxel and McCann, 1998); and top-down versus bottom-up control on food web structure (Power, 1992). While the prominence of detritus in estuarine food webs is less debatable, its role in shaping estuarine ecosystem dynamics and regulating the productivity of important consumers such as commercial fisheries is still somewhat controversial.

Cross-references