"Everything is connected
to everything else."
-- Barry Commoner
This simple quotation sums up the primary
reason that the science of ecology is so important: Human activities do not occur in a
vacuum. They are an intricate part of a.) complex biological systems, such as food chains,
and b.) large-scale abiotic processes such as weather, water and energy cycles.
The quote also explains why ecological changes are so hard to
predict and why so little consensus about these changes exists among experts. Consider
- Ecologic systems are highly complex and non-linear, making
many types of short-term prediction all but impossible.
- It is not possible to study these systems in controlled
- Cause-and-effect relationships become clear only over
centuries or millenia, rather than days or weeks.
These factors can make it hard to mentally grasp the results
of studies designed to improve our understanding of the environment. Yet grasp them we
must, if we are to develop truly sustainable economic and ecologic systems. There is far
too much at stake, both financially and environmentally, for us to do otherwise.
This module will improve comprehension of the environment by
exploring the more significant systems and cycles that occur here on Earth. It will also
provide some insight into how these processes are being impacted by human activities, and
what the related effects can be.
Environmentalism and Ecology
While these terms tend to be used interchangeably by the media, politicians and the
public, the two have profoundly different meanings. An understanding of this difference is
critical to making informed decisions about the environment, as it will help you determine
reality versus rhetoric and proof versus politics.
rhetoric that tries to cloak ideology within a scientific framework. Terms such as "ecologically sound" or "good for the
environment" are value judgements, not statements of scientific fact. This is true no
matter who makes the claims -- environmentalists, politicians, businesspeople or
advertising spokespeople. Regardless of stature and credibility, phrases such as "I
think that..." or "We should..." indicate that the person making a
statement is providing an opinion, not a fact.
- Environmentalism is a philosophical and ideological
movement based upon the belief that nature is sacred and should be treated as such.
Environmentalism thus includes spiritual and religious overtones, and is based largely on
values and other qualitative considerations. There is nothing wrong with the concept of
environmentalism, as long as one understands it for what it is -- a personal viewpoint of
what the world should be and not a statement of actual fact.
Ecology, on the other hand, is a science -- the science devoted to
studying the relationships between a.) members of living (biotic) communities and b.)
biotic communities and their non-living (abiotic) environment. As a science, ecology is
non-judgemental and amoral, concerned with what is, rather than what should be.
Ecology is one of a number of fields that make up the environmental sciences. Other
environmental sciences include meteorology, climatology and seismography.
An ecosystem is an organization consisting of groups of living and non-living components
that form a systemized whole. Ecosystems exhibit properties typical of biological
- Existence independent of specific components -- a plant
may die, but the prairie remains.
- Interdependency of components -- removed from the hive,
a bee will not usually survive.
- Activity -- changes constantly occur.
- Ranges of organization -- the relationship between
populations can range from total independence to total interdependence.
Within ecosystems, organisms are typically classified within three large groups:
- Producers -- Most energy within an ecosystem originates
from the sun, in the form of light. Through the process of photosynthesis, green
(chlorophyll-bearing) plants capture this energy, using it to drive the transformation of
atmospheric carbon dioxide (CO2) and water (H2O) into carbohydrates,
which are necessary for metabolism and growth. (For reference, this process also produces
water vapor and oxygen gas (O2), which the plants then "exhale".)
The carbon is also combined with additional elements to produce many other
compounds necessary for life -- amino acids, proteins and fats, for example. To produce
these, plants draw from the soil the other elements and minerals they need, including
nitrogen, phosphorous, sulfur and magnesium. Note that carbon is present in all of these
nutrients as well as in carbohydrates, hence the reason that life on earth is considered
to be "carbon based." In fact, carbon constitutes nearly 50 percent of the dry
weight in organisms. (This is why molecules that are "built" upon a carbon
"backbone" are considered to be "organic" molecules.)
As an important aside, animal life on earth was not possible
until plants developed and flourished. Animal metabolism is based upon a steady diet of
oxygen, carbohydrates, proteins and other nutrients. The oxygen that animals breath
originated as a byproduct of plant metabolism. Ultimately, all food consumed by animals
orginated from plants as well.
Consumers -- With a few exceptions, animals are the
consumers of these carbohydrates and other plant materials. Primary consumers are
those that receive all of their nutrition from the ingestion of plants (herbivores). Secondary
consumers, or carnivores, fill their nutritional needs by way of the herbivores. Tertiary
consumers are carnivores that eat other carnivores. (Organisms that receive nutrition
from both plants and animals are known as omnivores.)
Decomposers -- This group of organisms returns
valuable minerals and basic elements back to the environment, for use once again by
producers. Decomposers consist primarily of bacteria and fungi, secreting enzymes into
dead plant and animal material. Some of the newly degraded material is absorbed for food,
while much is left within the ecosystem.
Ecosystems thus encompass two types of flows: energy and
nutrients. The energy flow is unidirectional -- once it is completely used, energy cannot
be recovered. Nutrient flows, however, are cyclic -- the basic elements such as carbon,
hydrogen, oxygen and nitrogen are returned to the land, sea or atmosphere. (We will
explore the systems that utilize these flows in greater detail in the next sections.)
Energy and Nutrient Flows
As the primary source of the Earth's energy, the sun holds the key to all life. Sunlight
is captured and transformed ("fixed") from radiant to chemical energy during
photosynthesis by producers. Consumers and decomposers digest the plant matter and, using
the process of cellular metabolism, transform chemical energy into mechanical and heat
Areas of the earth where there is abundant sunlight, heat and
moisture would logically seem to provide the most abundant sources of energy fixation.
This is in fact the case: Given their high concentration of "biomass" -- the
weight of organic material within an ecosystem -- tropical seasonal forests and
rainforests are the two primary energy-fixing regions on Earth. Biomass is high in these
areas because a.) vegetation is abundant and fast growing per square meter and b.) there
are relatively large areas of the tropics covered by these forests.
It is important to note that the tropics do more than fix
large amounts of energy. In the process of doing so, they also absorb significant amounts
of carbon dioxide (thus fixing the carbon) while emitting both oxygen and water vapor. Thus,
the tropics play a disproportionately large role when it comes to capturing and storing
the sun's energy and in maintaining a balance between atmospheric carbon dioxide and
These unique abilities explain why rainforest destruction is
so disconcerting: the effects are complex and global, not simple and local. The difference
between the energy capturing (and carbohydrate producing) capabilities of tropical versus
other forests also explains why this concern remains even as land in temperate areas is
returned to forest: A square meter of rainforest produces 70% more energy than does a
square meter of temperate evergreen forest, and almost 85% more energy than does a square
meter of temperate deciduous forest.
One way in which scientists analyze energy flows is by
breaking them down into levels, starting with energy producers (plants), moving to primary
consumers (herbivores), and up to secondary and tertiary consumers (carnivores,
omnivores). This type of analysis produces a graph that looks like a pyramid, with the
largest energy level at the bottom (plants) and the smallest at the top (carnivores).
These analyses produce the following findings:
- Energy at lower levels is used to support the existence of
- All energy within the system is eventually dissipated and must
be supplied from external sources (i.e., the sun).
- Energy transfer up the chain is relatively inefficient, on the
order of 10 percent.
This last point is the reason why overconsumption of beef is
an issue with many environmentalists. It takes 10 times as much energy for us to receive
nutrients from meat than it does to receive them from plants.
The carbon cycle "begins" in the atmosphere, which acts as a reservoir for
producers (plants) and consumers (animals). Decomposers (bacteria, fungi) then act upon
both groups to return carbon to the atmosphere.
Contrary to what most people believe, the atmosphere does not
contain the largest pool of carbon. There are two "sinks" containing far more:
the sediments and oceans. Sediments alone account for over 99 percent of the Earth's
carbon, while carbon in the oceans is actually 500 times greater than the amount stored in
the atmosphere. Significantly, all of these inorganic sources of carbon combine to account
for more that 99.9% of the planet's carbon supply. Less than 0.1 percent is tied up in
organisms or fossil fuels.
The atmosphere's role as reservoir exists because its carbon
is most available for cycling. (The cycle time for carbon in sediments is on the order of 100
million years!) Atmospheric carbon exists primarily as carbon dioxide (CO2).
CO2 is a greenhouse gas, which means that it acts to trap heat in the
atmosphere and thus plays an important role in global temperature regulation.
Scientists believe that in the past, the cycle between the
amount of CO2 leaving the atmosphere and that returning to it was in balance.
This balance suggested that global temperatures should remain fairly constant, or that any
changes in temperature were caused by some other factor.
However, over the last hundred years, the amount of CO2
in the atmosphere has risen about 20 percent, due in large part to a.) the burning of
fossil fuels and b.) widespread deforestation. Burning fossil fuels produces relatively
large amounts of CO2 as a byproduct of combustion. On the other side of the
ledger, deforestation reduces the environment's ability to absorb the increased CO2,
as the volume of carbon-fixing foliage is significantly reduced.
Also over this time period, the earth's temperature has risen
about one degree Fahrenheit. Is there a relationship between CO2 increase and
temperature increase? Based on available evidence, there are three possible scenarios,
none of which can be totally discounted:
- Possibility 1 -- There is a cause and effect
relationship, in that the temperature increase was caused in part or in total by the
increase in atmospheric CO2.
- Possibility 2 -- The two factors are related, but not
in a cause and effect manner. Rather, the two are both dependent upon (e.g., controlled
by) some as-yet-unrecognized third factor which has caused them both to increase.
- Possiblity 3 -- There is no relationship at all, merely
coincidental increases in both factors.
Any of these three scenarios could be true. However, the
consensus among scientists is that the observed global warming which has occurred over the
last century has been at least partially caused by the concurrent increase in atmospheric
CO2. That being said, there is little agreement as to whether
warming will continue to occur, at what rate, and with what effect on the global
Those who try and negate the possibility of either continued
warming, or a relationship between CO2 and temperature change, cite a number of
arguments. Three of the most common are listed below, along with a reasonable rebuttal to
- The amount of carbon being talked about is miniscule in the
overall picture, and thus couldn't possibly have an effect.
The fact that so much of total carbon is inorganic and tied up for millions
of years is irrelevant. Little of the carbon cycling that occurs over the foreseeable
future involves the major inorganic repository -- sediments. Most involves organic
sources, which can recycle relatively quickly.
Further, since the systems involved are non-linear, cause and
effect relationships are non-linear as well: a very small change in one variable (i.e., CO2)
could have a very large effect in another (i.e., temperature). Think about building a sand
pile: As the pile gets higher and higher, it becomes more unstable and susceptible to
perturbation. At a certain point, just a few additional grains can cause a small
landslide, or even the entire pile to collapse. Thus, the additional CO2 added
by human endeavors might be "the straw that breaks the camel's back."
- We can't predict the weather tomorrow. How can we possibly
know what will happen in 10 years?
argument confuses short term fluctuations and long term trends. While it's true that there
is little chance of predicting what the specific weather will be four days from now, we
can use our knowledge of the seasons to make accurate assessments of general conditions
three months, six months, or even 100 years from now.
- The economic cost of change is too great to undertake
without a greater level of certainty.
counter to this is that the magnitude of change that might occur, and the costs involved
in remediating that change, are so great that we should begin to act long before we can
accurately quantify the risk involved.
Nitrogen is the most abundant element in the atmosphere, accounting for about 78 percent
of total gases. It is a significant biochemical element, important in the production of
nucleic acids and proteins.
Nitrogen is considered to be a limiting factor in many
processes, because the amount and form in which it is present has serious consequences for
metabolism: too little inhibits or prevents metabolism, the right amount helps create
healthy growth, and too much can once again inhibit metabolism or cause it to cease
altogether. (Limiting factors appear elsewhere in nature. For an example, see the
discussion in our science module on epidemiology.)
Unlike carbon dioxide, which represents a small (0.32
percent) portion of our atmosphere, nitrogen is plentiful. But while carbon is easily
fixed by plants, nitrogen is not. Plants must rely on a complex external cycle that turns
nitrogen gas into nitrates, the substances plants require for healthy metabolism. The
cycle involves four discrete steps:
- Nitrogen fixation, whereby nitrogen gas in the
atmosphere is converted to form other substances including nitrates, ammonia and organic
compounds. Natural fixation occurs primarily through algae and bacteria. Secondary natural
sources include the energy released by lightening and forest fires. Of no small concern is
fixation via man-made sources. These include the manufacture of industrial fertilizers,
which could soon surpass natural fixation methods, and the production of noxious air
pollutants as a byproduct of the use of internal combustion engines.
- Ammonification, which occurs when microbes metabolize
organic nitrogen. The byproduct of metabolism is ammonia, which is excreted into the soil
or aquative environments.
- Nitrification, a process whereby different bacteria or
fungi take in the ammonia and convert it first to nitrites and then to nitrates. These are
then excreted, and absorbed by plants through their root systems.
- Denitrification, a set of processes whereby bacteria
convert nitrates back to nitrites, ammonia and nitrogen gas that once again enter the
atmosphere, soil or water.
The long term effects of increased nitrogen fixation via
commercial processes are not widely understood. However, fixation through internal
combustion produces nitrous and nitric oxides, significant contributors to ground-level
Like nitrogen, sulfur plays a significant role in the regulation of other nutrients, and
is a key component of certain essential amino acids. The sulfur cycle is complex, and not
nearly as easy to break down into discrete steps as is the nitrogen cycle. The amount of
sulfur in various parts of the biosphere, and rates of turnover, are not as well
understood as with the carbon and nitrogen cycles.
The key part of the cycle involves the intake of sulfates by
plants through their root systems. Unwanted sulfur is excreted and acted upon by bacteria.
Decomposition also releases sulfur compounds. In either case, some forms are returned to
the soil, while others are released into the atmosphere.
Sulfur reaches the atmosphere via other methods as well: A
primary natural method is via volcanic eruptions. On the man-made side, the burning of
fossil fuels also releases sulfur compounds into the air.
Once in the atmosphere, sulfur compounds are washed back onto
land and aquatic environments, where they once again can be absorbed by plants and
microbes. The process is not completely benign, however: One of the substances formed when
sulfates and rainwater mix is a weak solution of sulfuric acid, the basis for acid rain.
While some acid rain is natural, scientists are concerned that the additional sulfur sent
skyward by manufacturing processes and automotive exhaust may produce an overabundance of
acid rain, stronger solutions of it, or both. This is a major concern, given the
potentially disastrous effects to both land and aquatic ecosystems, not to be mention the
costs of repairing buildings, bridges and automotive paint jobs.
Phosphorus is also an important biological element, found in nucleic acids and other
organic compounds. Like sulfur, the cycle is not as well understood as the carbon or
Organic phosphates are created or absorbed by producers
(plants), transferred to consumers ( animals), and ultimately to decomposers (bacteria,
fungus). Once deposited back in the soil or water, phosphates are again available for
plant absorption. (Note that there is no atmospheric portion of this cycle.)
Major disruptions to the cycle occur when too much phosphate
is pumped somewhere into the system. The results can have disastrous consequences anywhere
within the particular ecosystem.
Consider what happens when a heavy rain occurs just after fertilizer is applied to local
farms, fields, lawns and golf courses. Rather than leaching into the soil, phosphates are
washed away into storm sewers, which empty into local waterways. The phosphates trigger
algal blooms, which clog ponds and streams. As the algae die, severe oxygen depletion
occurs, since decomposers require oxygen to break down the dead material. This lack of
oxygen, in turn, suffocates the fish, effectively creating a "dead" lake or
Other Key Concepts
There are a number of other ecology-related concepts
that are frequently bandied about during debates, discussions and in the media. Here are a
few of the more important topics.
The number of species that call Earth home is not known, with estimates ranging from 3 to
70 million. This means that the presently identified species -- 1.4 million of them --
represent somewhere between 2 and 45 percent of the total species that exist today.
Biodiversity is a major concern to ecologists and
environmentalists, with good reason:
- Species are not evenly distributed throughout the biosphere,
but are highly concentrated in areas of abundant light and moisture. Only 7 percent of
the land surface contains more than 50 percent of the world's species.
The 7 percent in question is the tropical rainforests. Thus,
along with their value as carbon-fixers and oxygen-producers, rainforests are also major
species producers. (This is another good reason to be very concerned about rainforest
What's more, the relationship between rainforest destruction
and diversity has severe consequences. Studies have shown that when an area is reduced to
one tenth of its original size, the number of species will fall by half.
- Even the smallest, or as-yet-unknown organism in a particular
environment may play a pivotal role in the health of the entire ecosystem. For example,
humans tend to think of bacteria as tiny, insignificant, or even dangerous. However, if we
were to eradicate a particular bacteria which adheres to the root nodules of plants and
allows the latter to fix nitrogen for use in cell metabolism, we would end up eradicating
the plants as well. What's more, we would also eliminate the insects, animals and fungi
that rely on the plants for food or shelter. (For reference, removal of a species critical
to ecological stability is known as a keystone crisis.
- There are two other arguments favoring a very cautionary
approach to diversity reduction. The first is utilitarian - an extinct species is one that
we will never again be able to use for food, medicines, etc. The second is based upon
esthetics and ethics - biodiversity has value in and of itself, and thus should not be
Carrying capacity can be defined as the maximum population of a particular species that an
environment can support. In regards to humanity, the carrying capacity of the Earth is
open to considerable debate. The answer depends upon many factors, including population
size and growth rate; material standards of living; agricultural production and methods;
resource availability and costs; and even cultural norms. Some environmentalists and
scientists believe we have already exceeded the planet's carrying capacity, while others
believe the chances of us ever doing so are slim.
Discussions of carrying capacity ultimately involve
discussions of population control, lifestyle changes, government intervention and freedom
of choice. Debates tend to be colored by religious, ideological and philosophical
considerations. However, regardless of human opinions, nature will ultimately make
adjustments. Thus, a key concern is whether we are collectively willing (or even able!) to
accept the consequences of natural corrections, or should instead work to prevent them,
and the related effects, from ever occurring.
Ecology and Evolution
Obviously, changes in the environment can significantly affect evolutionary patterns. But
evolution can change the ecology as well. As noted previously, the early proliferation of
bacteria and plants radically affected the environment -- the proportions of carbon
dioxide and oxygen in the atmosphere ultimately reversed themselves. In turn, this change
allowed for the evolution of oxygen-breathing animals.
The fact that the Earth's biotic and abiotic processes are
intricately related and dependent formed much of the basis for the well-known Gaia
hypothesis. There are two interpretations of this theory, one being that the Earth
forms a huge self-regulating system, the other that the Earth itself is not simply a
system, but a living organism.
Regardless of whether either interpretation is correct, the
relationship between living organisms and their environments is irrefutable: The fate of
the environment, and of the life that exists within it, are completely intertwined.
Together, ecology and
environmentalism form the quantitative and qualitative study of the biotic and abiotic
aspects of the planet. Both are concerned with the relationships between
and among living and non-living systems. A key issue is the effect that humanity has had,
and will continue to have, on the rest of the planet, and thus on itself as well.
A fundamental principle of environmental science is the vast
connectiveness and interrelationships that exist within nature. Natural systems are thus
highly complex, with all of the attributes associated with complexity: self-organization,
non-linear relationships, high levels of unpredictability and constant change.
In the last 100 years or so, human endeavors have
significantly altered many of the planet's macro cycles including those associated with
carbon, nitrogen and sulfur. While the fate and effects of these changes are constantly
being debated, the potential magnitude of the consequences that might occur warrants
caution, further study and concern.
References On the Net...
- CIESIN, Earth
science databases and a world data center for human interactions in the environment
for the National Institute for the Environment, with a mission to
"improv the scientific basis for making decisions on environmental issues.
- Conservation Ecology, a publication of the
Ecological Society of America
Society of America
News Network, the self-professed "environmental science supersite"
Science Journals, Cambridge Scientific abstracts
- Internet Environmental
Biological Service, National Biological Information Infrastructure
States Environmental Protection Agency
the periodic table from actinium to zinc
- World Wide Web Virtual Library,
References Off the Net...
(Clicking on the link will take you to the appropriate
catalog page of Amazon.com, where you can learn more about the book and/or order it.)
Environmental Science, Michael Allaby (Routledge, 1996).
of Ecology, Edward J. Kormondy (Prentice Hall, 1996).
Science, C. Kupchella and M. Hyland (Allyn & Bacon, 1986).
Fragile Species, Lewis Thomas (Collier Books, 1992).
Earth, Stephen H. Schneider (BasicBooks, 1997).
within Limits: Ecology, Economics, and Population Taboos, Garrett Hardin (Oxford
University Press, 1993).
Views on an Old Planet: A History of Global Change, Tjeerd H. Van Andel (Cambridge
University Press, 1994).
Truths: Essays on Gaia, Symbiosis, and Evolution, Lynn Margulis and Dorion Sagan
Unified Ecology, Tomothy F.H. Allen and Thomas W. Hoekstra (Columbia University
Ground: Rethinking the Human Place in Nature, William Cronon, editor (W. W.
Less Stuff: Environmental Solutions for Who We Really Are, Robert Lilienfeld
and William Rathje (Ballantine Books, 1998).
the Earth: Economics, Ecology, Ethics, Herman E. Daly and Kenneth N. Townsend,
editors (MIT Press, 1996).
"It is the job of every citizen to choose how to balance
the environmental risks of certain kinds of economic development against the perceived
benefits, and to become well enough informed to be able to make the value judgments
implied in that balancing act."
-- Stephen H. Schneider
©1998 The Center for Informed Decision Making