Ecosystems are nonequilibrium dissipative
processes. The kind of energy required for organisms to maintain their
bodies, their metabolism, is strictly limited. The list includes
light (photoautotrophy), organic chemical energy (heterotrophy), and
a very limited number of inorganic energy-yielding chemical reactions
(sulfide to sulfur or sulfate, methane to carbon dioxide, ammonia to
oxidized nitrogen compounds, hydrogen to water). Heat, a thermodynamic
waste product roughly equivalent to entropy, is not on the list. Organisms
also need food, which forms the stuff of their bodies. Energy gets used
up; food is transformed into matter and materials of the body.
Ecology has been called an orphan science. In contrast
with theoretical physics, space sciences, and the human genome project,
ecology attracts little money. The federal budget for theoretical ecology
is in the tens of millions of dollars annually whereas tens of billions
of dollars support these other fields. Why does ecology, the study
of the home that supports us, receive such scant attention? Perhaps
it is because ecosystems are so ordinary. Familiarity can engender
contempt: human-supporting ecosystems are neither wondrously huge nor
enchantingly tiny. They hark to no ancient time nor storybook place.
They are right here now, at our scale, in our face…..
Odum's 1969 paper was given as the presidential
address to the Ecological Society of America annual meeting at the
University of Maryland. During this period of great political unrest
in the United States because of the war in Vietnam, there was also
an industry- and labor-supported backlash on recent national environmental
victories. The college campuses were in an uproar over both issues.
Even though this paper is the most important synthesis of phenomenological
observations of ecosystems ever completed, more than a third of it
is dedicated to environmental issues. Here was the president of the
Ecological Society of America speaking out on crucial political-environmental
issues of his time: "Thus the preservation
of natural areas [is not a] peripheral luxury for society, but a capital
investment from which we expect to draw interest. Also, it may well be
that the restrictions in the use of land and water are our only means
of avoiding overpopulation or too great an exploitation of resources,
or both.” It was a brave lecture for its time.
Odum's classic text Fundamentals of Ecology, first published in 1971,
linked energy flow to succession. In both his 1969 paper and his text
Odum made some basic observations. With regard to ecological succession,
he made three points: (1) "It is an orderly process of community
development that is reasonably directional and therefore, predictable." (2) "It
results from modification of the physical environment by the community." (3) "It
culminates in a stabilized ecosystem in which maximum biomass (or high
information content) and symbiotic function between organisms are maintained
per unit of energy flow.”
Odum's ecology fits comfortably into our thermodynamic paradigm. We
have attempted to synthesize Odum's basic ideas in the
Figure. This
diagram maps changes in ecosystem characteristics discussed in this
and the next chapter. We have made additions and attempted to distill
Odum's ideas. Across the top of the figure is the flora associated
with the archetypal New England succession, from fallow field to oak-hickory
forest. In the western United States the sequence would look similar,
with sagebrush instead of blackberries and bunch grasses playing the
role of crabgrass. The final stages of the western forest would be
fir, spruce, or white pine instead of oak. The same successional processes
are seen in grasslands, where there are no forests of oaks and fir.
Grasslands go through their own succession, with annual grasses replaced
by perennials. The ecosystem is doing its "best" under its
constraints to prosper. Many of the steppes and grasslands of the world
are water-limited, and the grasslands have evolved to develop a stable
community that degrades incoming energy as completely as possible.
A climax ecosystem should be looked at as a mosaic of different stages
of ecosystem succession. Each time one of these major setbacks happens,
for example a big forest fire, the ecological structure and process are
set back to the beginning like a player sent to Go on a Monopoly board,
the initiation point for the game. The penalty is not being thrown out
of the game, but being set back to zero and starting the progression
again.
Because of the patchiness in ecosystem disturbance, ecosystems tend
not to die. But then, they are not organisms (no organism recycles all
its materials), although, like organisms, they are complex thermodynamic
systems. Ecosystems represent biologically stable patterns across many
scales of a space-time hierarchy. Climax systems are vulnerable to rapid
change. The systems are often at an unstable point, like dry wood load
and biomass in a dry forest. A very small perturbation, a spark from
metal to rock of a passing horseshoe, can reset the successional clock
within seconds. Nonlinear dynamics and catastrophic events are common
throughout these biological systems.
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