Florida Benchmark
SC.912.L.17.9: Use a food web to identify and distinguish producers, consumers, and decomposers. Explain the pathway of energy transfer through trophic levels and the reduction of available energy at successive trophic levels.
Food Webs
Trophic Levels
The word ‘Tropho-‘ is a variant of the prefix/suffix ‘Troph-‘ and means “nourishment“. As previously discussed in our article here “autotrophs” are organisms that make food/nourishment for themselves while ‘heterotrophs’ get nourishment from other organisms. Plants, algae, some protists, photosynthetic bacteria, and some extremophile bacteria are examples of Producers/ autotrophs, and examples of producers in various ecosystems.
Organisms that feed on producers are called “Primary Consumers“, organisms that feed on primary consumers are referred to as “Secondary Consumers“. This pattern can continue indefinitely with Tertiary, and “Quaternary Consumers” and so on. Diagrams can be created that reflects the predatory prey relationship observed between producers and consumers in an ecosystem, we call these diagrams “Food Webs“.
The arrows in a food web or food chain are always going from prey to predator, showing the flow of nutrients and energy.
Energy Transfer
To summarize the first law of thermodynamics, energy is not created or destroyed it only changes from 1 state or position to another. In a food web the energy contained in the producers is not lost when they are eaten. 90% of the energy contained in the cells of producers is used in chemical reactions or escapes into the environment as heat while 10% of it is transferred from the the producers to the primary consumer. This process repeats as you move up each trophic level, 10% of the energy is transferred from one organism to another (The 10% Rule). The initial energy in an ecosystem is usually the sun if it is located on the surface of the Earth, but there can be other environmental energy sources. Deep sea ecosystems on the ocean floor may get the initial energy of their environment from hydrothermal vents releasing large amounts of heat and inorganic chemicals to nourish chemoautotrophs.
Energy can be measured in many units like; Daltons, Jules, and Kilocalories/Kcals/calories. Kcals are the unit of measurement used to communicate how much chemical energy your cells can extract from food at the grocery stores, this information is on the nutritional labels on the back of most prepackaged foods.
Concentrations of Chemicals in a Food Chain
Just as energy and nutrients can move up trophic levels and through a food web so too can toxic and dangerous chemicals like “pollutants“.
Bioaccumulation
Often due to human actions large amounts of a toxic pollutant may build up in the water, air, or soil. These chemicals can build up in the tissues and cells of organisms, especially those at the bottom of the food chain. As the organisms age and grow the levels of these chemicals can often increase significantly. This buildup over time is known as “Bioaccumulation“. The ‘concentration‘ or amount of a chemical is often measured in “ppm” or “parts per a million“.
Biomagnification
As organisms at the bottom of the food chain are consumed by larger heterotrophs the prey will pass on the chemicals and pollutants that build up in their tissues. What may have been insignificant levels of poison in the body of a single shrimp may increase to extremely dangerous levels in the body of a large fish. This is due to the fact that the large fish could possibly consume thousands of shrimp over the course of its life, each time increasing the levels of poison in its own tissues. The higher up the trophic level you go the higher the “concentration” or amounts of poison will be. This process of “biomagnification” usually has the greatest negative impact on the apex predatory of the food web, this is usually human beings.
A infamous example of a major pollutant is ‘mercury‘, element 80 with the atomic symbol ‘Hg‘ on the periodic table. There are currently large amounts of Mercury present in the ocean. The mercury is at low enough levels where a person may be able to swim their whole lives and be unaffected by it. The small organisms at the bottom of the ocean food webs build up mercury concentrations in their tissues over the course of their short lives. As these organisms are killed and eaten by predators the amounts of mercury in each trophic level will significantly increase. For this reason people can develop mercury poisoning from consuming too much fish that contain mercury. This is especially important for pregnant women as a developing fetus may be more susceptible to the damaging neurological effects of heavy metals such as mercury. To read more about mercury containing fish and the effects on a developing fetus read our article here.
DDT or (Dichlorodiphenyltrichloroethane) is a chemical pesticide that was heavily promoted and used in the United States during 50s and 60s. It was eventually banned in the United States in 1972 after it was discovered to accumulate in the fatty tissues of humans. DDT is believed to be a possible carcinogen, adversely effect the liver, seizures, and cause birth defects in developing fetuses. DDT also caused egg shell thinning in birds that are at the top of food chains like bald eagles. This was a contributing factor that helped to threaten certain species of birds.
