The Arithmetic of the Food Chain
The Desert Food Chain
At first, you might think that arithmetic would not have much to do with biology and the food chain. After all, you go to one class to learn about math and another to learn about living things. As we will see, however, arithmetic plays an essential part in understanding the relationships among the levels of the organisms of the food chains of the desert and, for that matter, of the earth itself.
Recalling the Links and Energy Flow of the Food Chain
Before we begin to consider the arithmetic, you may recall that we have described a food chain as a sequence of living organisms through which energy passes as the driving force in the life of a community of plants and animals. A food chain always begins with the plants, called “producers.” It always ends with the animals, called “consumers.”
In the process called “photosynthesis,” the plants capture energy from the sun and combine with it water and carbon dioxide to create “glucose,” a form of sugar required for reproduction and growth among the organisms. Plants are the first link in the food chain.
The herbivores animals that eat plant tissue are the second link.
Carnivores animals that eat other animals are a third link.
Omnivores animals that eat both plants and animals can be both second and third links. Scavengers eat dead and decaying organisms.
Decomposers bacteria and fungi feed on any remaining dead plant and animal tissue and animal waste. They break down the decaying organic matter. They convert it into carbon dioxide and water, making those compounds available to the plants for photosynthesis and beginning anew the cycle of life.
You can think of the organic matter that flows through the food chain link by link as blocks of energy used to fuel reproduction, growth and movement and waste production. As it passes through the food chain, much of the energy dissipates into the environment and, ultimately, into space in the form of heat.
The Arithmetic of Energy Flow Through the Food Chain
The plants use only about one percent of the total solar energy that strikes the earth to create living matter through photosynthesis. That means that for every 100,000 units (that is, any increment of measurement) of solar energy available, the plants use only 1000 units.
The animals in each succeeding link of the food chain convert only about 10 percent of the energy available to them as food into living matter. For example, if herbivores ate 1000 units of energy in the form of plant tissue, they would convert only 100 units into in the form of animal tissue and bone.
They spend the other 900 units in the form of waste or dissipated heat. Similarly, if carnivores ate 100 units of energy in the form of herbivore tissue and bone, they would convert only 10 units into new energy in the form of tissue and bone, spending the remainder as waste and heat. If a Golden Eagle ate 10 units of units in the form of another carnivore, it would convert only one unit into energy in the form of eagle tissue and bone.
This means that at least 1000 units of plant tissue energy would be required to support a single unit of carnivore-eating eagle flesh and blood energy, even though the plants are not eaten directly by the eagle. If you think of the links of the food chain as a pyramid, plants would form the base and eagles and other carnivores would form the tip.
The Relative Abundances of Organisms Within the Food Chain
The percentage of solar energy converted into living matter by plants during photosynthesis sets a limit on the total living matter or “biomass” on the earth. The percentage of food, or stored energy, that animals can convert into living matter, or newly stored energy, sets broad limits on the biomass at each link of food chain or level of the food pyramid.
The total biomass on earth equals according to the Southwest Renewable Energy Agency’s Internet site more than a trillion tons of dry (that is, water-free) organic matter. The plants the producers account for well over 90 percent of the earth’s biomass. The animals the consumers account for most of the remainder. (Bacteria, fungi, protozoa, algae and other life forms make up relatively small percentages of the biomass and the species population.)
The more complex organisms make up a very small percentage of the earth’s biomass. For instance, the earth’s human population, with some six and one-half billion individuals, accounts for only a small fraction of one percent of the earth’s total biomass. Small, more simple organisms make up a much larger percentage of the biomass. Microbes ancient microscopic plant and animal organisms that live even in the earth’s most extreme environments, including, for instance, the earth’s poles, its geysers, its deepest sea floors and its subsea floor structure may account for as much as 50 percent of earth’s biomass.
How small is a microbe? Well, as Hilaire Belloc said in his book More Beasts for Worse Children:
The microbe is so very small
You cannot make him out at all,
But many sanguine [optimistic] people hope
To seem him through a microscope
How many microbes are there on earth? The answer is, “an awful lot.” One scientist, Martin Fisk, of Oregon State University, estimates that “there are about 28,000,000,000 microbes in [a single] ounce of mud” on the ocean floor.
All these have never yet been seen
But Scientists, who ought to know,
Assure us that it must be so...
The Relative Abundances of Organisms in Different Environments
Our Southwestern deserts, compared with, say, an equatorial rain forest, seem like biological wastelands, primarily because the productivity and diversity of our food chains is more constrained by the harsh environmental conditions.
In an average year, our deserts receive only a few inches of rain, which fall in a random pattern. We experience evaporation rates that exceed rainfall rates by ten times or more. We have seasons when plants bloom and grow and seasons when, effectively, they sleep. Our daily air temperatures range from mild to blistering hot in the summer and cold to moderate in the winter. Desert soils contain little organic matter, or nutrients. In fact, soils in dry desert lake beds may contain concentrations of minerals, for instance, alkali salts, that are poisonous to most plants.
By comparison, in an average year, a tropical rain forest may receive as much as 20 to 30 feet! of rainfall, which fall more or less uniformly across the region. Water lost to evaporation returns rapidly in the form of rainfall. Located near the equator, rain forests have a never-ending growing season. Air temperatures range from the high 60’s to the low 90’s (in degrees Fahrenheit) throughout the year. Although the rain forest soils contain relatively little organic matter, nutrients from organic matter are freed rapidly by decomposers, allowing it to return to the food chain almost immediately. Additionally, rain forest soils are basically free of harmful minerals.
Because of the differences, the total organic matter, or biomass, produced by the food chains of our Southwestern deserts amounts to no more than a small fraction of that produced in comparably sized tropical rainforests. Moreover, the different species of wild plants and animals supported by our Southwestern deserts are measured in the tens of thousands. The different species supported by the rain forest might number in hundreds of thousands or even millions.
Our desert food chains, with their many spiny plants and venomous animals may appear to be hostile and indestructible, but they are, in fact, among the most fragile on the earth. Given their relatively low productivity and limited diversity in the desert environment, they lack biological and environmental resources to repair themselves when links are damaged or broken.
Already, food chains across the Southwest have been altered by overgrazing, land clearing, municipal and agricultural development, industrialization, road construction, recreational use, human water consumption and invasive plants and animals. They may never recover.
by Jay Sharp
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