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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.
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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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