- G. Tyler Miller, Jr., American Chemist (1971)
Summary: Nature consists of food chains, scientifically known as trophic levels : organisms eat other organisms that are lower down the food chain, and in turn get eaten by organisms higher up the food chain, forming a pyramid. At the base of the pyramid is the energy of the sun. Above that are found the primary producers — plants and algae. They use the energy of the sun to grow food — for themselves and everyone else — via photosynthesis. They are called autotrophs (Greek for “self - feeders”) because they can feed themselves. Above the primary producers are the organisms that cannot manufacture their own food and need to eat plants to stay alive. These are called heterotrophs (“other - feeders”). Heterotrophs include herbivores (plant eaters) which feed directly on autotrophs; they run the gamut from bacteria to elephants. Still higher up the pyramid are specialized heterotrophs called carnivores (“flesh eaters”): animals that eat other animals. There are also omnivores ("all" eaters); they will eat just about anything. Along with bears and rats, that describes us pretty well, although we like to fancy ourselves as the ultimate predators , residing at the very top of the food chain. Not shown in the diagram are detritivores , which consume and recycle dead matter, putting some of its energy back into the ecosystem. Because every thermodynamic process must produce a certain amount of waste heat, consumers at each trophic level convert only about ten percent of the chemical energy they get from their food into their own organic tissue. What this means is that an animal three levels up the food chain (say, a lion, that eats gazelles, that eat grass) can convert no more than about one tenth of one percent of the original sunlight energy into its body mass and metabolism. This embodied energy is a tiny fraction of the original solar energy input, because of the large thermodynamic loss at every stage of the life process. This is also why it’s classically represented as a pyramid: due to the Second Law of Thermodynamics effects, each trophic level can support only a small fraction (~10%) of the biomass of the one below it. The nature of food chains, even at the microscopic level, determines what sorts of life can exist at the large scale. For instance, in the Antarctic seas there are unusually few trophic steps from phytoplankton to krill, enabling a huge biomass of large complex creatures which can eat krill – including blue whales, the largest animals to ever exist on this planet. If there were more trophic steps at the microscopic level, this abundance of macro species biomass like whales would not be possible.
It is thermodynamic energy flows that shape and control the life systems of
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