A food chain is an ecological term that describes a series of organisms dependent on each other for food; a food web refers to an interconnected set of food chains in the same ecosystem. Organisms that eat similar foods are assigned to a particular trophic level, or feeding level, within a food web. Food web is generally considered a more accurate term because food chains are overly simplify ecological relationships. Feeding habits are often quited complex because organisms feed on different trophic levels. For example humans feed on the producer level when they eat plants but they also eat organisms from higher trophic levels when they consume meat.
Charles Elton, Raymond Lindeman, Stuart Pimm, Stephen Carpenter, and James Kitchell are some of the major figures in the development and exploration of food web concepts. Charles Elton was an English ecologist who first described the characteristic shape of ecological trophic interations which he called the pyramid of numbers. Elton observed that most food webs have many organisms on their bottom trophic levels and successively fewer on the subsequent, higher levels. His pyramid of numbers is now called the Eltonian Pyramid and is the basis for the classic ecological food pyramid.
The American ecologist Raymond L. Lindeman published a seminal paper in 1942 that re-examined the Eltonian pyramid in terms of energy flow. By using energy as the currency of the ecosystem Linderman quantified and explained that the Eltonian Pyramid was a result of successive energy losses at each trophic level. This loss is due to thermodynamic inefficiency in the transformation of energy and is referred to as ecological efficiency. Later, researchers discovered that ecological efficiency varies from 5-30% with an average of about 10%, depending on the species and the environment in which it lives.
Stuart Pimm published his classic book Food Webs in 1982. This book consolidated various aspects of food web theory and has become a reference for ecologists. The book’s many topics include food web complexity and stability and hypotheses on food chain length.
More recently, Stephen Carpenter and James Kitchell have become leaders in aquatic food web research. Their theory regarding the trophic cascade in aquatic food webs has been central to the current debate on top-down and bottom-up control of food webs.
Within food webs there are three main categories of organisms: producers, consumers, and decomposers. Producers are organisms that synthesize their own organic compounds using diffuse energy and inorganic compounds. Producers sometimes are called autotrophs (self-feeders) because of this unique ability. For example, green plants are autotrophs because they manufacture the compounds they need through photosynthesis. Photosynthetic organisms are called primary producers and they are the first trophic level of the food web. Their rate of productivity determines how much fixed energy in the form of potential energy stored in the chemical bonds of plantbiomass is available to higher trophic levels.
At higher trophic levels are all of the consumers, also called heterotrophs (other feeders). Heterotrophs feed on other organisms to obtain their energy and are classified according to the types of food they eat. Consumers that eat plants are called herbivores. Herbivores comprise the second trophic level of the food web and are called primary consumers because they are the first consumer group. Grass-eating deer and cows are primary consumers as are seed-eating mice and birds.
At trophic levels higher than that of the primary consumers, the food web fans out to include consumers that eat other animals, called carnivores, and consumers that eat both plants and animals, known as omnivores. Some of these are secondary consumers meaning they eat primary consumers. For example, wolves that eat deer and snakes that eat mice are secondary consumers. Tertiary consumers eat the secondary consumers, and so on. For example, an eagle that eats a snake that has preyed upon a mouse is a tertiary consumer.
In addition to this grazing food web there is another trophic section known as the decomposer food web. There are two main types of consumers of dead biomass: detritus feeders and decomposers. Both are called detritivores since they utilize dead plants and animals, or detritus. Detritus feeders, such as earthworms, ingest organic waste and fragment it into smaller pieces that the decomposers such as bacteria and fungi can digest. Unlike organisms in the grazing part of the food web, decomposers are extremely efficient feeders. Various types of decomposers rework detritus, progressively extracting more fixed energy. Eventually the waste is broken down into simple inorganic chemicals such as H20 and CO2 and nutrients such as nitrate, nitrite and sulfate. The nutrients then may be re-used by the primary producers in the grazing part of the food web. A well-known example of the decomposer food web is a compost pile, which turns kitchen wastes into a soil conditioner. Decomposers are active in all natural ecosystems.
As a result of human impact on the environment many food webs have become contaminated by insecticides and other manufactured chemicals. Some of these compounds have a profound effect on the reproduction and behavior animals at higher trophic levels. In many cases, chemicals were released into the environment because it was believed that their concentrations were too small to have an effect on organisms. However, ecological studies have shown that these contaminants are passed from organism to organism thought food webs. The most persistent are hydro-phobic (water-fearing) contaminants that accumulate in the fatty tissues of organisms such as PCBs and DDT. PCBs, or polychlorinated biphenyls, are a suite of about 209 different compounds, each with slight variations in their chemical structure. PCBs were widely used as insulating material in electric transformers and for other purposes. There is currently a worldwide ban on their production and use but large quantities still persist in the environment. DDT, or dichloro-diphenyl-trichloroethane, is an insecticide that has been dispersed all over the world. Unfortunately, both PCBs and DDT now occur in all plant and animal tissues even in remote areas where they were never used (for example, in animals such as polar bears and seals). Humans are also contaminated, and mothers pass these chemicals to their babies in their milk, which is rich in fat where DDT and PCBs concentrate.
Bioaccumulation refers to the tendency of persistent hydrophobics and other chemicals such as methyl mercury to be stored in the fatty tissues of organisms. When these compounds are spilled into the environment they are rapidly absorbed by organisms in food webs. It is estimated that 99% of pesticides do not reach the target pest which means the chemical ends up in the general environment. If these pesticides are hydrophobic they build in the tissues of non-pest organisms.
Once inside the fatty tissues of an organism, persistent hydrophobics are not excreted easily. Each time the organism is exposed to the contaminant, more is taken in and deposited in the fatty tissues, where it accumulates faster than it is excreted. Bioaccumulation is particularly acute in long-lived species because the period during which pollutants can accumulate is longer. Some governments have recommended against consuming fish over a certain age or size because the older and larger they get, the more contaminated they are likely to be.
Biomagnification (also called food web magnification or food web accumulation) is the progressive increase in the concentration of contaminants in organisms at higher trophic levels. Biomagnification occurs because of the ecological inefficiency of food webs and because of pollutant bioaccumulation in individuals. These two factors combine so that each subsequent trophic level has a larger concentration of contaminants dissolved in a smaller amount of biomass than the previous level. For example, DDT in Lake Ontario is biomagnified up the food web, so that top predators like herring gulls have tissue concentrations that are 630 times greater than primary consumers like zooplankton.
Dolphins have been studied by Japanese researchers as a model species for biomagnification because their migratory routes are known, they live in relatively unpolluted waters, and they live a long time (20-50 years). DDT has been found in dolphin blubber in greater concentrations (100 times greater than sardines) than would be expected given the small concentrations present in the water and in sardines, their favorite food. These unexpectedly large concentrations are the result of DDT biomagnification up the food web.
Biomagnification has serious consequences. It is particularly dangerous for predator species especially if they are at the top of long food webs. The degree of biomagnification is high by the time it reaches higher trophic levels. Also, top predators usually consume large quantities of meat, which is rich in fatty tissue and, therefore, contaminants. Polar bears, humans, eagles, and dolphins are examples of top predators, and all of these organisms are vulnerable to the effects of biomagnification.
Bioaccumulation —The tendency of substances, like PCBs and other hydrophobic compounds, to build in the fatty tissues of organisms.
Biomagnification —Tendency of organisms to accumulate certain chemicals to a concentration larger than that occurring in their inorganic, nonliving environment, such as soil or water, or in the case of animals, larger than in their food.
Food chain —A sequence of organisms directly dependent on one another for food.
Food web —The feeding relationships within an ecological community, including the interactions of plants, herbivores, predators, and scavengers; an interconnection of many food chains.
Hydrophobic compounds —“Water-hating” chemical substances, such as PCBs and DDT, that do not dissolve in water and become concentrated in the fatty tissues of organisms.
Photosynthesis —The conversion of radiant energy into chemical energy that is stored in the tissues of primary producers (e.g., green plants).
Primary consumer —An organism that eats primary producers.
Primary producer —An organism that photosynthesizes.
Trophic level —A feeding level in a food web.
Currently there is much debate over what forces control the structure of food webs. Some ecologists believe that food webs are controlled by bottom-up forces referring to the strong connection between primary production and the subsequent production of consumers. For example, adding large amounts of nutrients like phosphorus causes rapid growth of phytoplankton, the primary producers in lakes that subsequently influences consumers in the food web. Other ecologists believe that food webs are controlled by top-down forces meaning the predators near or at the top of the food web. For example, in the Pacific Ocean researchers have found that when sea otters disappear from an area, sea urchin (the favorite food of sea otters) populations increase, and these invertebrates dramatically overgraze the kelp beds. Removing top predators causes changes all the way down to the primary producers. Carpenter and Kitchell have called this type of control the trophic cascade because such food webs are controlled by forces that cascade down from the top trophic level. Understanding the roles of top-down and bottom-up forces within food webs will allow more effective management of ecosystems.
de Ruiter, Peter C., Volkmar Wolters, John C Moore, eds. Dynamic Food Webs: Multispecies Assemblages, Ecosystem Development and Environmental Change. Burlington, MA: Academic Press, 2005.
Polis, Gary A., Mary E. Power, and Gary R. Huxel, eds. Food Webs at the Landscape Level. Chicago, IL: University of Chicago Press, 2004.