Food-Web Theory. Trophic Pyramids

Charles Elton (1927) made the original observation that food chains tend to be short. The inefficiency of energy transfer between trophic levels places constraints on the number of top consumers that can be supported by the base of producer biomass (the amount of living matter). More importantly, as a food chain becomes longer, it becomes unstable.

Trophic pyramids are diagrams that present the amount of energy or biomass in each successive trophic level, with producers at the bottom. Such diagrams suggest that food-web dynamics are controlled by "bottom up" processes in which the original amount of primary production controls all other interactions.

The "Green World" model developed by N. Hairston, S. Smith, and S. Slobodkin in 1960 suggested there is always an excess of primary production, so each trophic level must be controlled by its consumers, reflecting "top down" control. The "Exploitation Ecosystem Hypothesis" first described by Fretwell (1977) suggested that variability in the number of trophic levels determines numbers or biomass. Food webs with even numbers result in low levels of plant biomass, whereas those with odd numbers result in excess biomass because predation is suppressed.

This model also suggests that food webs are limited to only three or four trophic levels. Trophic cascade models, like those described for aquatic systems by S. Carpenter and J. Kitchell (1988) and M. Power (1990), describe systems in which primary productivity is controlled by bottom-up factors like nutrient availability, but it is the number of trophic levels and predators that then controls food-web dynamics.

R. J. Williams and N. Martinez (2000) have recently introduced the "Niche Model." This model is based on the idea that each trophic species fits into a specific niche; this model seeks to predict food-web characteristics more accurately than do earlier models.

Trophic Pyramids. Karl Semper (1881) introduced the concept of energy transfer through food webs, which was later formalized into the food pyramid or "pyramid of numbers" concept by Elton in 1927. Any trophic interaction represents a transfer of energy in the form of calories from plants, detritus, or animals to other animals, so food webs can also show the movement of energy or biomass through a particular community.

These are sometimes called "Eltonian" or "Lindeman" pyramids in recognition of the pioneering work done by Elton (1927) and Lindeman (1942) in formalizing the trophic pyramid concept, but again it was Semper in 1881 who first recognized that the transfer of energy is incredibly inefficient and hypothesized that only 10 percent of the energy present at the lower trophic level is assimilated and converted to growth, activity, or reproduction, the other 90 percent is lost mainly through egestion as feces or other waste products or as heat.

Experts know from numerous studies on real food webs that this inefficiency varies considerably but is generally less than 20 percent. As energy is transferred from one consumer to the next there is the same loss of energy. So, in a food chain with one producer and just three consumers, only about 0.1 to 0.2 percent of the original energy is available to the top consumer in the food chain. Consumers that feed "low on the food chain" are able to capture more of the original total energy produced and often support populations with larger total numbers or biomass than can consumers that feed at higher levels.

Keystone Species. Keystone species are unique because their effects on the dynamics of a given food web are out of proportion to their abundance or biomass and they have strong interactions with other species. Removal of a keystone species, as its name suggests, results in changes in the food web. R. Paine introduced the concept in 1969 to explain the results of a series of experiments where removal of predatory starfish from a rocky intertidal area greatly changed the rest of the food web. Other examples of keystone species are sea otters, fruit-eating bats, beavers, termites, and harvester ants.

Applications in Environmental Science. The most important uses of food-web theory in environmental science are in the biomagnification (accumulation of toxins up the food web) of toxins and in the introduction of invasive species. Toxins such as DDT and PCBs (polychlorinated biphenyls) are not typically broken down when ingested. Instead they are sequestered in fatty tissue, where they accumulate over the lifetime of the organism.

When predators eat many toxin-containing organisms they sequester a higher total dose. Predators at the next level of the food web ingest the accumulated toxin of their prey, so that species at the top of the food web are, in fact, ingesting all the toxins ingested by all the individuals that fed before them. Biomagnification effects of toxins were first brought to the public's attention in 1962 in Rachel Carson's influential book, Silent Spring.

People have been deliberately or accidentally introducing exotic species into novel ecosystems for millennia. However, only recently have people begun to focus on the effects that those species can have on food-web dynamics. Because an exotic species is often free from the predators, diseases, or parasites of other native species feeding at its trophic level, it can exert strong interaction effects on the food web, often with catastrophic effects. For example, the continuing introduction of exotic species into the Great Lakes, which began in the 1930s, has resulted in destabilization in the lake food web as well as extinction of several species.