The Big Four of An Ecosystem
Ecologists recognize at least
four components in an ecosystem, each of which is essential and probably
has been essential since the beginning of life. A discussion of each
component will provide background for reconstructing mid-Pleistocene
conditions and events.
The Physical Environment
We are fairly certain that
during the Pleistocene epoch the earth rotated on its axis as it
revolved around the sun-as it does today. Phases of the moon
preceded and succeeded each other than as now. Atmospheric
pressures and temperatures were influenced by the same physical factors
in existence today and produced winds, clouds, fogs, rains, thunder,
lightning, dew, and humidity. Rains were seasonal, but the amount
of rainfall was probably greater during the summer months.
Water and wind erosion reduced
elevations and filled depressions. Streams were muddy or clear,
depending upon the amount of runoff from seasonal rains and the amount
and kind of sediment. As water percolated through the soil, it
dissolved mineral substances such as lime, phosphate, and potash and
carried them, together with other mineral substances, into larger bodies
of water which contained such organisms as plankton.
Amount the plankton were
diatoms-microscopic, free-moving plants that extracted silica from their
environment and incorporated it into their cell walls, transforming them
into protective test. Grasses, reeds, and other plants obtained
silica, which as generally deposited along the edge of the leaves.
Mineral-containing water was absorbed through plant roots by osmosis and
capillarity. Because these processes and organism exist today, we
assume that they also existed in the past.
A second component of ecosystems
is plant life. Plants extract minerals from water and carbon
dioxide from the air to manufacture food. Aided by sunlight in
this process, they produce food not only for themselves but for other
organisms as well. In this food is the energy used by all animals,
from paramecia to elephants. This is why ecologists refer to
plants as producers.
We were not in California some
1.3 million years ago, but we have fossil evidence to prove that plants
were there and that they were producers. Even without plant
fossils, we have to assume that plants were there. If carnivores
(meat-eaters) were the only fossilized animals found, we would have to
assume that herbivores (plant-eaters) had been an essential part of the
meat-eaters' environment. Without such assumptions, could anyone
reconstruct an ecosystem of the past?
Animals are the chief consumers.
herbivores are first-order consumers-they consume producers. Mice,
deer, horses, camels, and other plant-eating animals are in this group.
Carnivores are second-order consumers if they eat consumers of the first
order. For example, a fox is a second-order consumer if he eats a
mouse. If a wolf eats the fox, the wolf is a third-order consumer,
and so on. Higher orders of consumers exist, but each higher order
moves a step farther from the producers. Try to imagine a
Pleistocene situation in which a wolf is eaten by a sabercat. In
which consumer order would you place the cat?
Although remains of insects have
not been found in the California deposits, we assume that both
carnivorous and herbivorous insects were there We assume that tree
leaves were eaten by leaf-eating insects, wood and bark by boring
insects. Predatory insects consumed other predators and
herbivores. Today, throughout California, different species of
ladybird beetles feed extensively on aphids, scales insects and mites.
There is no evidence to prove that this did not happen during the
Assuming that there were
mid-Pleistocene insects, do you suppose that honeybees were among them?
If so, would it be illogical to further assume that bears were there to
raid the beehives? Wouldn't the presence of honeybees require the
presence of flowering plants?
The fourth major component of an
ecosystem, represented by bacteria and fungi, is a group known as the
decomposers. These organisms break down the complex organic
materials of dead protoplasm (both plant and animal) and release simpler
substances that can be used by producers. For example, certain
kinds of bacteria convert dead plant and animals proteins into ammonia.
Other kinds, called nitrifying bacteria, convert the ammonia into
nitrates that will be used by a new generation of plants. Can you
imagine ancient ecosystems without fungi and bacteria? If they had
not been present, wood would not have rotted, and dead flesh would not