Nature Notes: Diminishing Returns

Eugene Odum and a host of ecologists studying food chains discovered that about 90 percent of energy is lost when one trophic level feeds on another

    The only butterfly I’ve seen to date is the cabbage white, the one long from Eurasia that lays its eggs on members of the cabbage family, to wit, garlic mustard, wild radish, and the like, also from Eurasia.

    Butterflies and moths are part of the second trophic food level; they feed on the first level, the “producers.” In fact most insects — grasshoppers, various ant species, bees and other nectiferous species, almost all beetles and almost all bugs — feed on plants.

    Dragonflies, praying mantises, cicada killers (certain hornets), and a few other insect groups, as well as spiders that feed on the insects that feed on plants, are at the third trophic level in the food chain or “food pyramid.”

    There is a law in thermodynamics, part of the discipline of physics, that when you convert one form of energy to another form, say, solar to heat, you always lose some to entropy. Energy conversions are far from 100 percent efficient. Otherwise you would be able to run the world and all its countless activities on perpetual motion. In the last half of the 20th century, Eugene Odum and a host of ecologists studying food chains discovered that about 90 percent of energy is lost when one trophic level feeds on another.

    So if you start out with solar energy trapped by green leaves of green plants by way of the process of photosynthesis, you trap about 10 percent of that energy in the form of protein, sugars, and starch. When the gypsy moth larva comes along and eats the freshly opened leaves, say, of a white oak, it feeds on those stored foodstuffs and uses about 10 percent of the energy to make proteins, sugars, starches, and fats to enable it to grow up and pupate. By the time it busts out of its cocoon and becomes an adult, it has used 10 percent of the 10 percent of the original solar energy manufactured by the oak leaves.

    If a member of the third trophic level, say a blue jay or grackle, comes along and feeds on the gypsy moth larvae just before they pupate, it converts 10 percent of the ingested food into molecules for growth, proteins, and flying and carrying on the rest of its activities that characterize its overall lifestyle. By now, only 10 percent of 10 percent of 10 percent of the original solar energy has been captured and utilized.

    What if a bird hawk such as a Cooper’s hawk, a member of the forth trophic level, comes along and catches a grackle in its talons, takes it to the ground, plucks it and feeds on it? It manages to save 10 percent of 10 percent of 10 percent of 10 percent of the original energy. A bobcat, level five, could catch the hawk on the ground and eat it. Another 90 percent is lost, but the bobcat has the energy and musculature to catch another hawk or a rabbit in the future, or, if it’s a female, to nurse its kittens. In other words, only one millionth of the original solar energy falling on the plant is saved by the time the needs of the fifth trophic level are satisfied.

    Such a dog-eat-dog world sounds very inefficient, but the sun is a generous orb, it throws out more light energy every day than all of the plants on all of the world’s land and in all of the world’s oceans, rivers, and lakes could ever use. When the sun falls on a solar panel and is thereby converted to electrical energy, the loss, mostly in heat generated, is much less than the 90 percent waste energy lost by the plant or the rest of the trophic levels up through the food chain. But even so, the most efficient devices that change one form of energy to another waste more than 50 percent in the conversion.

    There is enough radiant energy from the sun to run all of those energy transducers that change one form of energy to another to do mechanical work. Humans are just figuring that out, a bit late in our development, to say the least.

    There is another trophic level, the reducers, they feed on dead stuff — leaves, rotting tree trunks, animal carcasses, and the like, which are ultimately broken down into the very stuffs from which new life will spring. Thus, the Biblical notion “ashes to ashes, dust to dust” is right on.

    Humans feed at each trophic level, two through five. In a world with more than seven billion of us, we would all starve to death if we fed only at level five, i.e., ate the meat of large carnivorous mammals, birds, and fish, say, sharks. “Give us this day our daily bread.” Wise words, indeed. They suggest that, at the very least, we should dine at the second trophic level, the one occupied by those that feed on plants. It would be hard to feed on the primary producers, phytoplankton. We don’t have the sieving apparatus that zooplankton, barnacles, clams, and certain fish species such as herrings, baleen whales, and basking sharks are equipped with.

    That is exactly why we are called terrestrial mammals, ones that evolved from earlier terrestrial mammals. We are given to live off the land. Nonetheless, a little seafood is good now and then, especially on Fridays.

    Larry Penny can be reached via email at