Worlds of David Darling
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The basic unit of ecology. An ecosystem is a dynamic complex of plant, animal, and microorganism communities and their non-living environment interacting as a functional unit. The limits of any ecosystem, such as a forest or a stretch of sea-shore, are always arbitrary because they are drawn by man; the whole planet is one huge ecosystem.

Producers, consumers, and decomposers

Energy is the basic essential of all ecosystems – without a ready supply of usable energy life can't continue. Green plants make their own foods by the process of photosynthesis – using the plentiful atmospheric components carbon dioxide and water, and energy from sunlight, they are able to synthesize sugars. These sugars retain, in their chemical bonding, some of the Sun's energy and this can be released and used to make the more complex chemical compounds needed by the plant for building its structural and reproductive elements.

Unlike plants, animals can't make their own foods. Instead they take energy from plants and other animals by eating them – they can live only at the expense of other living organisms. Within an ecosystem green plants are thus known as producers or fixers of energy, while animals are consumers. The organisms that feed on the dead bodies of plants and animals, causing their decay, are decomposers. In any particular ecosystem producers, consumers, and decomposers live together, rely on each other, and adapt to each other. The more well defined their boundaries of influence become the more the ecosystem is said to be closed.

Complex ecosystems may include many thousands of species, all interrelating with each other to some degree. Their interactions include feeding and the provision of food, but plants and animals can also provide various kinds of shelter or protection, nesting material and homes for each other.

Links in the food chain

The fundamental food-providing relationships between organisms in an ecosystem are often expressed in terms of diagrams known as food chains. Most commonly these consist of a sequence of species that are related to each other as prey and predators and most chains are tied to each other by cross linkages to form food webs that quickly become too complex to be mapped.

The energy flow within each food chain is, worldwide, remarkably constant and nearly always conforms to the "ten percent rule". According to this rule, ten percent of energy is transferred at every link in the chain. Thus a herbivore obtains ten percent of the calories in the plants in eats, and so on. This 90 percent reduction in available energy determines the length of the food chain: the animals at the end of the chain, such as foxes, yield too few calories to make preying on them a worthwhile method of obtaining energy.

Ecosystems do not arise suddenly but rather mature over many years, gradually becoming more complex. Generally speaking, the older the ecosystem the more species it is likely to contain. At its height, and in its final and most long-lived form, the ecosystem is known as a climax community.

The formation of an ecosystem

New environments (such as a newly formed pond, a field recently devastated by fire, or a glacier bed from which the ice has retreated, develop by succession. At first they attract only the hardy species of plants that can survive without shelter and a few animals capable of living among them. These pioneers – opportunists or fugitive species – are usually capable of rapid reproduction and the plants among them include lichens, mosses, and many plant species. These early settlers in the chain of succession modify the environment by adding humus and nutrients to the soil, provide shelter from the Sun and wind and make it more hospitable to other creatures. As more organisms move in, more habitats (known as ecological niches) become available. In a maturing ecosystem the early colonizers may die as stable species exert their superiority.

The surface of the globe can be divided ecologically into ten broad regions determined by their natural vegetation. These regions along with such areas as the sea and the polar ice caps, are often referred to as biomes. Some of the world's most complex ecosystems are to be found in the tropical rain forest where productivity is high and conditions have been stable for many millions of years. Some of the most simple ecosystems occur in polar regions where there is less energy available and where few organisms have had time to adapt to the new environment exposed after the retreat if the ice sheets

Just as ecosystems are built up so they may be destroyed, either naturally or, more probably, through the interference of man. The ecosystem is disturbed because one or more species is either wiped out or drastically reduced in numbers, thus changing the whole energy balance of the community.

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