Food Webs

Food web ecology has been a driving force in studying the interconnections among species (e.g., MacArthur, 1955; Paine, 1980; Cohen et al., 1990; Polis, 1991; Pimm, 2002). In fact, we typically think of the abundance and distribution of species in an ecological community as being heavily influenced by the interactions with other species (Andrewartha and Birch, 1984), but the species is more than the loci of an envirogram; it is those interactions, that connectivity, with other species and with the environment, which construct the ecosystem. The diversity, stability, and behavior of this complex is governed by such interactions. Here we introduce the standard food web treatment, discuss some of the weakness, while suggesting improvements, and end with an overview of the general insights gained from understanding ecosystem connectivity as revealed by ENA.

A food web is a graph representing the interaction of "who eats whom", where the species are nodes and the arcs are flows of energy or matter. For example, we show a food web diagram typical to what one would find in an introductory biology or ecology textbook (Figure 5.1).

Figure 5.1 Typical ecological food web.

The energy flow enters the primary producer compartments and is transferred "up" the trophic chain by feeding interactions, grazing and then predation, losing energy (not shown) along each step, where after a few steps it has reached a terminal node called a top predator (also known, in Markov chain theory, as an absorbing state). This picture of "who eats whom" has several deficiencies if one wants to understand the entire connectedness as established by the matter-energy flow pattern of the ecosystem:

• First, the diagram excludes any representation of decomposers, identified above as a more fundamental element of ecosystems than more familiar trophic groups like herbivores, carnivores, and omnivores. While decomposers have been an integral part of some ecological research (e.g., microbial ecology, eutrophication models, network analysis, etc.), their role in community food web ecology is just now gaining stature. Prejudices and biases often work to shape science; what food-web ecologist, for example, would a priori classify our species (Homo sapiens) as detritus feeders as our diet of predominantly dead or not freshly killed organisms (living microbes, parasites, and inquilants in our food aside) in fact rules us to be?

• Second, the diagram shows the top predators as dead-ends for resource flow; if that were the case there would be a continuous accumulation of top predator carcasses throughout the millennia that biological entities called "top-predators" have existed. Nature would be littered with residues of lions, hawks, owls, cougars, wolves, and other "top-predators", even the fiercest of the fierce like Tyrannosaurus rex (not to mention other non-grazed or directly eaten materials such as tree trunks, feces, etc.). It would be a different world. Obviously, this is not the case because in reality there is no "top" as far as food resource and energy flow are concerned. The bulk of the energy from "top-predator" organisms, like all others, is consumed by other organisms, although perhaps not as dramatically as in active predation. Although there have been periods in which accumulation rates exceed decomposition rates, resulting in among other things formation of fossil fuels and limestone deposits, but much organic matter is oxidized to carbon dioxide. For our purposes, the relevancy of these flows from top-predators to detritus is that they provide additional connectivity within the ecosystem.

• Third, when decomposers are included in ecosystem models, as there has been some recent effort to do, they are treated as source compartments only. Resource flows out to exploiting organisms, but is not returned as the products and residues of such exploitation. For example, in a commonly studied dataset of 17 ecological food webs (Dunne et al., 2002), 10 included detrital compartments but all of these had in-degrees equal to zero, meaning they received no inputs from other compartments. In reality, all other compartments are the sources for the dead organic material itself (Fath and Halnes, submitted). It is easy enough to correct these flow structures by allowing material from each compartment to flow into the detritus, but this introduces cycling and gives a significantly different picture of the connectance patterns and resulting system dynamics.

The point is that while food webs have been one way to investigate feeding relations in ecology, they are just a starting point for investigating the whole connectivity in ecosystems. Other, more complete, methodologies are needed.

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