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The Arithmetic of the Food Chain

The Desert Food Chain


Arithmetic plays an essential part in understanding the relationships among the levels of the organisms of the food chains of the desert and, even of the earth itself.

Click here to read the introduction to the Food Chain.

Bison

Large herbivore, the bison, or buffalo, which grazed in the Sonoran and Chihuahuan desert grasslands until about the 13th or 14th century.

The Arithmetic of Energy Flow Through the Food Chain

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 in the form of another carnivore, it would convert only one unit into energy in the form of eagle tissue and bone.

Prairie Dog

A legendary small herbivore, the prairie dog, which lived in vast colonies, was the favored prey for many carnivores until the 20th century.

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. The links of the food chain are a pyramid, plants form the base, and eagles and other carnivores 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 more than a trillion tons of dry (that is, water-free) organic matter. (Southwest Renewable Energy Agency’s Internet site) 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. 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 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 see 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...

Red-tailed Hawk

The red-tailed hawk feasted on ground-dwelling animals, including prairie dogs, whose populations have plummeted.

The Relative Abundance of Organisms in Different Environments

The Southwestern deserts, compared with an equatorial rain forest, seem like biological wastelands, primarily because the productivity and diversity of its food chains is more constrained by harsh environmental conditions.

In an average year, Southwestern deserts receive only a few inches of rain, which fall in a random pattern. Evaporation rates exceed rainfall rates by ten times or more. There are seasons when plants bloom and grow and seasons when, effectively, they sleep. 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 60s to the low 90s (in degrees Fahrenheit) throughout the year. Although 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 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 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. 

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 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

Food Chain Introduction
How Do Green Plants Manufacture Their Own Food?
How Do Desert Plants Survive?
How Do Desert Animals Survive?
The Arithmetic of the Food Chain
The Classification of Desert Plants & Animals


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