Ecosystems are dynamic interrelated collections of living and non-living components organized in self-regulating units.  Some degree of biodiversity exists in all ecosystems. An ecosystem is a unit because it has boundaries and can be distinguished from its surroundings. The living and non-living components affect each other in complex exchanges of energy, nutrients and wastes.  It is these dynamic exchanges, both fast and slow, which provide ecosystems with their distinct identities. Because of these distinct features ecosystems themselves represent part of the earth’s biodiversity. The characteristic exchanges within an ecosystem are called ecosystem functions and in addition to energy and nutrient exchanges, involve decomposition and production of biomass. The complex interdependencies which develop within or among ecosystems often create emergent properties, or characteristics that cannot be predicted from the component parts alone. 

Ecosystems are often characterized by one or more equilibrium states. An equilibrium state is a mildly fluctuating, relatively stable set of conditions that maintain a population or nutrient exchange at specific levels. Each equilibrium state is dynamic and undergoes periodic decline and resurgence depending on such factors as energy and nutrient inputs, predator-prey relationships (including diseases), or irregular disturbances. Even in its relatively stable condition an equilibrium state is dynamic in terms of the exchanges of nutrients and energy which occur, as well as the activities of its living components.  As ecosystems incorporate more components into their functioning, for example as a result of energy, nutrient or waste accumulations, or the introduction of new species, their biodiversity can increase. More biodiversity increases ecosystem complexity, allows for the provision of more ecosystem functions, and may contribute to the occurrence of emergent properties (see Panarchy, and Millenium Ecosystem Assessment).

Multiple stable states characterize most ecosystems. If disturbances or perturbations occur from either internal or external sources which tend to drive an ecosystem away from its current equilibrium state, then the ecosystem’s regulatory feedback mechanisms work to maintain the current state, or to bring the ecosystem to one of its other typical equilibrium states. Which state is prevalent at any particular time has an impact on related ecosystems. Depending on which equilibrium state is prevalent, there will be more or fewer plants or animals in that ecosystem (or more of one type and less of another), more or less food available, more or less waste absorption, more or less nutrient cycling, or more or less energy. 

Ecosystems can be small or large.  Our entire planet is covered with a variety of different, sometimes overlapping, and often interdependent ecosystems. Major global ecosystems are referred to as biomes.  A tropical forest is a mid-sized ecosystem, which itself contains a diverse array of smaller ecosystems, and which in turn connects with global ecosystems.  These layers of ecosystems are in dynamic interactions with each other, and influence which equilibrium state each maintains (see Scale Levels). 

Ecosystems are said to be “self-regulating” or “self organizing” in that each contains feedback mechanisms which function to maintain the components of the system in one or other of its equilibrium states. An equilibrium state demonstrates the stability of ecosystems. Even in these stable states the components of ecosystems are in dynamic exchanges, and these exchanges involve the predictable build up of energy or materials which cycle the ecosystem either within a single equilibrium or between various equilibrium states.  Ecosystems tend to cycle between these states of change and stability. Ecosystems of different sizes are interconnected and affect each other.  As ecosystems at one level ebb and flow between different stable states, they each create fast and slow cycles relative to their neighbours. Smaller ecosystems are generally characterized by faster cycles of change and stability, and larger ecosystems by slower cycles, with timeframes as long as a millennium or more.

Disturbances  In addition to the relatively predictable ebbs and flows of ecosystem cycles, less frequent and predictable external disturbances also occur (lightning induced fires sweep through a forest or grassland; a volcanic eruption spews tons of material into the atmosphere; a desert riverbed is flooded).  These disturbances stimulate ecosystems to change within their existing equilibrium state, or if the disturbances are great enough the system may move to one of the other typical equilibrium states.  When disturbances occur with regularity (although their timing and extent may be unpredictable, such as with fires) they are incorporated into the ecosystem’s self regulating mechanisms.  These adaptive mechanisms may either provide protection against the disturbance (eg. fire resistant bark) or rely on the disturbance to maintain itself (eg. fire induced bursting of pods to release seeds). 

An ecosystem is described as having resilience to the extent that it is able to return to its current equilibrium state following a disturbance.  Ecosystems that have evolved over long periods of time have incorporated numerous adaptive mechanisms to various disturbances which provide them with the resilience to maintain their structures and functions, and to cycle between typical equilibrium states.  Ecosystems which have survived for long periods of time have done so because they have been successful at maintaining their own structures and functions, as well as adapting to the fast and slow cycles of interconnected ecosystems, and to the unpredictable disturbances that are an inherent part of nature. 

If a disturbance pushes an ecosystem beyond its resilience capacity to adapt, the system will change into a chaotic state; eventually, a new equilibrium state will emerge from its components. Because of the complexities involved, and our relative ignorance of how ecosystems work, it is impossible to predict the characteristics of such new equilibrium states.

It is the adaptive capacities of ecosystems that have provided both the stability and the enormous range of diversity on earth (see Areas of Concern).  It was the ecosystem involving the oxygen producing micro organisms which emerged  billions of years ago that provided earth with its unique atmosphere. Continued evolution of these ecosystems allowed the protective atmospheric ozone layer to develop, making the planet’s surface safe from UV radiation for more complex life forms to emerge. The evolution of forest ecosystems and ocean plankton contributed significantly to the development of the greenhouse effect which provides us with climate stability. Soil fertility is dependent on complex ecosystems of insects and micro organisms creating rich top soils by cycling nutrients from both decaying organic matter and deeper mineral-rich sources.  The wonderful varieties of plants and animals we enjoy as foods all evolved from unique ecosystem environments with specific requirements of moisture, temperature and nutrients. 

The specific ecosystem functions that are apparently beneficial to human civilization are called ecosystem services.  However, given the early stages of human knowledge regarding ecosystems, it would be both premature and imprudent to exclude any ecosystem functions from this category. Ecosystem services (see box) clearly provide life support services for both humans and other species. Our dependence on ecosystem services is complete but poorly understood.  Even in simplistic economic terms, the value of ecosystem services is larger than the global economy¹. Ecosystem services go beyond the direct economic benefits derived from exploitation of very specific ecosystem functions such as timber from forests. It is ecosystems’ ongoing capacities to provide a stream of life supporting and life enhancing services that are vital to human well being. Many of these services are non-market services by virtue of their inherent characteristics (eg. both the atmospheric ozone layer, and the climate stability provided by the global carbon cycle, cannot be owned by anyone who can control their use by others; both ownership and control are conditions for a good or service to be traded in a market.²

There are also many ecosystem services that are thought to have intrinsic value, for moral, ethical or aesthetic reasons.  It is becoming increasingly important to understand the many different types of benefits that ecosystems provide for human well being, and to reconcile the various approaches to valuing these benefits³(see Moral Approaches).

Over the 20^th century human activities have placed increasing demands on certain ecosystem services, particularly those affected by the market economy.  For example, the earth’s forest cover has been significantly reduced to provide wood; ores have been mined for both fuel and to build things; plants and animals have been domesticated, bred and commoditized to provide food. The success of the human species has affected virtually every major ecosystem on the planet (see Biodiversity). Humans are omnivorous, capable of establishing settlements anywhere on the planet’s surface, and make extensive use of the earth’s renewable and non-renewable resources.  While ecosystem science may be relatively new, human domination of the earth’s ecosystems has been gradually expanding ever since the beginning of agriculture; it has grown exponentially with the ready availability of fossil fuels over the last 150 years.  For a civilization that has sought to dominate nature, our impacts on ecosystems all over the planet are clearly indicating that we are very much a critical feature of these ecosystems which we are just beginning to understand.

 

Costanza, Robert, C. Perrings and C. Cleveland (eds.). The Development of Ecological Economics. Brookfield: Elgar, 1997.

 

2 Daly, H. E. and Farley, J.  Ecological Economics: Prinicples and Applications, Island Press, Washington, D.C., 2004. 

 

3. Ackerman, F. and Heinzerling, L.  Priceless: On Knowing the Price of Everything and the Value of Nothing, The New Press, New York, 2004.

 

 

Daily, Gretchen (ed.). Nature’s Service. Washington: Island Press, 1997.

 

Ehrlich, Paul and A. Ehrlich. One with Nineveh. Washington: Island Press, 2004.

 

Gunderson, Lance and C. S. Holding. Panarchy: Understanding Transformations in Human and Natural Systems. Washington: Island Press, 2002.

 

Melillo, Jerry, C. Field and B. Moldan. Interactions of the Major Biogeochemical Cycles. Washington: Island Press, 2003.

 

Millennium Ecosystem Assessment. Ecosystems and Human Well-being. Washington: Island Press, 2003.

 

Smil, Vaclav. The Earth’s Biosphere. Cambridge, MA: MIT Press, 2002.