12.1 Ecosystems, Food Chains, and Food Webs

Food Chains and Food Webs

The term “food chain” is sometimes used metaphorically to describe human social situations. Individuals who are considered successful are seen as being at the top of the food chain, consuming all others for their benefit, whereas the less successful are seen as being at the bottom.

The scientific understanding of a food chain is more precise than in its everyday usage. In ecology, a food chain is a linear sequence of organisms through which nutrients and energy pass: primary producers, primary consumers, and higher-level consumers are used to describe ecosystem structure and dynamics. There is a single path through the chain. Each organism in a food chain occupies what is called a trophic level. Depending on their role as producers or consumers, species or groups of species can be assigned to various trophic levels.

In many ecosystems, the bottom of the food chain consists of photosynthetic organisms (plants and/or phytoplankton), which are called primary producers. The organisms that consume the primary producers are herbivores: the primary consumers. Secondary consumers are usually carnivores that eat the primary consumers. Tertiary consumers are carnivores that eat other carnivores. Higher-level consumers feed on the next lower trophic levels, and so on, up to the organisms at the top of the food chain: the apex consumers. In the Lake Ontario food chain shown in Figure 12.3, the Chinook salmon is the apex consumer at the top of this food chain.

Figure 12.3. These are the trophic levels of a food chain in Lake Ontario at the United States-Canada border. Energy and nutrients flow from photosynthetic green algae at the bottom to the top of the food chain: the Chinook salmon. [Image Description]

One major factor that limits the length of food chains is energy. Energy is lost as heat between each trophic level due to the second law of thermodynamics. Thus, after a limited number of trophic energy transfers, the amount of energy remaining in the food chain may not be great enough to support viable populations at yet a higher trophic level.

The loss of energy between trophic levels is illustrated by the pioneering studies of Howard T. Odum in the Silver Springs, Florida, ecosystem in the 1940s (Figure 12.4). The primary producers generated 20,819 kcal/m²/yr (kilocalories per square meter per year), the primary consumers generated 3368 kcal/m²/yr, the secondary consumers generated 383 kcal/m²/yr, and the tertiary consumers only generated 21 kcal/m²/yr. Thus, there is little energy remaining for another level of consumers in this ecosystem.

Figure 12.4. The relative energy in trophic levels in a Silver Springs, Florida, ecosystem is shown. Each trophic level has less energy available and supports fewer organisms at the next level. [Image Description]

There is one problem when using food chains to accurately describe most ecosystems. Even when all organisms are grouped into appropriate trophic levels, some of these organisms can feed on species from more than one trophic level; likewise, some of these organisms can be eaten by species from multiple trophic levels. In other words, the linear model of ecosystems, the food chain, is not completely descriptive of ecosystem structure. A holistic model—which accounts for all the interactions between different species and their complex interconnected relationships with each other and with the environment—is a more accurate and descriptive model for ecosystems. A food web is a graphic representation of a holistic, nonlinear web of primary producers, primary consumers, and higher-level consumers used to describe ecosystem structure and dynamics (Figure 12.5).

Figure 12.5. This food web shows the interactions between organisms across trophic levels in the Lake Ontario ecosystem. Primary producers are outlined in green, primary consumers in orange, secondary consumers in blue, and tertiary (apex) consumers in purple. Arrows point from an organism that is consumed to the organism that consumes it. Notice how some lines point to more than one trophic level. For example, the opossum shrimp eats both primary producers and primary consumers. (credit: NOAA, GLERL) [Image Description]

A comparison of the two types of structural ecosystem models shows strength in both. Food chains are more flexible for analytical modeling, are easier to follow, and are easier to experiment with, whereas food web models more accurately represent ecosystem structure and dynamics, and data can be directly used as input for simulation modeling.

Figure Descriptions

Figure 12.3. This vertical diagram shows the trophic levels of a food chain in Lake Ontario, moving from the bottom (primary producer) to the top (tertiary consumer). On the left side, a column titled “Organism” lists and depicts each species; on the right side, a column titled “Trophic Level” categorizes each organism’s ecological role. At the base is green algae, labeled as the Primary Producer. Above that are mollusks, labeled as Primary Consumer, which feed on the algae. Next is the slimy sculpin, labeled as a Secondary Consumer, which eats the mollusks. At the top is the Chinook salmon, labeled as the Tertiary (apex) Consumer, which preys on the slimy sculpin. Arrows between the organisms indicate the upward flow of energy and nutrients through the food chain, consistent with trophic structure. The diagram visually supports the caption’s explanation that energy begins with photosynthetic algae and moves up to the top predator, the Chinook salmon. [Return to Figure 12.3]

Figure 12.4. This horizontal bar graph is titled “Relative Energy Content in Trophic Levels” and shows energy content in different trophic levels in a Silver Springs, Florida, ecosystem. The x-axis represents energy content in kilocalories per square meter per year (kcal/m²/yr), ranging from 0 to 20,000. The y-axis lists trophic levels from bottom to top as: Primary producers, Primary consumers, Secondary consumers, and Tertiary (apex) consumers. The graph shows a large purple bar for Primary producers, reaching just under 20,000 kcal/m²/yr. Primary consumers show a much smaller bar, around 3,000 kcal/m²/yr. Secondary consumers have an even smaller bar, and Tertiary consumers have the smallest bar, barely visible in comparison. The visual clearly demonstrates the rapid drop in energy available at each ascending trophic level, supporting the caption’s point that higher levels support fewer organisms due to reduced energy availability. [Return to Figure 12.4]

Figure 12.5. This diagram shows a food web representing trophic interactions among aquatic organisms in the Lake Ontario ecosystem. Organisms are organized into four trophic levels, each outlined with a different color: green for primary producers, orange for primary consumers, blue for secondary consumers, and purple for tertiary (apex) consumers. The bottom level of the illustration (outlined in green) shows primary producers, which include diatoms, green algae, blue-green algae, and flagellates. The next level (outlined in orange) includes the primary consumers that eat primary producers. These invertebrates include zebra/quagga mussels, chironomids, mollusks, calanoids, native waterfleas, raptorial waterfleas, rotifers, amphipods, and invasive waterfleas. Primary consumers are in turn eaten by secondary consumers, (outlined in blue) which include fish like alewife, emerald shiner, rainbow smelt, round goby, and invertebrates like opossum shrimp and cyclopoids. The small fish are eaten by larger fish, the tertiary, or apex consumers, including sea lamprey, lake trout, Atlantic salmon, Chinook and coho salmon, brown trout, burbot, and walleye. Finally, all fish are eaten by the sea lamprey. Arrows indicate the direction of energy flow from the organism being consumed to the one consuming it. For the most part, every organism at a given level eats every organism in the level below and nothing else, but there are exceptions. For example, not all secondary consumers eat all primary consumers. Lake whitefish only eat zebra and quagga mussels, round goby eat all primary consumers, and the other secondary consumers eat every primary consumer except for the zebra and quagga mussels. The secondary consumer yellow perch eats two other secondary consumers from its own level, the slimy sculpin and the rainbow smelt, in addition to eating primary consumers from the level below. Some organisms can also skip a level. The secondary consumer opossum shrimp eats most primary consumers in the level immediately below and all of the primary producers two levels below. Sea lamprey eat all tertiary and secondary consumers. Thus, the food web is complex with interwoven layers. [Return to Figure 12.5]