17.15: Food Chain Efficiency

Essential to Know

• All organisms require food for respiration, growth, and reproduction.
• All organisms lose some of the food as waste products.
• The rate of biomass production by an organism that is available as food for the next trophic level equals the total amount of food ingested, minus the proportion used for respiration and reproduction, and the proportion lost as waste products.
• The percentage of ingested food converted to new biomass is the food chain (or trophic) efficiency. It is usually approximately 10% (ranging from 1% to 40%), regardless of species or food source.
• Approximately 90% of the available food biomass is used in respiration and reproduction or lost as waste products at each trophic level in a food chain.
• Long marine food chains that lead to consumers at higher trophic levels, such as tuna, utilize primary production hundreds of times less efficiently than short marine food chains, such as those that support anchovies or sardines, or most terrestrial food chains used to produce food for human consumption.

Understanding the Concept

All organisms require food, which they can obtain by performing photosynthesis or chemosynthesis or by ingesting organic matter. In all species, food is distributed and used in four ways: (1) to provide energy through respiration for the life processes of the organism, (2) to provide the basic materials for production of the myriad compounds contained in any organism’s body, (3) to provide the basic materials needed to produce offspring (including eggs and sperm), and (4) as waste products that are excreted. Hence, not all food is incorporated into new body tissues or biomass. The amount converted to new biomass equals the total food intake minus the amounts used for respiration and reproduction and lost as waste. Food used for reproductive processes does create new biomass, but the vast majority of eggs, larvae, and juveniles of most species are consumed by predators. Consequently, most new organic matter produced during reproduction does not contribute to the species’ adult biomass.

The percentage of food ingested by a particular species and converted to new biomass is often referred to as food chain efficiency or “trophic efficiency.” The total biomass of a species can be estimated if the total amount of available food and the food chain efficiency are both known. For example, studies of Loch Ness have determined that it would be virtually impossible for a viable population of “monsters” to live there, because the total amount of food available in the loch would not be enough to sustain even one very large animal, unless it converted all available food into new biomass at an impossibly high food chain efficiency.

The efficiency of converting food to biomass is an important parameter in managing food supplies for human populations. For example, much of the corn fed to beef cattle is used by the cattle in respiration, reproduction, and waste generation. Hence, although beef has a higher protein content than corn and therefore is a desirable food, the same amount of corn could feed more people if eaten directly than if first fed to cattle, which are then eaten. This concept is particularly important in many parts of the world where famine persists, but where cattle are raised on corn and other grains that could be consumed directly by people.

Terrestrial food chains used for human food are generally very short. Humans consume primarily plant material, which is at the primary production or first trophic level, or plant-fed animals, which are at the second trophic level. Ocean food chains that lead to human foods are, in many cases, much longer. For example, tuna generally feed at the fifth trophic level (Fig. 12-11). The overall efficiency of the food chain from primary production (phytoplankton) to tuna is very low because each step is inefficient.

Food chain efficiency is difficult to measure precisely because it varies with many factors, such as the amount of food available, food quality, the nature and timing of the reproductive cycle, and the level of physical activity required to avoid predators. As an example of this variability, consider that two human children of the same height who ingest approximately the same amount of food, but one may be much thinner than the other. We might conclude that the thinner child has a “faster metabolism.” What we mean is that the thinner child uses more food for respiration (because of either greater physical activity or a genetic disposition to utilize food less efficiently in respiration) or that the thinner child sends a greater proportion of food to waste (because of genetic differences in the ability to digest and assimilate food). Of course, if the food ingested by these two children was not equal in quantity, the thinner child could also be thinner simply because of a limited food supply and eating less. Another illustration of the variability of food chain efficiency is the difference in efficiency between children, whose biomass is increasing, and adults, whose biomass stays relatively constant, even though they may ingest approximately the same amount of food as children.

Food chain efficiency is variable. It has been determined that, if averaged over a population and over time, this efficiency ranges from about 1% to 40% and, in most instances, is approximately 10% between any two trophic levels. Hence, on average, all species above the primary producer trophic level convert only about 10% of their food into new biomass, which then is available to be consumed at the next trophic level. The following relationships summarize marine food chains:

1 kg of biomass at trophic level 1 (phytoplankton) 

produces 0.1 kg of biomass at trophic level 2 (zooplankton)

produces 0.01 kg of biomass at trophic level 3 (e.g., small fishes, baleen whales)  

produces 0.001 kg of biomass at trophic level 4 (e.g., larger fishes such as mackerel, squid)

produces 0.0001 kg of biomass at trophic level 5 (e.g., predatory fishes such as tuna)

produces 0.00001 kg of biomass at trophic level 6 (e.g., killer whales)

In this example, certain types of organisms are designated as feeding at a particular trophic level. However, some of them, such as killer whales, may feed at more than one trophic level.

Many seafood species captured and eaten by people feed at a much higher trophic level than terrestrial animals used as human food. Consequently, human use of ocean primary production is grossly inefficient compared with terrestrial primary production.