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Ecological System
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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.



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.

Name any animal populations that have become extinct in your lifetime.



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.



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 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.



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 two animals from your state on the endangered species list.
  • 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


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.

CLASSIC CASE: Tennessee Valley Authority vs. Hill, 1978 court decision. A federal agency wanted to build the Tellico Dam on a segment of the Little Tennessee River. A citizens' group wanted to block the project and tried to do so under NEPA. NEPA required the agency to do an assessment of the environmental impacts caused by the proposed dam. In 1973, a small endangered fish known as the snail darter was found in the Little Tennessee River. The citizens' group filed a lawsuit claiming that the dam would destroy the fish's habitat. The court agreed and after many appeals, the 1978 Court of Appeals stopped the project.

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,


- Measures of Ecosystem Well Being (Bork)





  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.