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Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment
Environmental Stability andSustainable Development
Miguel A. Santos*Baruch College, City University of New York, USA
ABSTRACTMany scholars have advocated that the cornerstone of sound environmental man-agement is an effective control of stability of the human life-support system. Acommon theme running through these suggestions is that we should maximize theinherent stability of the life-support system. This essay proposes a new scheme ortechnique of classifying the stability of systems. Then the essay describes how thestabilizing mechanisms may be considered as a force that holds the human life-support system intact. Stabilizing energy is the energy available to do work, withoutcompromising the integrity of the configuration. The anthropogenic processes of harvesting or using the system as a sink for pollutants are the counterforce that tends to destabilize the system. The basic conclusion is that if society is using asystem, then the maximum energy of the anthropogenic processes cannot exceed thestabilizing energy. If this occurs, the system reaches its metastate. Copyright © 2005John Wiley & Sons, Ltd and ERP Environment.
Received 17 June 2003; revised 24 March 2004; accepted 29 April 2004
Keywords: stability; homeostasis; homeorhesis; stabilizing energy; metastate
Introduction
THE BALANCE OF NATURE IS AN INCIDENTAL BY-PRODUCT OF THE INTERACTION OF NATURE’S FORCES
(Brewer, 2000; Calow, 2000). As these forces push and pull matter and energy without con-
scious plan or purpose, we as moral beings in quest of the optimum conditions for human life
must develop environmental systems that have a rate or speed of change that buttresses the
stability of the human life-support system (Kay, 1991; Santos, 1995; Holling and Meffe, 1996; Ludwig
et al., 1997; Carpenter et al., 1999; Levin, 1999; Whitford et al., 1999).
Moreover, according to some environmental statutes and treaties, the need to maintain the integrity
(structure and function) of environmental systems has become public policy. To that end, indicators of
stability have been proposed that serve as a measuring rod or yardstick of sustainability (Santos, 1983;
Liverman et al., 1988; Kay, 1991; Whitford et al., 1999; Barrett and Odum, 2000). Most indicators
* Correspondence to: Dr. Miguel A. Santos, Department of Natural Sciences, Box A-506, Baruch College of the City University of New York, NY10010, USA. E-mail: [email protected]
Sustainable DevelopmentSust. Dev. 13, 326–336 (2005)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sd.259
Environmental Stability and Sustainable Development 327
concentrate on assessing the dysfunctions of environmental systems rather than gauging the inability
of the system to maintain its stability in the face of perturbations. Just as the maintenance of a stable
internal environment relative to some established measurable state is referred to as homeostasis, the
controlled change and development has been termed homeorhesis (White et al., 1992). Systems with
high stability tend to have high control, homeostatic or homeorhetic properties.
This essay includes an analysis of the stability of environmental systems and proposes a new scheme
of classifying their stability. The essay concludes with perspectives on some unifying principles, directly
related to the understanding of stability of environmental systems.
Stabilizing Mechanisms
In our discussion of stability mechanisms, we will classify them in three general types: primary, sec-
ondary and tertiary. Despite these separate considerations, we must not lose sight of the commonality
of purpose – the preservation of the integrity of the system. Insofar as each of these strategies function
to maintain a stable system, it can be considered as a control, homeostatic or homeorhetic mechanism.
Primary Stabilizing Mechanisms
Primary stabilizing mechanisms are based on the size, mass and mass distribution of a system. In a
non-living system, the more matter a system contains, the more the system can resist any change in its
integrity. In living systems, stability is associated with an optimum mass. In a sense, this stabilizing
mechanism overlaps with the concept of ‘congeneric homotaxis’. The latter term was coined originally
by Hill and Durham (1978) to describe stability based upon redundancy in functional parts of the system.
Generally speaking, suppose we ignore the other forces applied against a system; if a given force acting
on a system that weighs 100 grams changes the system by 10%, the same force applied to a system that
weighs 200 grams should change the system by 5%. It follows that the matter of a system, which has
redundant functions, is a measure of its inertia or its resistance to changes in its integrity. This notion
aids in understanding why oxygen’s relatively large atmospheric volume of 21% provides it with a rela-
tively high inertia. Indeed, if photosynthesis were to stop, it would take approximately 2.5 million years
for all the oxygen in the atmosphere to be exhausted (Raven and Johnson, 1986). On the other hand,
since carbon dioxide has a small atmospheric volume of 0.04%, this molecule has a relatively low inertia.
Therefore any small shift in the concentration in this molecule (a force) readily perturbs its relative
concentration in the atmosphere. Indeed, the amount of carbon dioxide can be affected by volcanic
eruptions, time of day, season and anthropogenic processes (Schneider, 1989).
The stability of atmospheric oxygen and carbon dioxide is much more complicated than described.
It is affected by a complicated biogeochemical cycle series of transformations in the ecosphere in
which these molecules are interconnected between various states of organisms and abiotic reservoirs
(American Geophysical Union, 1989; Schneider, 1989).
To further illustrate the correlation between stability and mass, consider the following. Almost all the
mass of our solar system (99.9%) is concentrated in the sun. Therefore, the ultimate source of primary
stability for all life on earth is the sun.
Biomass increases during ecological succession (Brewer, 2000; Stilings, 2002). The sheer biomass
of the forest provides a more stable habitat than the biomass of the grasslands. Thus, the K-adapted
species that typically dwell in the climax forest community live in a much more stable habitat in com-
parison to the species that inhabit the pioneer grassland community. The K-strategists are so called
because such organisms tend to maintain a population size close to their environment’s carrying
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)
328 M. A. Santos
capacity (K ). The K-adapted species have lived in the climax community for many generations and are
at a steady state. They fluctuate a little with the vagaries of environmental forces. Apparently, the biomass
of the community tends to produce steady-state species that occupy more stable habitats and do well in
relatively constant, predictable conditions.
Another example concerning primary stability relates to one of the most fundamental environmental
concerns, the extinction of species. Most species that are rare tend to have relatively little primary sta-
bility. Their biomass is relatively small in comparison to other species. The small size of their biomass
may be attributed to two encompassing factors: food level and tolerance range of species. The higher
the food level a species occupies, the less energy is available to it. Thus predators, because they are at
the pinnacle of the energy pyramid, tend to be few in numbers. They are the most prone to extinction
(Brewer, 2000; Stilings, 2002).
Some organisms, called euryecious species, have wide tolerance ranges. They thrive in a variety of
habitats and are active at wider ranges of environmental conditions. For example, they are active at wider
ranges of temperatures. Organisms with relatively narrow tolerance ranges are stenoecious species.
Since stenoecious species have less ability to survive and reproduce in the face of changing factors, they
are more prone to extinction than euryecious species.
Other variables being equal, the higher the food level a species occupies or the more stenoecious a
species is the more rare it is. A small population is inherently more prone to become extinct than a
larger one (Brewer, 2000; Stilings, 2002). First, unpredictable environmental conditions may lead to
very low densities that increase the probability of extinction. A hurricane can more readily eradicate a
small population than a larger one. Second, inbreeding depression will reduce the population ability to
produce viable offspring.
Another example of a primary stabilizing mechanism can be observed in homeotherms or warm-
blooded animals. The homeotherms have various heat conservation adaptations in order to adjust energy
expenditures to available food supplies. Heat conservation measures include migration and hibernation
or falling asleep during the winter. Some forms of adaptation are based on size and mass distribution.
For example, many small homeotherms maintain their temperature by clustering together to reduce the
surface-to-volume ratio. That is, they reduce the proportion of their surfaces exposed to the cold tem-
perature. (Some poikilotherms or cold-blooded animals, such as bees, also huddle together for warmth
during the winter.) In some small homeotherms (e.g. hummingbirds and bats), body temperature drop
every evening upon resting, thereby decreasing their metabolic requirements and fuel consumptions.
There is an ecological principle that states that races of the same species from colder climates tend
to have a smaller surface-to-volume ratio than races of the same species from warmer climates (Ashton
et al., 2000). This rule applies more rigidly to homeotherms, for example, the jackrabbit is rather rangy
with relatively long hind legs and long ears as compared with the globular shape of Arctic hare. This
rule is generally followed by humans. In colder climates, humans such as the Eskimos have stocky bodies
that do not give up heat readily. In warmer climates, however, humans tend to have long, slender limbs,
through which body heat escapes more readily. The small size of the Congo Pygmies and Bushmen
is believed to be somewhat related to heat loss – as volume decreases, we recall, surface area increases
relative to the volume.
Secondary Stabilizing Mechanisms
Secondary stabilizing mechanisms are based on the amount of momentum in the system. Generally,
the more momentum a system contains, the more the system can resist any change in its integrity. Let
us apply this line of reasoning to the interaction between the heat generated by human activity and the
average global temperature of 15°C. In this situation, the heat generated by human activity is the force
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)
Environmental Stability and Sustainable Development 329
moving toward the atmosphere. The atmospheric heat is the system that meets the human heat in a
head-on collision. Consequently, the heat generated by human activity can be considered as a destabi-
lizing force that can change the average global temperature.
In our world, energy transfer between systems occurs constantly. If the transfer of energy is due to
differences in temperature, the form of energy transferred is heat (Atkins and Jones, 1997; Serway and
Faughn, 1999). Heat always flows from a high temperature system to a low temperature system. In
highly populated urban centres, this waste heat is enough to cause inner-city temperatures to be 5–20°C
warmer than adjacent rural areas. On a worldwide scale, this waste heat is ‘negligible’ (Goudie, 2000).
Recent studies indicate that in order to raise the global temperature by one degree Celsius, humankind
would have to increase its heat generation 100-fold. Thus, the momentum of the atmospheric heat has
a strong inertia, which overwhelms the momentum of waste heat from human activity.
Another example of a secondary stabilizing mechanism is that provided by the r-adapted species. Such
organisms tend to increase in numbers at a rate close to the organism’s intrinsic rate of increase (r).The terms ‘K-selection’ and ‘r-selection’ were coined by MacArthur and Wilson (1967). r-strategists are
opportunistic and dwell in short-lived areas like a pioneer community. When these species find a suit-
able environment, they reproduce and mature rapidly and expand throughout the area, exploiting its
resources. They may only have a few years to live there. These opportunistic species then migrate to find
other suitable environments, where the process is repeated. Thus, their biomass is constantly emigrat-
ing and colonizing. This constant movement generates a stability based on the momentum of these
species. From the point of view of a single area, r-adapted species are highly unstable. However, from
the point of view of the entire ecosphere, these species are quite stable.
Interesting hypotheses emerge from considering the stability of systems based on primary and
secondary stabilizing mechanisms. In order to survive, every system must adapt to live within a certain
range of variables, where its activities are adapted to function best. This range may be very broad for
some systems and quite narrow for others, but there are always limits beyond which a system may be
destroyed. (For example, no society survives at absolute zero or at the surface of the sun.) The more
primary stability the system possesses, the less it is subjected to severe changes in environmental forces.
However, some systems live in environments with extreme shifts in their environmental conditions.
In order for these systems to meet these challenges, the systems must possess secondary stabilizing
mechanisms. Once the mass encounters a suitable resource, it exploits the resource and keeps on
moving to other transitional resources. On other words, the system is kept stable as it moves from one
resource to another by the momentum it possesses.
One more point: the interaction between the force and the system is a model and depends on the
type of force, change, matter and energy of the system. For example, when the forces and the changes
are alike, the change in the system is more predictable since the effect of the force tends to be the
difference between the force and the change. Thus, the effect of an increase in carbon dioxide and oxygen
on the atmospheric masses of these two molecules is simply a problem of arithmetic. Also, the
effect of heat generated by human activity on global temperature is simply the difference between the
two.
On the other hand, when the force and the change are not alike, the change is less predictable. As
the amount of atmospheric carbon dioxide increases (a force), it is suspected that the global atmosphere
temperature may change. However, scientific uncertainties make this change less predictable. Major
sources of uncertainties are the roles of the atmosphere, oceans, photosynthetic organisms and soil.
Thus, interactions between force, change, substance and energy are bound to occur.
Moreover, once the force is greater than the primary and secondary stabilizing mechanisms, the
change tends to be proportional to the applied force. As long as the force is applied, when a system with
a high resistance starts to change, it may be hard to curb without tertiary stabilizing controls in it.
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)
330 M. A. Santos
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)
Tertiary Stabilizing Mechanisms
Tertiary stabilizing mechanisms derive from the information contained in matter and energy. By infor-
mation we mean more than the opposite of randomness (entropy). In this sense, information is a new
property that arises out of matter and energy when the two qualities interact with each other. Tertiary
mechanisms are emergent properties, which buttress the stability of systems. The following paragraphs
describe three mechanisms: binding energy, negative feedback and feedforward.
The binding energy of a system helps to hold the system together, and can generate a tertiary level of
stability in the system. The origin of the binding energy may be explained by using a system formed by
the interaction of the earth and a rock. Both the earth and the rock have kinetic energy that drives them
apart, thereby increasing the entropy of the earth–rock system. When the earth and rock are infinitely
apart, their energy is zero. The force of gravity causes the earth and rock to attract each other. As they
get closer together, they lose potential energy. If they are pushed too close, then the energy rises again.
This is caused by the contact force, which keeps the earth and rock separated. The most stable distance
of separation between the earth and the rock occurs when the energy is lowest. At this point, the attrac-
tions and repulsions are balanced. The amount of energy that must be supplied to separate the earth
and rock is called the binding energy (Figure 1). The distance between the earth and rock when the
energy is at its minimum is called the bond length or bond distance. (In this illustration, we are ignor-
ing other forces such as the earth’s atmosphere and the sun.)
Due to the interaction of forces, there is an energy well at the bottom of the system, where energy is
at its lowest. Recall that open systems, from a thermodynamic perspective, tend to be most stable at the
point of least energy. Similarly, at the point of contact between the earth and rock where energy is least,
the earth–rock system forms a stable bond. To break this bond, we have to add a destabilizing force
strong enough to destabilize the system. The work or energy that must be supplied to the earth–rock
system to move it to infinity against the opposition of this attraction and thus enable the rock to escape
ENERGY WHEN THE EARTH AND
ROCK ARE INFINITELY APART
BOND
ENERGY
BOND LENGTH
0
DISTANCE BETWEEN EARTH AND ROCK
BO
ND
EN
ER
GY
Figure 1. Energy of the earth-rock system as a function of distance. As the force of gravity pulls the earth and rock closer together,they lose potential energy. When the earth and rock get too close, the situation is somewhat reversed. Due to the contact force,the potential energy rises again
Environmental Stability and Sustainable Development 331
from earth is called its binding energy. We say there is energy ‘in’ a system, but notice that a deficiency
of energy is what actually keeps the system together. Therefore, a threshold must be crossed before the
integrity of the system is disrupted (Figure 1).
The concept of minimum potential energy has been applied to explain the binding energy of stable
molecules, the need for streams to flow downhill and the capacity of living systems to maintain complex
structures (Leopold and Langbein, 1962; Lehninger, 1966). At the point where the destabilizing force
is equal to the binding force, the system is said to reach its metastate. When a system is at its metas-
tate, any slight external force may cause the system to change (White et al., 1992; Mackenzie, 1998).
The metastate hypothesis suggests that as long as the threshold is not crossed, the system will be stable.
From this inference and our review of binding energy, we can elaborate on the binding energy dimen-
sion of sustainability.
An example of the binding energy concept concerns the management of economically valuable wild
species (many harvested fish, game birds and mammals). An important principle that emerges from the
logistic growth curve is that per capita growth rate tends to be higher when N = K/2 (Graham, 1935). In
other words, the largest number of individuals is added to a population when its size is approximately 50%
of K. This is also the population size that provides the optimal yield or maximum sustainable yield. It is
also the greatest rate at which organisms can be removed without further reducing the population size.
This principle provides a basis for population management policies (Kahn, 1998; Bush, 2000;
Townsend et al., 2000). In theory, if humans want to achieve maximum long-term productivity in har-
vesting of a particular type of economically valuable species, the population should be harvested when
N = K/2. Under the logistic equation, the biotic potential is a force that drives the species’ biomass to
increase. The environmental resistance is the counterforce that diminishes the biotic potential. When
these two forces are equal, DN/DT has a maximum value. At this point, the population has intermedi-
ate population size and K/2 is equal to N. Moreover, when N = K/2, the sum of the two forces (biotic +environmental resistance) is equal to zero.
This article proposes another way of obtaining the optimum yield of an economically viable wild
species. Recall that a species is part of nature’s food web, whose existence implies that the forces of
nature have operated for it to reach its present configuration. There is a binding energy that holds the
configuration together. This idea may be illustrated with an example of a predator–prey system in which
a predatory fish feeds on a prey, a shrimp for example. There are certain forces of nature such as the
principle of entropy, avoidance behaviour of the prey and competition that tend to disrupt the energy
flow from the shrimp to the fish. When these forces drive the prey and predator infinitely apart, the
energy flow is zero (predators die from starvation). However, there exists a counterforce, the predator’s
searching behaviour that drives it closer to the prey. As the predators consume more prey, the energy
flow increases. With fewer prey available, there is less energy flowing. After all the prey is consumed,
the energy flow is zero.
In nature, habitat complexity stabilizes predator–prey interactions by providing an area that permits
the prey to survive free from predation. As a result, a few prey always survive under high predation.
With fewer prey available, many predators die and energy flow declines. When predation is at a
minimum, the prey’s biomass expands, causing the cycle to repeat.
Thus, in nature, the most stable (lowest energy) habitat configuration occurs when the energy flow is
at its maximum. In order to break the predator–prey system, that is, to separate the predator and prey
populations, one has to add a destabilizing force strong enough to destabilize the system. The work or
energy that must be supplied to the prey to move it to infinity against the opposition of predatory attrac-
tion, and to enable the prey to escape from the predator, is called its binding energy (Figure 2).
We can estimate that the binding energy may be approximately 10% of the energy contained in the
prey population. This percentage is derived from the observation made by many ecologists that the
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)
332 M. A. Santos
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)
amount of energy available to organisms at each food level steadily diminishes at the number of energy
exchange increases. Although the actual efficiency of energy transfer varies widely, the 10% rule of
thumb is accepted as an average by most ecologists (Pauly and Christensen, 1995). This is due to the
consumption of energy in life processes, the loss of energy as waste heat, loss of energy in feces and
accumulation in sediment.
An important principle of this binding energy model is that, if society is economically harvesting this
predator–prey configuration, the maximum energy that it harvests must be less than 10% of the binding
energy. If the harvested energy were equal to 10% of the binding energy, the configuration would be at
a metastate. As mentioned earlier, a configuration at its metastate is highly unstable.
A further implication of the model is that for each specific predator–prey interrelationship there is a
specific habitat configuration that provides maximum stability. Today, many governmental and institu-
tional organizations are making worldwide efforts to protect these habitats. Organizations such as the
World Wildlife Fund, Nature Conservancy, the National Audubon Society, the Sierra Club and the
Environmental Defense Fund all contribute to protecting natural habitats. Efforts to save biodiversity
include private purchase of pristine environments for preservation and the ‘debt-for-nature swaps,’ in
which developing nations receive money if they protect ecologically sensitive ecosystems.
The binding energy model suggests that flora and fauna interact with their habitat so as to arrive at
the most stable configuration with the lowest energy content, under their specific physical conditions.
This is the configuration that requires the least expenditure of energy to maintain; therefore, efforts to
protect natural ecosystems should include research to identify these configurations.
Feedback refers to the return to the system of some of the output as input. It occurs whenever a
response to a stimulus feeds back to alter the original stimulus. If the output augments the system, then
the feedback is positive; if it inhibits the system, then the feedback is negative. Organisms usually keep
conditions optimum within their bodies through negative feedback control. One of the many examples
of feedback regulation in biological processes is the automatic maintenance of our internal bodily func-
tions by the nervous and endocrine systems. A negative feedback loop usually regulates the population
MINIMUM OPTIMUM MAXIMUM
HABITAT COMPLEXITY
PE
RC
EN
T E
NE
RG
Y F
LO
W
Figure 2. For a specific habitat, the most stable (highest energy flow) configuration occurs when energy flow is at its maximum.Prey’s defenses decrease energy flow while predator’s offenses may increase it
Environmental Stability and Sustainable Development 333
of wild species. According to the logistic equation, when N = 0 or is very small, (K - N )/K = 0 or is very
small, the population is allowed to increase. As N approaches K, (K - N )/K becomes zero and the pop-
ulation stops increasing. In a negative feedback fashion, (K - N )/K varies in such a way as to decrease
the population size when the population begins to become too large and to allow the population size to
increase when it becomes small. Over time, the birth rate is balanced with death rate, therefore mini-
mizing the difference between actual population size and the carrying capacity. When population size
is plotted against time, the curve is S-shaped.
As mentioned, species that tend to produce a stable population are termed K-adapted, such as Cali-
fornia condor, whooping crane, mountain sheep, elephant and human. Typically these organisms have
a much greater chance of survival, especially once maturity is reached. They produce relatively few off-
spring that mature slowly, and generally provide parental care. K-adapted species tend to live in more
stable, long-lived habitats. Their populations are consequently at or near the saturation or the carrying
capacity (K ) of their environment. These species live under crowded conditions in which intraspecific
competition plays an important role. However, such species may not be able to recover from very low
population densities. Their populations may be too low for maximal reproductive success (or mortality
decreases) with rising density. This generalization is known as Allee’s effect (after W. C. Allee), and
affects many species of birds and other organisms.
Many endangered species such as whooping crane and California condor, for instance, are probably
well below their actual limit. In fact, the now-extinct passenger pigeon did not breed successfully in
small groups. Consequently, it disappeared and became extinct. It seems that these animals require the
interaction of large groups of organisms for survival and/or reproductive purposes.
However, other species do not stabilize and overshoot the carrying capacity. These species exhibit a
J-shaped growth curve and are termed r-adapted species. The J-shaped curve is a characteristic of the
population growth of small insects with short life cycles and of annual plant populations. In such a
curve, the population increases in density at a rapidly accelerating rate until the environmental resis-
tance suddenly comes into play. At this point, the population density falls dramatically. In this growth
pattern, the population grows exponentially until it strikes the limit set by the habitat and/or nutritional
resources. Thereafter, the population suffers a reduction that is independent of density. During decel-
eration, the population declines abruptly due to exhaustion of available resources.
Recall that r-adapted species rely on their momentum for growth to stabilize their global population.
These organisms generally produce many small offspring that mature rapidly. The parents provide little
or no parental care. While most of the offspring die, a few reach maturity to continue the population of
the species. They possess remarkable reproductive and dispersal powers. They live in uncrowded con-
ditions and thus intraspecific competition is rarely important. Such species quickly increase beyond the
carrying capacity. They wipe out resource supplies and/or become susceptible to predation and disease.
Consequently, their growth curves are usually J shaped.
An example of the feedforward mechanism is when sensory cells in human skin detect a drop in air
temperature. They send signals to the brain to ‘expect’ a modification in blood temperature. The brain
then sends an impulse to metabolic and muscular systems that can function in raising the body tem-
perature. With feedforward mechanisms, corrective measures can sometimes commence even before
the external environment significantly changes the internal environment.
Concluding Perspectives
The preceding pages described primary, secondary and tertiary mechanisms separately. However, in
nature we may find that these mechanisms may interact with each other, as well as external forces, to
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)
334 M. A. Santos
affect the stable configuration of the system. If society is to properly maximize the inherent stability of
its life-support systems, it should not push or pull the configuration beyond its stability threshold. The
following pages consider these fundamental issues.
The first fundamental issue, whether stabilizing mechanisms interact with each other and with exter-
nal forces to affect the configuration of a system, can be resolved with two illustrations: Archimedes’
principle and salmon population.
Archimedes’ principle states that any object completely or partially submerged in a fluid is buoyed up
by a force whose magnitude is equal to the weight of the fluid displaced by the object (Serway and
Faughn, 1999). For example, a system composed of a canoe with four occupants can float on water if
the system can displace a volume of water whose weight is greater that that of the system. If the density
of the system is less than that of the water, then it will float even if the material is less than that of the
water. If the density of the system is greater than that of water, such as an aluminium canoe with four
occupants with metal armour, the system can be made to float, provided that the system is not placed
in such a way that they form a uniform solid. Since the non-uniform distribution of the weight displaces
more water than a solid, the system floats.
If the occupants in the canoe suddenly shift position, their momentum may cause the system to
become unstable and sink. The range of possible momenta of the people that do not lead to tipping is
termed the domain of stability (Ludwig et al., 1997). However, by rationally using mass distribution,
momentum, negative feedback and feedforward strategies, the occupants may be able to shift position
without destabilizing the system.
As a second example, consider the stability of a salmon population. After hatching in freshwater
streams, salmon migrate hundreds of miles away in the ocean, where they grow and mature. After a
few years, the adult fish migrate back to the same stream. In order to overcome the force of the current,
the animals rely on primary stability: mass and body shape. The salmons have an optimal body size.
This helps them push their way through the rapids. The body shape of a salmon forms a bilateral sym-
metrical system. Bilateral symmetrical animals have two similar sides (mirror images) with distinct
upper and lower surfaces as well as a head and tail. They are especially suitable to active movement.
Most animals that swim or crawl rather poorly are radially symmetrical. Species such as hydra, starfish
and corals have identical body parts that radiate from the centre.
In addition, the secondary stability provided by the salmon’s muscular systems gives the fish the
momentum required to propel their bodies through the rapids. Finally, the salmon has tertiary mecha-
nisms such as olfactory senses that recognize one combination of chemicals, and directs locomotion
toward it. In a synergistic fashion, the three mechanisms cooperate with each other to contribute to the
stability of the salmon population.
The second fundamental issue is why environmental changes go naturally in a particular direction to
form a stable configuration. To answer this question, we need to look at the principle of minimum
potential energy and evolution by natural selection. Some natural processes are driven more by increas-
ing entropy and others more toward lower energy (Atkins and Jones, 1997; Ebbing, 1993). This notion
of increasing stability is termed the minimum potential energy principle.
In non-living systems, the force that drives a system towards a stable configuration is the tendency
of matter to achieve its minimum potential energy. Matter arranges itself in relation to its surroundings
in order to minimize the energy content of the system. The apparent organization is an incidental
by-product of the minimum potential energy principle. In the ecosphere, there is no goal-oriented
mechanism and its apparent configuration involves the mechanistic interactions of the four spheres
(biosphere, atmosphere, hydrosphere and pedosphere). What we perceive as a ‘balance of nature’ is no
more than the product of the uncoordinated activities of the four spheres. There is no central control
that uses primary, secondary or tertiary mechanisms to maintain the balance of nature. For example,
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)
Environmental Stability and Sustainable Development 335
during ecological succession some species of large trees create heavy shade under the canopies and thus
build a stable microclimate. Afterwards, the shade-adapted plants invade the cooler climate. The micro-
climate is an emergent property, but an incidental by-product of the invasion of the large trees. Thus,
the large trees plus the shade-adapted plants, and all other species in that particular environment, unin-
tentionally form a climax community. Through facilitation, competition, mutualism, feeding associa-
tion and other types of interaction, the climax community emerges.
However, for some organizations such as organisms and obligate mutualistic species (such as some
lichens), the driving force is natural selection. In these systems, the interactions between parts (organs
or constituents) are optimized for the good of the entire system. Hence, there are stabilizing mecha-
nisms that are due to the law of natural selection (Brewer, 2000; Calow, 2000). However, goal-oriented
behaviour is acceptable in explaining human decisions to form a stable society because humans can
anticipate the future and act rationally. For organisms, including humans, whether progressive organi-
zation occurs would depend on the adaptive or social value of the primary, secondary or tertiary stabi-
lizing mechanism.
A third fundamental issue concerns the anthropogenic effect on human life-support systems. These
servient systems serve as a source of vital resources and a sink for pollutants. The stabilizing mecha-
nisms may be considered as a force that holds a servient system intact. The anthropogenic effect is the
counterforce that tends to destabilize the system. According to the metastate hypothesis, at the point
where the destabilizing force is equal to the stabilizing energy of the system, the system may be said to
be at the metastate. When a system is at its metastate, any slight external force may cause the system
to change. The metastate hypothesis suggests that as long as the threshold is not crossed, the system
will be stable. From this inference and our review of the primary, secondary and tertiary stabilizing
mechanisms, we can elaborate on the stabilizing energy dimension of sustainability. In theory, if
humans want to achieve maximum long-term stability in exploiting a particular type of life-support
system, then the maximum energy of the anthropogenic effects must be less than the energy that main-
tains the stability of the system. If the destabilizing energy were equal to the stabilizing energy, then
the configuration would be at a metastate. As mentioned, a configuration at its metastate is highly unsta-
ble. Thus, the stabilizing energy is the energy available to do work without compromising the stability
of the configuration.
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Biography
Dr. Miguel A. Santos is a professor of ecology and environmental science at Baruch College of the City
University of New York, where he is the coordinator of environmental studies. Since 1975 he has taught
a wide variety of courses, focusing especially on ecology, environmental science and law. He is the author
of The Environmental Crisis (Greenwood, 1999) and seven other books. He can be contacted at the Depart-
ment of Natural Sciences, Box A-506, Baruch College of the City University of New York, New York,
NY 10010, USA.
Tel.: +1 212 802 3091
E-mail: [email protected]
Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment Sust. Dev. 13, 326–336 (2005)