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

What is an ecosystem engineer?

Contents of one clump of G. salicornia from a shallow reef at Ala Moana. These include sponges, ascidians, larval fish, invasive and native algae, and crabs. Other clumps contained juvenile sea cucumbers and other fish species. A recent introduction to the islands has not prevented G. salicornia from becoming both an effective invader and a new habitat for benthic species.

Though I use the term frequently, deciding whether an organism is an ecosystem engineer is difficult. The term "ecosystem engineer" itself is problematic: almost every organism modifies its environment in some way, and in the face of indirect effects, quantifying this modification is nearly impossible. However, this category is useful because it can help us distinguish species which have strong physical impacts on ecosystems from those who affect the community mostly through competition, predation, or other biological pathways. Jones et al. (1997) define ecosystem engineers as "organisms that directly or indirectly control the availability of resources to other organisms by causing physical state changes in biotic or abiotic materials. Physical ecosystem engineering by organisms is the physical modification, maintenance, or creation of habitats." Essentially, ecosystem engineers create, modify, or destroy physical habitat.

Famous examples of ecosystem engineers include beavers, which fell trees and build dams, creating habitat for other organisms and altering patterns of water flow, and prairie dogs, whose burrows create nest habitat for birds. Plant examples abound: terrestrial forests are ecosystem engineers, as are many invasive plants. The cordgrass Spartina anglica has converted soft-bottom nearshore communities in the northeastern US to poorly drained swamps. In Hawai'i, the nitrogen-fixing shrub Morella faya has taken over areas of native forest, and because it fixes nitrogen, has significantly increased nitrogen concentrations in the areas where it has taken over. Mangroves have invaded much of Hawai'i's nearshore habitats and are expected to have significant and varied community impacts (Simberloff 2011). The Invasive alga Gracilaria salicornia alters nutrient concentration and sedimentation and flow rates. Okay, organisms can have physical effects on ecosystems. Why do we need to know whether or not they are engineers, or how much engineering they can do, exactly?

There are at least two reasons: 1) determining the extent of an organisms physical impact on a system is key in deciding whether or not the species will flourish and how it will affect the invaded community. This is particularly important in Hawai'i, which has endured a number of invasions and continues to be on the lookout for new, dangerous potential invasive species. 2) If we study these systems we may be able to build predictive models that tell us not only whether a species will be successful but where it is likely to colonize (Cuddington and Hastings 2004).

Information about invasive engineers can be difficult to sort, and sometimes difficult to find in the first place. But understanding their impacts can be a useful tool for management, and an ecological lesson.

Dinoflagellates and diatoms removed from the surface of a frond of Acanthophora spicifera, another structure-forming alga. A. spicifera is a physical host for microalgae, which grow on its surface and take advantage of localized high nutrient concentrations. In the Caribbean, it hosts Gambierdiscus toxicus, the dinoflagellate that causes Ciguatera Fish Poisoning (CFP). Interactions like these are important and can be vital to human health. (Note: G. toxicus does not grow well in waters with low salinity, so it's unlikely to show up in the fishpond).

Jones, C., Lawton, J., & Shachak, M. (1997). Positive and Negative Effects of Organisms as Physical Ecosystem Engineers Ecology, 78 (7) DOI: 10.2307/2265935

Jones, C., Lawton, J., & Shachak, M. (1994). Organisms as Ecosystem Engineers Oikos, 69 (3) DOI: 10.2307/3545850

Daniel Simberloff (2011). How common are invasion-induced ecosystem impacts? Biological Invasions : 10.1007/s10530-011-9956-3

Cuddington, K. (2004). Invasive engineers Ecological Modelling DOI: 10.1016/S0304-3800(04)00152-8