Dead, Alive, or Excreted: The Mysterious Role of Detritus in Food Webs

Suppose I ate a moldy fish, and my friend ate a fresh one. Though both food items are fish, food web ecologists would probably call my friend's meal "fish" and mine "detritus." Detritus is a catch-all term for dead plant tissue (e.g. leaf litter, dead wood, dead algae), dead animal tissue, feces, mucus, and even things exuded from plants and cells like nectar or extracellular matrix. Particles can vary in size and chemical composition. Detritus is often in some state of decomposition, so it is hard to tell exactly what it is, or where it came from.

A food web in which detritus is a nutrient source can potentially support more diversity, longer food chains, and larger predator biomass than one based only on living material (Hairston & Hairston 1993). It can subsidize the diets of consumers who feed mainly on living material, and it can provide dissolved nutrients to producers (like phytoplankton). Detritus can also be an avenue by which invasive species affect energy flow in native systems, sometimes to the benefit of natives (see Wolkovich 2010). However, there has been a separation in ecology between "brown world" (detritus-based) models, and "green world" (primary-production based) models. The most common food web models dwell in green world: they only incorporate living material, such as algae, as nutrient sources. So why is the ecosystem ecology spotlight still on the living?

Moore (2004) attributes this to an early divide between community ecology and ecosystem ecology. Back in the day, community ecology focused on individual organisms, while ecosystem ecology was nutrient-centric, focusing instead on how carbon, nitrogen and phosphorous move through an ecosystem. Detritus ended up in the nutrient-centric category, and since then has rarely been included in "green" food web models.

Taking a nutrient-centric approach to account for detritus makes some sense because detritus is a) made of dead things b) often made of a mixture of those dead things and c) highly variable in its composition. Not accounting for spatial and temporal variability in detritus sources, and simply counting it as a carbon source, is simply easier.
Imagine a pond with algae in it, that is continually dying and being fed on by bacteria, some of which are also continually dying (Lindeman 1942). An herbivorous fish that eats that algae will certainly also be eating some of those bacteria: so how do we know where the fish's nutrients are coming from? Much easier to measure carbon and nitrogen than to sort out where those molecules came from.

But often we want to know nutrient sources, and chemical/nutrient tracers often fail to reveal them. A detritivore's diet is highly variable, even if it eats the same thing every day. Brown muck with cellulose might occur under, say, a mangrove canopy, but the same brown muck will suddenly become loaded with fishy fatty acids if a fish dies under that canopy. Therefore, if you use chemical tracers to determine food sources, the diet of this brown-muck-eating fish still looks highly variable. I have found this in natural carbon isotope abundances from detritivorous crabs: blue pincher crabs with a < 5 m range, sampled at the exact same site on the same day can have very different carbon isotope signatures. Given this kind of data, we don't know whether the crab has been eating plants, live fish, dead fish, or other crabs, or which of these nutrient sources is most important.

One way to overcome this issue is to work with label addition experiments-- these are popular in seagrass and algal ecology, and can at least distinguish whether things are eating living or dead plant matter. Living plants take up nitrogen, so you can label plant material by adding an isotope tracer, like isotopically heavy nitrogen, to your study site. The live plants and algae will take up the tracer, while detrital material (especially if it's washing in from somewhere else, like a stream) will not. Then you can measure consumers to see if they ate labeled material (if they ate something labeled with heavy N, it will show up in their tissue nitrogen values). Compound-specific stable isotope measurements can also track material through detrital loops as well.

The incorporation of detrital variability and green-brown interactions into food web models is only just beginning, though there is promise in compound-specific and stable isotope methods. These advances will be of utmost importance to food web ecologists.

Where green world and brown world collide: mats of benthic microalgae are glued together with detritus, but fish eat the mats indiscriminantly, obtaining nutrients from both detritus and microalgae.

Hairston, Jr., N., & Hairston, Sr., N. (1993). Cause-Effect Relationships in Energy Flow, Trophic Structure, and Interspecific Interactions The American Naturalist, 142 (3) DOI: 10.1086/285546

Moore, J., Berlow, E., Coleman, D., Ruiter, P., Dong, Q., Hastings, A., Johnson, N., McCann, K., Melville, K., Morin, P., Nadelhoffer, K., Rosemond, A., Post, D., Sabo, J., Scow, K., Vanni, M., & Wall, D. (2004). Detritus, trophic dynamics and biodiversity Ecology Letters, 7 (7), 584-600 DOI: 10.1111/j.1461-0248.2004.00606.x

Lindeman, R. (1942). The Trophic-Dynamic Aspect of Ecology Ecology, 23 (4) DOI: 10.2307/1930126