ECOLOGICAL SYSTEM

Introduction

> INSERT SHORT PARAGRAPH ABOUT EASTER ISLAND

We live on a planet with finite resources, with only one input from the outside -- the energy from the sun. While our technologies can rearrange matter to better suit our convenience, and extract energy by reaching farther and farther into the depths of the Earth, the limits to this process have become evident. A return to "ecological thinking" is necessary if we are to survive. Our deep involvement and preoccupation with technological thinking to the exclusion of ecological thinking has been a major contributor to environmental degradation.

In this unit, we look at ecology -- the understanding of living systems in relation to their environment. Ecology is the study of the patterns and relationships of these systems. In Greek, the word "oikos" means "house," and "logos" means "pattern." The word "oecologie" was coined by Ernest Haeckel, German scientist and follower of Darwin, in 1866. From that time and throughout the 1890s European botanists studied systems of plants and land and their interdependencies, giving rise to the science of ecology. Thus the science of ecology has always had a holistic approach to nature, connecting communities and systems. The philosophical roots of ecology and the land ethic of Aldo Leopold are discussed in detail in the unit on Ethical Systems.

The early study of ecology was tilted towards moral philosophy. As a science, it grew in parallel, more as a description of the distribution of plant communities, and their patterns of succession. In the 1920s and '30s ecology became more of a discipline of science. In 1927, Charles Elton, a colleague of Aldo Leopold, coined phrases such as "food chain" and "niche" and began to work on the way nutrition started with the sun, and on the natural dependencies of organisms and "communities of plants." The English ecologist Arthur Tansley proposed the term "ecosystem" for the total system of relationships.

A comparison of ecological thinking with technological thinking is important to our understanding of why conventional technologies have worked with little regard for the environment, except as a source of raw material or a place to dump -- or even as conditions and constraints to conquer. Table 1 contrasts features of conventional technological thinking with those of ecological or systems thinking.

Technological Thinking Ecological Thinking PARTS PATTERNS Focus on parts and how they connect for immediate performance Focus on patterns, context, connectedness, and relationships ANALYSIS CONTEXT Problems reduced or taken apart for understanding Whole = sum of parts   Need understanding of parts in context of larger whole Whole > sum of parts OBJECTS RELATIONSHIPS World as collection of objects; relationships secondary.   Objects are networks of relationships HIERARCHIES NETWORKS Gives sense of rigid structures of domination and control   Multileveled order of interdependence STRUCTURE PROCESS Product- oriented thinking, often in terms of closed systems   Contextual thinking or weaving together to make sure there is free flow in network; structure follows. Often open systems.

Table 1: Technological Thinking vs. Ecological Thinking (adapted from FRITJOF CAPRA).

Technology is the result of human effort to transcend limits placed on us by space and time. Population explosion has made us aware that the extent to which we can overcome space constraints is limited.

Speed and efficiency are the main metrics of success in technology, which has led to a lack of appreciation of time -- that it takes time to build the complex intricate system that houses and nurtures us as part of it. This lack of respect for time that is embedded in our technological thinking has been one of the most salient factors in degrading environmental quality. To feed our technological ways of life, we currently destroy 24.7 million acres of ancient forest, pump 6.6 billion metric tons of CO₂ into the air, and pump 24.9 billion barrels of oil out of the Earth each year.^{3, 4, 5}

Technologies have typically focused only on narrow segments of the entire system to which the specific technology relates. For example, the design and marketing of the automobile had no forethought about the disruptions large numbers of automobiles would have on land, on air, on energy use, on social units such as cities and families, on all aspects of our ways of life. Technological thinking has traditionally had characteristics that are contrary to holistic, systems thinking -- but this has slowly begun to change. The emerging practice of industrial ecology looks at products as part of a larger cycle and attempts to reduce the environmental impacts of production, consumption, and disposal.

Ecology has traditionally dealt only with natural systems. The new field of industrial ecology is beginning to study industrial behavior and biogeochemical cycles as a part of a system, using the results to design environmentally friendly products and processes. For true integration, however, we need to merge the two ecological systems -- natural and industrial -- with the right consideration of space and time. We are still a long way off from this undertaking!

The sensitivity and response of various organisms to nature's cues is beautifully illustrated in a flower clock designed in 1751 by Carl von Linnaeus, who is considered the father of botany. Noticing that different flower species opened during different times of the day, Linnaeus designed a clock (Figure 1) using the characteristic time of opening and closing of the flowers. He found that once bees had found the flowers they preferred, they would return to the "clock" at the appropriate time, rain or shine! This is a beautiful example of the temporal behavior deeply embedded in the ecological system, including animal physiology. It has also been shown through experiments that certain medications or treatments for human illnesses are more effective at different times of the day (for reasons not fully understood by science). Technology has often tended to ignore these time-linked behaviors and effects.

Figure 1: Representation of the flower clock proposd by Linnaeus. The 12 hours of the clock run from 6 a.m. to 6 p.m.. Click for larger image. Source: The Clocks That Time Us, by Moore-Ede, Sulzman, and Fuller. 6

[1] Ponting, Clive. Green History of the World, Oxford University Press, 1991. p. 7

[2] Miller, Peter and William Rees. Ecological Integrity: Integrating Environment, Conservation, and Health. (Edited by D. Pimentel, L. Westra, and R.F. Noss). Island Press, 2000. p. 3

[3] Source: Greenpeace Video, Magnificent 7. Greenpeace USA Media Center, http://www.greenpeaceusa.org

[4] Global CO₂ Emissions from fossil-fuel burning, cement manufacture, and gas flaring in 1998, as measured by weight of carbon. Source: Carbon Dioxide Information Analysis Center, http://cdiac.esd.ornl.gov/ftp/ndp030/global98.ems.

[5] Global Crude Oil Production, 2001. Source: Energy Information Agency, http://www.eia.doe.gov/emeu/ipsr/t11c.txt.

[6] Moore-Ede, M.C., F.M. Sulzman, and C.A. Fuller. The Clocks That Time Us, Harvard University Press, 1982. p. 12

Ecological Structures

In the mid-eighteenth century, the Swedish botanist Carolus von Linné (1707-1778) better known as Linnaeus, invented the classification scheme of the living world that we still use. Early scholars such as Aristotle and Pliny had also invented such classifications, some of which still hold. For example, Aristotle first classified dolphins as mammals! Pliny (23-79 AD) wrote a 37-volume Natural History, classifying all reported living beings! Linnaeus' scheme gave every living being two names. The first is its genus, the group to which it belongs, and the second the species, describing the subclass in the genus. Thus the present species of humans are Homo sapiens, others like Homo erectus and Homo habitis being extinct. The genus and species have Latin names, with the genus term written capitalized.

Several members of the same species in a particular area at the same time constitute a population, and the area is called the habitat of the species. Different species may live together in a habitat, forming a community. The different species in a community might interact through a food web or exist in symbiosis. Symbiosis is a state in which members of different species live in physical contact, mutually benefiting from each other's presence. Lichens that occur on exposed rocks throughout the world are a wonderful example of symbiosis: They are usually a fungus and an algae (or bacterium) living symbiotically. The photosynthetic algae provide nutrients for the fungus. The fungus seems to provide support and the ability to extract essential minerals from the rock. Because of this pairing, lichens can colonize extreme environments where the fungi or algae alone cannot exist. These include the rocks of Antarctica and of Donegal, Ireland. The lichens scraped off from rocks in Donegal is used to color the woolen material called Donegal tweed.¹

Biomes are the several habitats that co-exist in a particular climatic area. Tropical rainforests and coniferous forests are examples of biomes. Biosphere is the general term for the highest organizational level in which life exists, ranging from the very depths of the oceans to several thousand meters into the tropospheric region of the atmosphere, and including land masses.

Ecosystems and Ecological Balance

Ecosystems are living and nonliving components of an area that include the habitat and the physical and chemical environment. The classic definition of an ecosystem was stated in 1953 by Odum: any unit that includes all organisms (i.e., community) in a given area interacting with the physical environment so that a flow of energy leads to a clearly defined structure, biotic diversity, and materials cycles. [INDIRA - I don't have a reference for this, because it came from Sharon's modules. Should we just take it out?]

What do we mean by "ecological balance," "balance of nature," or "ecosystem stability"? Balance and stability in this context are different from a static condition in which there is no change. Nature is continuously changing, and especially over periods of thousands of years, changes substantially. In his book, Discordant Harmonies, Daniel Botkin writes, "...every thousand years a substantial change occurred in the vegetation of the forest, reflecting in part changes in the climate and in part the arrival of species that had been driven south during the ice age and were slowly returning."² The forest he is referring to is in the western region of northern Minnesota and southern Ontario, which Botkin studied in detail.

Recognizing this difficulty of defining balance, and the fact that balance or stability occurs over different time scales, ecologists talk of "ecological stability" or "resilience." For each of these terms, one may focus on one or two species and their change over time. Most ecologists study population ecology or community ecology. In general, the stability of 10 to 100 species over time scales of 10 to 1000 years is considered when talking of stability. Over this time, populations may remain in an equilibrium. Population resilience is defined as the rate at which the population return to equilibrium after it is disturbed.

Figures 1A-D show a representation of the progress of the Earth's ecosystems as we progress from prehistoric times, through hunter-gatherer societies and agricultural societies, to an ecosystem in which industrial activities dominate. In Figure 1A, the different levels of the ecosystem depend upon the plants, the primary producers of nutrients from H₂O and CO₂ using the sun's energy. As we go from 1A to 1D the role and impact of humans increase. In Figure 1D, human industrial activity and pollution dominate. As the human-dominated fraction of the system increases, we see the shrinking of the other levels, representing loss in biodiversity and even species extinction.³

Figure 1A: Trophic levels in an ecosystem.

Figure 1B: Ecology of a hunting-gathering economy.

Figure 1C: Ecology of an agricultural economy.

Figure 1D: Ecology of an industrial economy.

Source: Clark, Mary E. Ariadne's Thread. St. Martin's Press, New York, 1989. Reprinted with permission of Macmillan Ltd..

[1]McFadden, Johnjoe. Quantum Evolution, Norton: New York, 2000.

[2] Botkin, Daniel. Discordant Harmonies, Oxford University Press: New York, 1990. p. 62

[3] Clark, Mary E.. Ariadne's Thread, St. Martin's Press: New York, 1989.

Biodiversity

Biodiversity, or biological diversity, is generally defined as "the variety of life and its processes," and can be thought of as the full richness of life that exists on Earth. The term "biodiversity" can be applied on several scales. We often talk of the biodiversity of an ecological or climatic region, such as the biodiversity of the Arctic region, of tropical rainforests, of coastal regions, or of plains and prairies.

At a smaller scale, we sometimes talk of genetic biodiversity within a given species, or even a local population of a species. For example, even before the current biotechnology upsurge, genetic manipulation of plants by horticulture significantly decreased biodiversity in the world's corn crops. Species of corn were selected to be propagated specially for their desirable characteristics, such as amount of crop yield or low susceptibility to certain pests. Such "monocultures," however, are then all susceptible to the same diseases or pests. Monocultures have very little genetic diversity to ensure resilience of at least some of the species to certain stresses. This can lead to destruction of large corn crops all at once. Continued genetic engineering by large agricultural corporations will only exacerbate the problem.

The corn example is a good illustration of the fact that a species or ecosystem can exist on a very large scale but not exhibit biodiversity. Another similar example is the cultivation of plantations of genetically identical pine trees that are replacing the forests of the south. E.O. Wilson, a Pulitzer Prize-winning biologist at Harvard, estimates that a pine plantation has 90 to 95% fewer species than the natural forest it replaces.¹

Species richness also varies from place to place depending on the energy available for different species to share and the stability of climate. Solar energy and water availability are of course the most important factors for biodiversity. This is why tropical forest have the most species diversity.

A viable ecosystem therefore must have:

1. A source of energy
2. A supply of raw materials
3. Mechanisms for storing and recycling the necessary materials
4. Mechanisms that allow it to evolve at suitable rates

In general, biodiversity of a given ecosystem consists of three components: composition, structure, and function. The composition of an ecosystem includes the groups of organisms, species and the various organic and inorganic substances that are inputs and residues of the organisms. An ecosystem has two primary types of structures: architectural structure, consisting of spatial organization and patterns; and social structure, which includes the interdependence and relationships among the parts. Organisms, materials, and energy of the ecosystem function in relation to one another. They might interact to influence processes in the ecosystem or the structure of the ecosystem.

IMPORTANCE OF BIODIVERSITY

The importance of biodiversity stems from the fact that ecosystems evolved over thousands, hundreds of thousands, or even millions of years, and are therefore in delicate balance, with each species playing a vital role. Appreciation of biodiversity has come about as a result of an increased understanding of the interrelatedness of species in a given habitat.

Recognition of the importance of biodiversity represents a paradigm shift for conservationists. Within a biologically diverse community, each species -- no matter how small -- plays an important role in the ecosystem. Historically, humans have been moved to conserve and protect that which is beautiful and inspiring, and meets our narrow definition of "importance." To maintain biodiversity, it is necessary to protect species that we may not find beautiful, and some that may be barely visible.

There are varying ideas about how and what biodiversity must be protected or conserved in nature. As seen above, biodiversity as a whole includes soil fertility, water quality, and air pollution levels in addition to species diversity. So these qualities are as important as endangered species in understanding and maintaining biodiversity. Preserving these is central to the stability of an ecosystem. The three different types of stability that need to be preserved are: species stability, structural stability, and process stability. These three very general factors are in somewhat of a hierarchical relationship. Process (including inputs and ways to overcome shocks) interacts with structures to preserve species. But disturbing the species balance will affect the other two, so that this once again demonstrates the close interdependence of the components of this system - the ecosystem.

ECOSYSTEM STABILITY

Ecosystem stability is not a static property, but a dynamic balance. The two qualities (or properties) which characterize ecosystem stability are resistance and resilience. Resistance represents the potential that prevents tree and animal populations from succumbing to stresses such as drought or high pollution. Resilience is the capability that comes into play when organisms are weakened or killed. It is defined as the rate at which population density in an ecosystem returns to equilibrium after it has been disturbed away from equilibrium. Alternatively, it could be defined as how large a range of conditions a system can tolerate and still remain in equilibrium. Resistance and resilience depend on a variety of factors, each important on different temporal and spatial scales.

Biodiversity, the fact that typically there are a variety of species in an ecosystem, shows that natural evolution results in subtly complex systems that best preserve local habitats -- systems that can hardly be designed and engineered by human technologies. Local ecosystem change and undergo modifications through time. Certain niches became modified in time and space through small and large disturbances. Some species extinction may even be "natural." It is the rate at which technology induced change, or anthropogenic change in general, happens that might disturb an ecosystem beyond its own capacity to repair.

Interactions among organisms maintain diversity and in destroying or enhancing one species in a local ecosystem may destroy the whole system in time. While grazing elk normally reduce shrub dominance and promote diversity in early successional forest of the western United States, some of this same region now has had its biodiversity significantly reduced by overgrazing cattle. This type of phenomenon, "overgrazing" for example, occurs when humans intervene to pus the system out of equilibrium.

Keystone and Indicator Species
There are certain species whose role in maintaining the balance of an ecosystem is so significant that they are known as the "keystone species." A keystone is the stone at the summit of an arch that supports all the other stones and keeps the entire arch from collapsing. Therefore, the keystone species in an ecosystem is a species that supports many other species in that ecosystem. The removal of the keystone species would result in quick and noticeable change or degradation of an ecosystem.

Photo courtesy USGS.

The sea otter has been referred to as a keystone species in western Alaskan coastal ecosystems by the US Department of the Interior and the US Geological Survey. Because of a decline in the population of Steller sea lions and harbor seals in Alaskan waters, killer whales have been feeding on sea otters. The sea otter is considered keystone because it feeds on sea urchins, who in turn feed on kelp. Without the sea otter, sea urchin populations would rise, leading to probable destruction of the kelp forests, disrupting large portions of that coastal community. Without the otters to keep the sea urchin population in check, the habitat of the entire community would be altered significantly.

However, the designation of keystone species is sometimes controversial. For example, it could be argued in this case that since it was actually the decline in population of Steller sea lions and harbor seals that caused killer whales to feed on sea otters, the sea lions or seals are also a sort of keystone species. This case demonstrates that large disruptions in ecosystems can often be traced back even farther than disruptions in populations of the so-called keystone species, again underscoring the strongly interrelated nature of ecosystems.

But keystone species are those that play a role in the ecosystem that is much larger than their total number or biomass suggests. Their interaction and rate of consumption determines the tolerance of the system in an important way. Thus the sea otter is considered the keystone species in this chain because they consume sea urchins in large enough quantities and at a fast enough rate so that the relatively slow-growing kelp can keep up with its consumption by sea urchins.

Indicator species are species whose changes in behavior -- or more often, population -- alert us to environmental conditions that threaten ecological niches, or even the entire global system. These species serve as the "canaries in the coal mine," warning us that levels of something in the environment are increasing or decreasing beyond the resilience of the system.

Many scientists today believe that the hundreds of species of amphibians on the decline globally are indicator species, warning us of how human impacts on the climate and air/water quality are having cumulative effects. For more information on this topic, see Tracking the Vanishing Frogs, by Kathryn Phillips.

ECOSYSTEM "MANAGEMENT"

Land management, including forest management, is often chosen as a way of maintaining biodiversity. In certain cases, land (including forests) is managed by reducing diversity to maintain what is required for some economic crop, such as the earlier mentioned pine plantations of the south for paper and wood. This reduction of biodiversity eliminates habitat and sets in place a system that requires continuous maintenance. When laws and forest are managed to preserve natural diversity, the existing structures and processes have to be studied in some detail for a significant amount of time. Even then, "managing" always implies interfering with what would have occurred naturally.

Quite an amount of work has gone into understanding the forces that create and maintain biodiversity. Any management looks at habitat closely. Trees and shrubs provide the primarily habitats for animals as well as other plants and microbes. Biodiversity is also believed to play an important role in stabilizing an ecosystem against stress, such as climate fluctuations and pest outbreaks. So even in forest managed for an economic product, managers are beginning to work to preserve diversity. However, the complexity and the dependencies are never completely understood, and the disruptions caused often destabilize the system.

THREATS TO BIODIVERSITY

Threats to biodiversity are as numerous and varied as the sum of problems that face the overall environment. Symptoms of severe stress in ecosystems have been noted all over the world. Following are several main categories of threats to biodiversity. It is important to note that, although we've grouped the threats into several main categories below, almost all threats facing ecosystems today are the result of human and industrial activity.

As the human population increases, and we use up more and more land area for residences, industry, and commercial or recreational activity, habitat loss becomes a greater threat to biodiversity. Species are forced to live in higher concentration, or move into habitats to which they are not adapted.

Humans often also bring with them exotic or invasive species -- species that are not native to a region or habitat. These invasive species sometimes carry with them viruses or disease to which the local population is not adapted, causing a direct harmful effect. The exotic species present competition for food and habitat, and sometimes "edge out" native species due to their pervasiveness.

As we mentioned before, there is a high level of interdependency among species in an ecosystem, and a reduction of population or loss of one species often leads to population changes among other species. An ecosystem, once altered, can take years to return to a state of equilibrium after a disturbance.

Common environmental problems -- like air pollution, ozone depletion, and global climate change -- are also often serious dangers to the survival of threatened species. Tracking the Vanishing Frogs, by Kathryn Phillips, documents the research of scientists into the disappearance of many populations and species of amphibians. Scientists are more and more convinced that many of these disappearances are related to increased UV exposure (due to the thinning of the ozone layer) or seemingly slight changes in weather and precipitation patterns that affect the frogs' reproductive behavior.² Within the last twenty years, 5100 amphibian species (including 2300 frog and toad species) have disappeared. As we mentioned earlier, amphibians are considered by many scientists to be an indicator species for damage from ozone depletion or global climate change. We say that this is an indicator of global, rather than local, change because amphibians in totally unrelated niches are disappearing concurrently!

Damage to trees from industrial pollution in Germany provide another example of ecosystem stress due to environmental problems. In 1982, the former West Germany noted that 8% of its forests showed decline. In 1983 it was 34% and by 1985, 50%! Dying of forests from pollution has become a sever problem Acid precipitation, causing an imbalance in soil chemistry, has been identified as the reason in Germany and in the Great Smokies Nature Park in the U. S.. Pollution is believed to have stressed the ponderosa pines in the San Bernardino National Forest of California so that they could not produce then natural digestive chemicals. This made them susceptible to bark beetles. Trees can also get overpowered by fungi that cause root rot when they are stressed.

The causes of soil degradation are deforestation, over-exploitation, overgrazing, industrialization, and large-scale agricultural activities maintained through artificial fertilizers. This leads to loss of natural cycles - decay of organic matter, nitrogen fixation, etc. - and decline in soil fertility. Soil fertility in Wyoming, Panama, Thailand and other regions have been destroyed by clear cutting forests which lead to loss of topsoil and of soil compaction that preserves nutrients.

LOSS OF BIODIVERSITY

There are different types of losses of species as follows:

• Buffalo from a wildlife refuge in Nebraska being reintroduced to Theodore Roosevelt National Park in North Dakota. (1956) Photo courtesy of the NPS.

  Depletion of a once common species - the population of a species is greatly reduced, but the habitat still exists and the species could be replaced though there is still some loss of variety in the gene pool.
  
  Example: Buffalo on the American plains, whose population faced near extinction due to large-scale slaughter... Buffalo populations were tremendously reduced, to near extinction, the end of the 19th century. Buffalo were slaughtered for reasons of commerce, sport, and even political reasons. It was the policy of the U.S. Military (in practice, if not officially) to kill as many buffalo as possible. "In 1874, Secretary of the Interior Delano testified before Congress, 'The buffalo are disappearing rapidly, but not faster than I desire. I regard the destruction of such game as Indians subsist upon as facilitating the policy of the Government, of destroying their hunting habits, coercing them on reservations, and compelling them to begin to adopt the habits of civilization.'"³
  
  
• Local or global species extinction - the species is gone (either from its habitat or from the Earth) forever and all current and potential adaptations are lost. Species extinction has regularly occurred since the beginning of life on Earth. Historically, losses occurred at a slow enough rate that ecosystems to adapt; however, losses due to human activity are happening at a much higher rate, causing concern among scientists and conservationists.
  
  Example: Deforestation...The rate of extinction due to deforestation is now 10,000 times that before human civilization.

<<LIST FROM STUDENTS OF SPECIES LOST SINCE 1980'S>>

• Ecosystem disruption - this is the most serious of the three because it is not just the loss of several species, but of an entire ecosystem.
  
  Example: Three Gorges Dam... ecosystem loss that will result from the construction of the Three Gorges Dam in China. This is a project whose goal is to build the world's largest hydroelectric dam on the Yangtze River, creating a 400-mile long reservoir and displacing up to 1.9 million people -- threatening the entire ecosystem.⁴

POLICY
Ideally all of our environmental regulations and policies protect ecology by preventing pollutants from degrading the habitats of species. However, in the USA there is one federal regulation that specifically discusses species protection: the Endangered Species Act (ESA).

Endangered Species Act
The ESAwas enacted in 1973 to place the highest priority on the protection of endangered species. It is administered by the US Fish and Wildlife Service, the National Marine Fisheries Service.

The ESA prohibits government agencies from authorizing, funding, or carrying out any activities that might harm an endangered species, or its habitat, and prohibits individuals from taking an endangered species (taking can be broadly defined as causing any harm) without regard to economic consequences.

The ESA, in conjunction with the National Environmental Policy Act (NEPA), is the main law that can prevent large civil infrastructures from being built when ecosystems or species are threatened. NEPA was enacted in 1969 with the goal of ensuring public input regarding actions that affect their local environment. NEPA requires all agencies to complete an environmental impact statement (EIS) analyzing the effects of any major project that it plans to implement.

The ESA works as follows:

The ESA works as follows:

1. Listing: The Secretary of the Interior maintains a list of endangered species, and a list of threatened species (likely endangered in the future). A species is listed if any of these conditions applies:
   a) present or threatened destruction, modification, or curtailment of its habitat,
   b) over utilization for commercial, recreational, scientific, or educational purposes,
   c) disease or predation impacts,
   d) inadequacy of existing regulatory mechanisms, and
   e) other natural or anthropogenic factors affect the existence.
   
   There is no economic consideration at this stage.
   
   
2. Critical habitat: The relevant agencies define a geographical area with physical and biological features that are essential to species survival. At this stage the agencies can consider economic impacts to limit the area, therefore the area is not necessarily equal to the entire habitat.
   
   
3. Recovery Plans: These are developed to include specific steps that must be taken to help the species populations to increase in size.
   
   
4. God Squad: overruling authority was added to help negotiate conflicts.

[1] Williams, Ted. "False Forests," Mother Jones (Magazine). May/June 2000.

[2] Phillips, Kathryn. Tracking the Vanishing Frogs,

[3] Wooster, Robert. The Military and United States Indian Policy 1865-1903 , Yale University Press, 1988.

[4] Source: International Rivers Network, http://www.irn.org.

OTHER SOURCES:

- Measures of Ecosystem Well Being (Bork)

- http://www.iucn.org/

Life and the Earth's Environment

While we can characterize living organisms by various chemical, physical, and biological parameters, the harmony that puts it all together and forms the spark we call life is far from understood. We see live organisms that may be stages toward more complete organisms. Viruses, for example, are essentially a piece of DNA or RNA coated with protein. It is by inserting this unit into cells of other animals that viruses reproduce and continue their lives. Infective viruses have been synthesized from elements. Several scientists have synthesized DNA and RNA molecules that replicate themselves in a test tube. But while we can concoct many of the molecules that must have been in the primeval soup in which life evolved, we cannot yet compose from elements a group of "cells that crawl out of a test-tube" on their own, to paraphrase Margulis and Sagan.

We do know a lot about the environmental conditions that sustain life. We have begun to understand that not only does the environment sustain life, but life in turn has made environmental conditions what they are today. We now discuss some peculiar features and compounds on Earth that make our environment particularly "fit" for our kind of life. Much of this discussion is based on three sources:

1. Microcosmos: Four Billion Years of Microbial Evolution, by Lynn Margulis and Dorion Sagan, 1986.
2. Time's Arrow and Evolution, by Harold F. Blum, 1951.
3. Gaia, by James Lovelock, 1979

Blum discusses the "fitness of the environment," a concept originally proposed by Lawrence J. Henderson in 1913. Certain aspects of the environment make the Earth particularly advantageous for living organisms to live, develop, and evolve. The Earth's size and its distance from the sun (a medium yellow star), and the nature of the sun itself, determine the gravitational force of the Earth and the amount and type of electromagnetic energy we receive. These factors provide the conditions under which life evolved, and that sustain life on Earth.

[Does this go here? Or should it be a notecard on symbiosis?]

Life can adapt to extreme conditions through symbiosis. Photosynthesis is an original adaptive mechanism developed in a species of bacteria called cyanobacteria a billion years before plants evolved. These seem to have eventually partnered with fungus-like organisms to evolve into the cells of plants. The cyanobacteria have now become the part of the plant called chloroplasts which are the photosynthetic organelles of plants! This gives evidence to a symbiotic theory of evolution. A group of bacteria live in animals intestines. Another species live in the seabed seven hundred meters below the surface of the Gulf of Mexico. Bacteria are responsible for nitrogen fixing -- extracting nitrogen from the air and making it into nitrates and NH₃, which can then react with water to provide nutrients.

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What is Life?

What is special about living systems and their relationship to the environment? In this unit, we seek to understand some of the organizing principles of living systems, recognizing with humility what Manfred Eigen and Mae-Wan Ho say--that we can only understand what happened, not why it did.

Reflection on the nature of life even for a short while brings to mind the awesome variety, coherence, and organization in the functioning of live organisms. Exchange of materials with the environment and adaptation to environment are also evident. While we can try to understand these interactions, and guess at how life might have evolved on Earth, we can only guess at how life did begin and survive. In the words of the Nobel Laureate Manfred Eigen, "life is historical reality."

[1] Eigen, Manfred. Steps Towards Life: A Perspective on Evolution.

[2] Mae-Wan Ho. The Rainbow and the Worm, Singapore: World Scientific Publications, 1994. p. 10

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Materials for Life

Carbon, hydrogen, oxygen, nitrogen, phosphorus, and calcium form the major chemical scaffolding of biological molecules. Hydrogen, nitrogen, oxygen, phosphorus, and sulfur combined with carbon generated the first group of compounds that eventually formed the chemical basis of life. Other elements, such as iron, magnesium, sodium, potassium, chlorine, and iodine also play specific and vital roles.

Hydrogen, oxygen, and carbon make up over 93% of the weight of the human body. Water is almost 80-90% by weight of all living organisms. Water has several physical and chemical properties that make it important in maintaining conditions fit for life on Earth.

Water
<student exercise on water?>
The large amount of water on the Earth's surface and the fact that at the average temperature over most of the Earth, water is in a liquid state, are both important to life. Water constitutes the largest fraction of most organisms.

If spread evenly over the Earth, the water present on Earth could form a layer about 2.5 km (1.6 miles) thick. Water has a high heat capacity. It can absorb quite a bit of heat (1 calorie for every gram for each degree rise in temperature) before its temperature rises significantly. This provides a moderating influence that prevents sudden rises in temperature, which could be damaging to live organisms. Water bodies on Earth work to moderate atmospheric temperature changes, and internal water helps organisms maintain temperature ranges.

Water also has a large latent heat of vaporization because a lot of energy is needed to break hydrogen bonds among water molecules. It takes a lot of heat to change liquid water to vapor--560 calories/gram. Thus organisms (plants and animals) can dissipate a lot of heat by having some of the water in them evaporate. For example, we are able to evaporate water from our body, as sweat, cooling the body because of the heat removed by the evaporating water.

Water has a high latent heat of fusion as well. Eighty calories are required to convert 1 gram of ice to water. In addition, because of the peculiarities of the hydrogen bond, ice is less dense than water. It rarely happens that a solid material is less dense than its liquid state. Ice also does not conduct heat well. The high latent heat, low heat conductivity, and low density of ice causes ice to float on water, keeping the warmer water sealed below the insulating ice layer on lakes and other bodies of water, and keeping the water habitable for aquatic life.

Water is a "universal" solvent. It is capable of dissolving a variety of materials. Salts dissolve in water to form ions because of the polar nature of the H₂O molecule. As described in the Science Notes of the Energy System, water is a polar molecule. It has a positive and a negative end. The longer time spent by the covalent electrons near the oxygen atom makes the oxygen end negatively charged overall. Various ions play important roles especially in the conduction of nerve impulses. Balance of ionic flow across cell membranes (cell walls) are also an important mechanism of moving nutrients as well as in several other cell functions. <ornella animation?>

Finally, water vapor is one of the greenhouse gases that keeps the Earth's atmosphere at the temperatures suited to life.

Hydrogen
Many of the special properties of water come from hydrogen. The small size of the hydrogen atom makes it possible to fit into many more molecular configurations than a bigger atom can. Thus hydrogen can form numerous compounds. Hydrogen is light, so all hydrogen gas could have escaped the Earth's gravitational pull when the Earth was still very hot. However, its high chemical reactivity with nitrogen, oxygen, and carbon, and the abundance of these elements made it possible for the Earth to retain a large amount of hydrogen in combination with these elements as ammonia, water, and methane during the primitive days of the Earth.

Carbon
The carbon compounds that make up essential molecules such as proteins are described in the notes on biological molecules. Carbon is second only to hydrogen in the number of compounds it can form, oxygen being the third in this capability. Carbon can form more than 2500 compounds with hydrogen alone. The next elements that form most hydrides are boron and nitrogen; each of which can form only seven! The C-C bonds make possible a great variety of molecules with different chains and rings. C, H, and O combine together to from even a richer variety of compounds. The same number of atoms can yield completely different compound depending on the arraignment of atoms. For example, C can form butyl alcohol (the prefix butyl refers to four carbons) in two alternate forms with slightly different but similar properties.

Butyl Alcohol 1		Butyl Alcohol 2

The -OH group is the hallmark group of an alcohol. These same number of atoms could also form an ether characterized by the -O- bond between carbon groups. Thus these atoms can form diethyl ether, two ethyl (C₂H₅) groups bridged by O, as in:

Diethyl Ether

For much larger molecules with many C atoms, the possible arrangements become very large. Recall that carbon is the middle element in the first period of the Periodic Table. It has four electrons (1s²2s²2p²) in the outer shell, needing four more to complete the outer shell. This capacity to form four covalent bonds makes for the capability of carbon to form compounds. The four bonding electrons and various spatial arrangements give carbon its enormous versatility.

This versatility and the fact that carbon dioxide is a gas at ordinary temperatures are two important aspects in carbon being the chemical basis of life. Photosynthesis occurs because CO₂ is a gas and is soluble in H₂O, so that this mixture, with energy from the violet part of sunlight, can form sugars.

The versatility of carbon comes from its central position in the periodic table. People have conjectured why silicon in an analogous position and, being one of the most plentiful elements in the Earth's crust, did not become that centerpiece. SiO₂ is found in the solid form in plenty--as sand. But it becomes a gas at only 3000°, and it is not soluble in water. The chemical versatility of silicon is indeed the property we value for its use for computers.

Oxygen
With carbon and hydrogen, oxygen forms the third principal element in living systems. As we see later in this unit, our atmosphere was not always oxygen-rich. About 2000 million years ago, there was only about 0.0001% oxygen in the atmosphere. During the Archean and Proterozoic ages (when plants started to use photosynthesis), there was a radical increase of the O₂ concentration to the almost 20% that it is today. This resulted in a major extinction of some bacteria as discussed in a later section.

As silicon is to carbon, so is sulfur to oxygen. So we could imagine a material and life configuration where H₂S instead of H₂O was the basic "liquid" of life. (H₂S is actually a gas at Earth's average temperatures.) However the H-O bond is stronger than the H-S, and oxygen is 50 times more plentiful on Earth than sulfur. There are some organisms that use sulfur. This is discussed in the section on photosynthesis.

Other Elements
The other elements that play a vital role in living systems are N, Ca, P, Na, Fe, K, Cl, S, Zn, and Mg. Together these form about 1.72 atomic percent of the human body. Along with H, O, and C, these elements account for 99.96% of the human body. Lighter elements dominate this list and these elements have more specific roles in the function of biological molecules than the more general C, O, and H trio.

Nitrogen and sulfur are components of all proteins. Phosphorus is an essential component for the storage and use of energy in all cells. Cellular energy resides in phosphate bonds. Mg is a central component of chlorophyll, and iron is a component of hemoglobin and other respiratory enzymes. These elements serve very specialized but important functions.

Metals such as Fe and Na are rare in the body but play important roles either in very specific molecules as Fe in hemoglobin, Zn in gene transcription proteins, or Mg in chlorophyll; or with a specific function such as Na or K ions providing the flow of ions for conduction of information along nerves. Most metals, however, are toxic to most organisms. Examples of metal toxicity that have become significant environmental problems in the last half-century are cases of lead poisoning, mercury poisoning (Minamata disease), and poisoning by metals such as chromium (Cr), aluminum (Al), and cadmium (Cd). The amounts of chromium and cadmium in the environment have increased due to numerous technological uses ranging from steel production to household batteries.

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Capturing Energy for Life

We said at the outset that the ability of certain molecules to capture and use small amounts of energy efficiently is a central aspect of life. The continuous chemical reactions and changes that go on in an organism is called metabolism. Accumulation and use of energy is the basis of metabolism.

In the long run, almost all the energy for life on Earth comes from sunlight, through photosynthesis by plants. Energy is stored as potential energy in chemical bonds of carbon compounds. This is our "free reserve."

Figure X: Cycle of photosynthesis and oxidation -- energy capture and release.

Figure X shows the cycle of energy transformation overall.

Photosynthesis and Energy Storage
In most of photosynthesis, electromagnetic energy from sunlight is used to store chemical potential energy through oxidation of water, using CO₂ for the carbon.

CO2 + H2O + sunlight energy rich carbon compounds + O2 + heat
	(e.g. C12H22O11)

This equation may be written in general as:

CO2 + H2O + sunlight (CH2O)n + O2

where (CH₂O)_n is the generic name for the energy-rich compounds--sugars and starches such as C₆H₁₂O₆ [(CH₂O)₆] or C₁₁H₂₂O₁₁ [(CH₂O)₁₁].

Part of the energy captured is lost as heat. Photosynthesis is less than 1% efficient--only 1% or so of the total energy falling on leaves is used for photosynthesis--mainly the blue and ultraviolet region. Because photosynthesis only uses blue and ultraviolet wavelengths of energy, it only actually captures energy from a small fraction of the solar spectrum. Then again only 1% of this captured energy is converted into food.

Leaves appear green because they reflect most of the sunlight in the visible region. The main molecules responsible for photosynthesis is the family of pigments called chlorophyll (NOTE: Pigments are light absorbing molecules that by absorbing light preferentially of one or a set of wavelengths gives the color to the material that contains them.) Chlorophylls are formed in cell bodies called chloroplasts. Chlorophyll and other similar energy-transforming molecules (phycocyanin, fucoxanthin, phycoerythrin) are characteristic of autotrophic organisms. Autotrophs are organisms that are able to manufacture their own basic supply of energy-rich carbon compounds from CO₂. Heterotrophs (like us) on the other hand, have to be supplied with energy-rich carbon compounds from outside sources. All autotrophs reduce CO₂ to carbohydrates (written in general as (CH₂O)_n) or related organic compounds.

Photosynthesis may also occur through oxidation of compounds other than water. There are a few bacteria called chemoautotrophic bacteria that get their energy for storage not from light (photosynthesis) but from other inorganic chemicals (chemosynthesis). Many of these bacteria can also do without pure oxygen as long as they have CO₂ and energy from chemical bonds. Thus some use H₂S found in volcanic ash and make (CH₂O) according to:

CO2 + 2H2S + sunlight or chemical bond energy [CH2O] + H2O + 2S

Here sulfur can use bond energy from H₂S instead of sunlight. Recall how we said in the previous section that sulfur could have been a "contender" for the position oxygen holds in making life possible.

Some species oxidize H₂:

CO2 + 2H2 [CH2O] + H2O

or even some complicated molecule like isopropanol:

CO2 + 2CH3CHOHCH3 [CH2O] + H2O + 2CH3COCH3

In general, photosynthesis may be written as:

CO2 + 2H2A [CH2O] + H2O + 2A

where H₂A (e.g. H₂O, H₂S) is the compound that gets oxidized to A. The role of H₂A is to donate hydrogen to make the energy-rich carbohydrates (CH₂O)_n from CO₂. <<NOTE TO TEACHER>>

Biological oxidation is the process--often a series of processes--by which the energy in (CH₂O)n is eventually used by the organism with the final products being CO₂ and H₂O which are excreted. Glucose is a type of sugar made through photosynthesis. The oxidation of glucose can be used as an example to show the oxidation reaction:

C6H12O6 +6O2 6H2O + 6CO2 + 688 kcal/mole

The oxygen that comes into the body through respiration enables the biological oxidation (or "combustion) of carbohydrates with the release of energy. Note that this energy is of a different type than the original light energy which helped form the sugar.

Note the similarity of this reaction to the burning of coal described in the Energy System. We had to have high temperatures for the combustion of coal. Compared to that, our combustion takes place at low (body) temperatures. Glucose is a complex compound and it breaks down in steps. Actually, this breakdown happens very very slowly at normal temperatures. Certain catalysts--enzymes and coenzymes--speed these up as needed, in the absence of heat.

All compounds used for energy are not directly derivable from glucose. The vertebrate animals use carbohydrates, fats, and proteins to store energy. These are broken down into smaller units before oxidation begins. Adenosine triphosphate (ATP) and adenosine diphosphate (ADP) play a unique role in mobilizing and transforming energy in living systems. The phosphate bonds in ATP and ADP are used to transfer energy. It takes a long chain of processes to eventually complete the breakdown of sugars and starches to H₂O and CO₂, material being constructed and deposited along the way.

The ATP-ADP System

Figure Y: Adenosine monophosphate.

FigureY shows the structure of adenosine monophosphate, made of the protein adenine and the sugar ribose with a phosphate group hanging at the side. One or two more phosphate groups can be linked on to the phosphate in AMP to give ADP and ATP respectively. The ADP phosphate bond is about 10 kcal/mole and breaking and building this (the ADP-ATP cycle) transfers energy from glucose to where it is needed. The high energy phosphate bond is denoted by ~ph. This reaction, called phosphorylation, is represented in the equation:

glucose + ATP glucopyromose-6-phosphate + ADP
	(known as GP6P)

or

glucose + ~ph GP6P

GP6P has higher energy than glucose and we say the glucose has been phosphorylated. This compound can take part in reactions that glucose cannot, and use the energy in these reactions. One example is muscular contraction, where the ~ph bonds transfer the energy needed.

Part of the extraction of energy and sugar breakdown occurs through fermentation of sugars to alcohol and acids (alcohols have the -OH group, acids have the -COOH group).

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Evolution & the Environment

Evolution is defined as the gradual change of any object, in our case, the biological system, through time. We discussed briefly in the previous section some of the aspects of the complex intricate system of the chemistry of biomolecules. We now shift to the level of the organism--cells organized into tissues, organs, and organism--which have persisted and changed through time. Broadly speaking, the first single-celled organism did not have a nucleus. These prokaryotes persist today as microbes, such as bacteria and viruses.

Protein synthesis was an early step in the evolution of organisms who were then capable of reproducing themselves using their RNA. It is not clear when the first cells in the form we know now were formed. Somewhere, in the primordial soups, some molecules got enclosed inside a membrane to form a cell. RNA molecules within this cell could then facilitate reproduction of the cell. All we can surmise is that this aggregation of molecules started about 3.5 billion years ago. It is believed that RNA came after proteins and enzymes evolved. Once a cell was established, RNA evolved into DNA, making a subcellular structure we now call the nucleus. The DNA, which is a coil of two strands of RNA (with the bases A, C, G, and T, whereas RNA has A, C, G, U), is the molecule that enables protein synthesis.

Some of the prokaryotic bacteria we know like mycoplasms, as well as different varieties of bacillus, e-coli, and staphylococcus, still abound on Earth. Bacteria live in an enormous variety of environments, often symbiotic with other organisms. Thus benign forms of e-coli live in our guts and numerous bacteria live in soil and in plants. Species of bacteria can use a variety of organic molecules such as sugars and polypeptides as their food. Bacteria are the most abundant type of cell on earth. The early bacteria seemed to have used ATP to store the energy of sunlight. Blue-green algae is a type of bacteria that converts CO₂ and N₂ into its food.

As plants developed the capability of photosynthesis, the composition of the early atmosphere (mostly CO2) began to change over billions of years to this current composition of 20% oxygen and only traces of CO₂ as the plants "fixed" the carbon. Many of the early bacteria were used to an oxygen-poor environment and several of them then became extinct. Others formed a symbiotic association with oxygen--using (aerobic) types of cells to form the present day cells with nuclei (eucaryotes). The different organelles in the cell such as the chloroplasts and mitochondria are now believed to have been separate organisms that originally associated with each other to form mutually beneficial colonies we call cells.

Origin of Life

As we noted before, life on Earth has evolved around the chemical versatility of a few atoms, especially carbon. Some special features of chemistry are used by living systems. These features are:

1. the ability of carbon to combine in so many different ways;
2. the unique properties of water; and
3. the ability of organic molecules to use small amounts of energy efficiently.

A live organism is an open system, continuously exchanging energy and matter with the environment. It is "self-organizing," meaning it takes raw material and reassembles it into complex vital molecules. During this process, life increases internal order (decreases local entropy). Thus life builds up information (order) which is then duplicated.

In the Beginning

The early environment on Earth is a matter of conjecture. Piecing together evidence, it is believed now that the environment consisted of high energy events such as volcanic eruptions, continuous torrents of rain, and large amounts of lightning. It is believed that there was little if any oxygen in the atmosphere, and certainly no ozone layer. Therefore, ultraviolet from the sun could reach all the way to the Earth's surface.

Some of our models for the early environment of Earth come from the observation of the atmospheres of Mars and Venus, made by NASA.

Table X shows the major gases in the atmosphere of Venus, Mars and the Earth. Note the difference between the estimate of the composition with and without life on earth.

It has been shown in laboratory experiments that simple carbon-based (organic) molecules are formed under these early conditions that prevailed on Earth. In 1953, Stewart Miller, a graduate student of the famous chemist, Harold Urey, simulated the early (prebiotic) atmosphere on Earth--a mixture of ammonia (NH₃), water vapor (H₂O), hydrogen, and methane (CH₄). He bombarded the mixture with electrical discharges to simulate lightning. In a week, he saw some spectacular results: alanine and glycine, two amino acids that form proteins in life forms today (including humans) were formed in the resulting mixture. Under the conditions provided, more complex molecules such as formaldehyde (HCHO), formic acid (HCOOH), and hydrogen cyanide (HCN) had formed. In a water solution these molecules had then reacted with each other to form more complex organic molecules such as acetic acid (CH₃COOH), glycine (NH₂CONH₂), alanine (NH₂CHOHCOOH).

The richness of carbon chemistry and the plethora of carbon compounds form the basis of life on Earth. Carbon chemistry (called organic chemistry) and the function of biomolecules are explained in detail in the section on carbon compounds.

Environment and Life

In the late 1970's, Elso Barghoom of Harvard University was looking for the earliest evidence of life, and found it eventually in Swaziland, Africa. He found evidence of bacteria in 3.4 billion year old fossils. This means that life started very early on our 4.5 billion year old planet. The time it took to move from inanimate matter to the first forms of life was actually shorter than that to move from bacteria to larger organisms--the earliest of which appear to be only 570 million years old, as evidenced by hard-shelled fossils of that age that appear all over the Earth.

Early life then probably came from mixtures of materials combining to form biomolecules with the energy provided by ultraviolet light and lightning. Replication of DNA and mutation in rapidly dividing bacteria, as well as local variations in environment, then provided a route to diverse populations of bacteria. Development of metabolic pathways to store and convert energy--mechanisms of fermentation to break down sugars--was an early step. Along the way the bacteria also began to capture atmospheric nitrogen to begin the manufacture of amino acids and other organic compounds. To this day, we need bacteria to take the stable nitrogen gas N₂ from the atmosphere and convert it into usable compounds. This "nitrogen-fixing" is discussed under the nitrogen cycle in the Materials System.

Margulis and Sagan also state that "the evolution of photosynthesis is undoubtedly the most important single innovation in the history of life on the planet" (p. 78). The first photosynthetic organisms were bacteria that used H₂S rather than H₂O. H₂S must have been plentiful, emitted from volcanoes. The development of the successive stages of bacterial development is fascinating as described by Margulis and Sagan. Early adaptations included developing pigments to protect against ultraviolet, then top layers protecting the layers below and developing repair enzymes. Repair enzymes persist in us today. When ultraviolet or other ionizing radiation damages part of our DNA, these enzymes remove the damaged portion and replace it with new healthy DNA. Despite the fact that we have had an ozone layer to filter out the almost all ultraviolet for over 2 billion years now, we still have this repair system.

Xeroderma pigmentosum is a rare genetic defect inhibiting DNA repair mechanisms against ultraviolet radiation damage. It is characterized by severe sensitivity to all sources of UV radiation (especially sunlight), and often results in cutaneous lesions, premature aging of the skin, cataracts, increased risk of ocular benign and malign tumors, and sometimes neurological disorders such as mental retardation. To learn more about XP, visit the Xeroderma Pigmentosum Society, or read its disease description at the Atlas of Genetics and Cytogenetics in Oncology and Haematology.

Gaia: Co-Evolution of Climate and Life

Gaia is the Greek goddess of the Earth. While designing experiments for NASA to detect life on Mars, the atmospheric chemist James Lovelock developed the theory called Gaia. Gaia refers to the system of all life on Earth and the atmosphere which mutually regulates prevailing conditions to continue life on Earth. The name "Gaia" was suggested to Lovelock at his request by his neighbor, William Golding, author of Lord of the Flies.

The Gaia hypothesis states that the biota (group of all living organisms) regulate the temperature and gas composition of the atmosphere. Lovelock came to this conclusion because the 20-80 composition of O₂- N₂ in our atmosphere can not be explained by laws of physics and chemistry alone. If we were to make a simple mixture of these gases in the laboratory along with some of the other materials on Earth, the gases would react quickly and become compounds, and not remain as O₂ and N₂ in the gaseous state. Lovelock therefore postulated that this unlikely mixture must be aided by the continuous production of these gases by live organisms! If this were not true, our atmosphere would be a mixture of N₂, NH₃, SO_x, CH₄, methyl chloride (CH₂Cl), and others. These are indeed present but only in minute quantities.

In addition, the Earth's average temperature has remained relatively stable (around 22° C) despite the increase in the sun's temperature over the past 4 billion years. This too has been attributed by Lovelock to the feedback effects of life on the atmosphere.

(Need to add material)

Several scientists disagree with Lovelock's hypothesis. Lovelock has actually modeled a simple system, "Daisy World," a planet with black and white daisies circling a sun-like star. He and his co-author, Andrew Watson, have demonstrated the Gaia-like character of this world. The daisies act as thermostats, stabilizing the temperature. In our world, microbes can play the role of the daisies. Margulis and Sagan cite the discovery that about 20,000 years ago there was only two-thirds the amount of CO₂ that we have now, and that the rise of CO₂ to pre-industrial levels took place abruptly in a 100 year span. This cannot be explained by geophysical or chemical processes alone, but could be the result of a sudden species death of algae.

(Add stuff from Gaia)

(Find Gaia or Daisyworld links and add...)

Gaia

• http://erg.ucd.ie/arupa/references/gaia.html
• http://www.kheper.auz.com/gaia/Gaia.htm
• http://www.sprl.umich.edu/GCL/paper_to_html/gaia.html
• http://www.globalchange.umich.edu/gc1_lecture_schedule.html
• http://www.magna.com.au/~prfbrown/gaia.html
• http://www.gaia.org/
• http://www.lancs.ac.uk/users/philosophy/mave/guide/gaiath~1.htm
• http://www.ozi.com/ourplanet/gaia.html
• http://www.ozi.com/ourplanet/
• http://www.ecolo.org/lovelock/
• http://www.bangor.ac.uk/~iss023/gaiaweb/

Daisyworld

• http://www.pik-potsdam.de/~bloh/
• http://www.gingerbooth.com/courseware/eco/daisy/readers/discussion.html
• http://www.lawrence.edu/dept/environmental_studies/daisytwo.html
• http://www.cogs.susx.ac.uk/daisyworld/daisyworld.html
• http://www.uta.edu/geology/geol1425earth_system/1425chap9.htm