ecosystem

The complex interrelationships between animals and plants in a food web. A food web shows how different food chains are linked in an ecosystem. Note that the arrows indicate movement of energy through the web. For example, an arrow shows that energy moves from plants to the grasshopper, which eats the plants.

Fire over mountains near Grahamstown, South Africa. Fire is an important agent in ecosystems, whether naturally occurring or caused by human activity. It can have a beneficial impact on ecosystems by killing off weeds and invading species. This favours plants that have adapted to fire. Some have very thick barks, whereas others regenerate quickly after a fire. However, devastation can occur when the ecosystem is not adapted to fire, and this is often the case when the cause of fire is human activity.

In ecology, a unit consisting of living organisms and the environment that they live in. A simple example of an ecosystem is a pond. The pond ecosystem includes all the pond plants and animals and also the water and other substances that make up the pond itself. Individual organisms interact with each other and with their environment in a variety of relationships, such as two organisms in competition, predator and prey, or as a food source for other organisms in a food chain. These relationships are usually complex and finely balanced, and in natural ecosystems should be self-sustaining. However, major changes to an ecosystem, such as climate change, overpopulation, or the removal of a species, may threaten the system's sustainability and result in its eventual destruction. For instance, the removal of a major carnivore predator can result in the destruction of an ecosystem through overgrazing by herbivores. Ecosystems can be large, such as the global ecosystem (the ecosphere), or small, such as the pools that collect water in the branch of a tree, and they can contain smaller systems.

System diversity Ecosystems can be identified at different scales or levels, ranging from macrosystems (large scale) to microsystems (local scale). The global ecosystem (the ecosphere), for instance, consists of all the Earth's physical features – its land, oceans, and enveloping atmosphere (the geosphere) – together with all the biological organisms living on Earth (the biosphere); on a smaller scale, a freshwater-pond ecosystem includes the plants and animals living in the pond, the pond water and all the substances dissolved or suspended in that water, together with the rocks, mud, and decaying matter at the bottom of the pond. Thus ecosystems can contain smaller systems and be contained within larger ones.

Equilibrium and succession The term ‘ecosystem’ was first coined in 1935 by a British ecologist, A G Tansley, to refer to a community of interdependent organisms with dynamic relationships between consumer levels, that can respond to change without altering the basic characteristics of the system. For example, cyclical changes in populations can sometimes result in large fluctuations in the numbers of a species, and are a fundamental part of most ecosystems, but because of the interdependence of all the components, any change in one part of its nature will result in a reaction in other parts of the community. In most cases, these reactions work to restore the equilibrium or balance of nature, but on occasions the overall change or disruption will be so great as to alter the system's balance irreversibly and result in the replacement of one type of ecosystem with another. Where this occurs as a natural process, as in the colonization of barren rock by living organisms, or the conversion of forest to grassland as a result of fires started by lightning, it is known as ecosystem development or ecological succession. The maximum number of organisms that can be supported by a particular environment is termed its carrying capacity.

The human threat The biosphere, or ecosphere, is an interactive layer incorporating elements of the atmosphere, hydrosphere and lithosphere, and involving natural cycles such as the carbon cycle, the nitrogen cycle, and the water cycle. Human interference in the Earth's natural systems, which began with the transition of human society from nomadic hunter-gatherer tribes into settled agriculture-based communities, gathered pace in the 18th and 19th centuries with the coming of the agrarian revolution and the Industrial Revolution. The technological revolution of the 20th and 21st centuries, with its programmes of industrialization and urbanization and intensive farming practices, has become a major threat, damaging the planet's ecosystems at all levels.

Gaia hypothesis The concept of the Earth as a single organism, or ecosystem, was formulated in the mid-1960s by the British scientist James Lovelock, while researching the possibility of life on Mars for NASA's space programme. The Gaia hypothesis, named after an Ancient Greek earth goddess, views the planet as a self-regulating system in which all the individual elements coexist in a symbiotic relationship. In developing this hypothesis, Lovelock realized that the damage effected by humans on many of the Earth's ecosystems was posing a threat to the viability of the planet itself. The effects of this disruption are now becoming apparent in the changing landscapes and climates of almost every region or biome of the planet. They can be seen in the desertification of the Sahel, the shrinking of the Aral Sea in central Asia, the destruction of tropical rainforests, and the creation of the holes in the ozone layer over the Arctic and Antarctic because of the pollution of the atmosphere with greenhouse gases. These gases include carbon dioxide and sulphur dioxide emissions from the combustion of fossil fuels, and the chlorofluorocarbons (CFCs) widely used as propellants and refrigerants. The thinning of the protective ozone layer surrounding the planet, with its consequent threat of global warming, affects the basic functioning of energy flow within every ecosystem of the planet, from micro-organisms to the ecosphere itself.

Oldest ecosystem In 1999, palaeontologists discovered evidence in Western Australia, near the town of Marble Bar (1,200 km/745 mi) north of Perth, of what is believed to be the world's oldest ecosystem. The evidence consists of fossilized stromatolites that have been dated at 3.46 billion years old.