The Biosphere
The biosphere is that part
of the earth in which life exists (hutchinson, 1970). It may be thought of as a
biochemical system capable of capturing, converting, storing, and utilizing the energy of the
sun. Approximately three hundred thousand species of green plants and microorganisms
are recognized as primary producers which utilize inorganic elements and
compounds to synthesize the organic materials of life. Their productivity is consumed
by more than a million other species of organisms which convert this organic
storehouse into animal form, adding to the beauty and value of the biosphere as
well as its complexity. Still other species, primarily bacteria and fungi,
accomplish the re-cycling process by returning plant and animal wastes and
residues to inorganic form so the process may be renewed. In this cyclic
process of life, manv elements are shared from a common global pool and are
converted from inorganic to organic form and back again. Examples of such
elements in ah living organisms are carbon, oxygen, hydrogen, nitrogen,
phosphorus, sulfur, sodium, potassium, calcium, magnesium, iron, manganese,
cobalt, copper, and zinc. Perhaps 15 to 21 other elements, such as selenium, vanadium, and
boron, are less abundant
and not universal in all living forms, but still vitally important in some
species.
The oldest known
photosynthetic organisms are at least two billion vears old, and throughout
this vast period of time, the entire biosphere has matured and evolved as a
self-regulatory system with internal checks and balances. It has developed a
remarkable degree of homeostasis and ecological control within its various
components. At later points in this book, we will consider in more detail the
process of photosynthesis, the cyclic passage of elements, the flow of energy
through living organisms, the growth and development of populations, and the
maturation of communities and ecosystems.
In physical terms, the
biosphere is a relatively thin and incomplete envelope covering most of the
world. It represents a mosaic of different biotic communities from simple to
complex, aquatic to terrestrial, and tropical to polar. It does not exist in
the extremities of the polar regions, the highest mountains, the deepest ocean
troughs, the most extreme deserts, or the most highly
polluted areas of land and water. Its total thickness, including all portions
of the earth where living organisms can exist, is less than 10 miles. Its zone
of active biological production, in terms of photosynthesis, is much narrower,
and varies from a few centimeters to over 100 meters. This zone would, for example,
be only a few centimeters in muddy or turbid water, whereas in very clear ocean
water, it could be more than 100 meters in thickness. On land, the zone of
biological production might be only a few millimeters in a desert or rock
environment, where it might again be more than 100 meters in a sequoia or
tropical rain forest. Living organisms can exist, of course, beyond the range
of active biological production; some insects or birds may be airborne to
altitudes aihove 20,000 feet, and viable spores, seeds, and microorganisms may
ibe found in the atmosphere and mountain tops above 25,000 feet. In tlhe ocean
depths, many animals can exist well below a thousand feet one hydra-like animal
has been photographed at a depth of 15,900 feet in the south atlantic. In both
extreme altitude and extreme depth, however, the organisms depend upon the much
thinner zone of active biological production, that portion of the system which
converts the energy of sunlight into the chemical and physical energy of living
organisms. Figure diagrams some of the altitudinal limits of the biosphere.
A simple analogy may help us
to visualize the relative thickness of the biosphere. If the diameter of the
earth were represented by the height of an eight-story building, approximately
100 feet tall, the total biosphere would be represented by a board 1,5 inches
thick (less than 4 centimeters) on top of this building. On the same building,
the zone of active biological production within the biosphere would be
represented by a piece of paper, approximately 0.3 millimeters thick, and even
this thickness would represent the most favorable conditions, such as a
tropical rain forest or a clear coral sea.
It becomes apparent, then,
that we are dealing with, living in, and totally dependent upon a very thin envelope
of life-supporting environments which partially cover our globe. We might think
of them as a thin, irregular veil of living material. When we realize that this
is the ultimate life insurance system for all mankind, as well as for several
million other species of living organisms, it adds a new urgency to our
attempts to understand the biosphere and our effects on it.