A Pleistocene Ecosystem
by Wesley Gordon
page 5


Relating Matter and Energy

Now we need to look at three related concepts-food chains, loss of mass and energy, and population pyramids- that fit logically into the ecologist's organized body of knowledge.  They are concepts derived from interpretations of numerous bits of isolated data.  An understanding of each will be of great use when you begin visualizing and describing events that probably occurred in the 1.3 million year-old ecosystem.

A hypothetical population pyramid. Compare the number of each kind of predator with that of its prey.  Predator populations are generally smaller than prey populations.  Although predators are commonly bulkier than the organisms they eat, the golden eagle is the pyramid is an exception.  It is about equal in length and weight of the fox.

Food Chains

 The transfer of matter and the release of energy through food from one organism to another is called a food chain.  For example:  Plant lice pierce plant stems and suck the sap, only to be eaten by carnivorous insects, which , in turn, are eaten by spiders that are devoured by small birds.  the small birds are captured and eaten by hawks.  This chain consists of six links: plants, plant lice, carnivorous insects, spiders, small birds, and hawks.  (Describe a chain with only three links; with two, with seven.)

Loss of Mass and Energy

Loss of mass and energy occurs in each link of food chain.  Consider loss of mass first:  Part of the grass eaten by herbivores consists of water and indigestible plant fibers.  Some of the water and all of the fibers are eliminated as wastes.  Similar losses of mass occur when carnivores, the next links in the food chain, eat the herbivores.

It has been calculated that between 80 and 90 percent of the organic bulk (mass) of energy-giving food is lost in each consumer link.  Thus, the larger the chain the greater the total loss from producers to final consumers.  Specialists estimate that in a plant-steer-man food chain about 4,000 pounds of alfalfa is required to produce one yearling weighing 550 pounds, and almost five such yearlings would be needed to feed a boy from birth to the age of twelve.

Conversion of the mass of energy of one organism into the protoplasm of another also results in a loss of some energy in the form of heat.  As mentioned before, green plants are the producers in any ecosystem, and the energy stored by them comes from the sun.  Researchers estimate that only about 50 percent of the sunlight falling on a leaf is absorbed, and only a small fraction of this amount becomes available to herbivores and carnivores.  For example, when 3,000 units of solar energy fall upon a plant, 1,500 units will be absorbed by the plant.  But only 15 units will be available as net plant production.  Of these 15 units, only 1.5 units will be assimilated by herbivores.  of the 1.5 units, only 0.15 units will be assimilated by carnivores.  Organisms generally transfer to other organisms less energy than they initially receive.

Materials that do not yield energy-such as water, nitrogen, carbon, and other contained in protoplasm-may circulate repeatedly from link to link in a food chain.  It is quite possible that certain individual atoms in your body were also part of plant or animal protoplasm over a million years ago!

Population Pyramids

Imagine that in a particular area a producer, grass, supported thousands of insects and that about a dozen small birds were supported by the insects.  How many hawks could be supported by the birds?  It seems clear that the hawks would have to survey many such areas to survive.  This imaginary situation illustrates a reality of most ecosystems:  Predatory animals are usually larger than the organisms they eat.  Thus they require relatively large amounts of prey to survive.  This is the answer to the often-asked question, Why are there usually so many plant-eaters for every meat-eater?


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