The term ecosystem was coined by the British ecologist A. G. TANSLEY in 1935. He used it to define a unit that covers all organisms of a given area as well as their relationship to the inorganic environment. The organisms within an ecosystem form a biocenosis, their inanimate environment is called a habitat. The totality of all ecosystems on earth is called the biosphere.
Ecosystem is an operational term, since it describes units that are less easily captured than, for example, a molecule, a cell or a species. What is defined as an ecosystem is within the discretion of the person dealing with the ecosystem. A lake, for example, a reed belt, a wood, or a cornfield can be defined and described as ecosystems. Each system can consist of a number of partial systems as can be deduced from the system theory. Which level of complexity is studied is therefore mainly dependent on practical considerations.
It is distinguished between simple and complex, between aquatic and terrestrial, between natural and human-influenced ecosystems. Among the astonishingly complex systems, i.e. those systems that are especially rich in species are tropical rain forests and coral reefs.
Systems are more than just the sum of the system elements' performances. Between their elements exist numerous, often specific and usually always regulated connections. Controlled systems, in contrast, are characterized by feedbacks and thus by a high degree of stability. They are largely independent against disruptions. The higher the number of system elements and the amount of interactions with each other, the more effective fluctuations can be balanced out. But still each system has only a limited capacity. When the limit of this capacity is exceeded, then the system will not return to its original situation or will even be destroyed. Ecosystems with only a few system elements are extremely liable to break down. Just think of a spruce monoculture or a corn field. They can only be kept in balance with the use of stabilizing, energy-consuming measures like the use of insecticides. They can, on the other hand, easily be rebuilt after complete destruction. Complex systems are very resilient, they have a large buffer capacity, but their destruction is an irreversible act. A destroyed tropical rain forest or a destroyed coral reef is lost forever.
Natural or almost natural ecosystems were able to absorb human influences for centuries, but it becomes clear that the limits of the strains on them are reached or even exceeded. Ecology has therefore during the last decades been in the public eye. Environmental protection has become a major issue of the domestic policy of many industrial nations. Many students of biology study this subject only, because they are interested in environmental issues.
Ecology cannot solve political problems, it is especially unable to offer a fast aid to decision-making in order to answer short-term questions. Changes in a human society require a knowledge about its structure, its functions, its decision-makers, the chances to alter something, and the most effective ways to initiate changes. Successes that can be achieved by organizations like Greenpeace helped considerably to create an environmental awareness among general public and politicians.
Ecology is a holistic science. In ecological work, long-term experience, a detailed insight into plants, animals or microorganisms, a sound knowledge and/or mastery of physical and chemical measuring are more important than spontaneous political activity. A co-operation between scientists of different disciplines is therefore the best way to approach the aim. A solid scientific management is necessary in order to co-ordinate the activities of all scientists participating in the project.
Ecosystems change permanently. Changes and disruptions are not only caused by human activities. The evolution of organisms and their adaptation to variable environmental conditions are the main reason of the changes and thus also of the evolution of ecosystems. A sequence of differently structured systems under constant abiotic conditions is called succession. In Central Europe, for example, certain types of forests are the most stable systems. Such a system is called a climax association. Such final links of developments exist only as long as the environmental conditions are stable. In the history of the earth, several drastic climatic changes occurred that caused the extinction of whole groups of plants and thus also to the replacement of once stable ecosystems by others. The pre-Ice Age vegetation of Central and Northern Europe, for example, or the carboniferous swamp forests became irreversibly extinct. After the ice had gone, the landscape was inhabited again beginning in the south and slowly advancing towards the northern part of Europe. A comparison of today's Central and North European flora with that of North America shows that the European flora is far poorer in species. The variety of species of North America that was only to a small extend covered by ice the variety of species that existed before the Ice Age in Europe, too, remained.
The transformation of the natural landscape into a cultivated landscape had a serious impact on all ecosystems. Often, new ecosystems possibly worth protecting developed that could not exist under natural conditions. The Calluna vularis areas of the Lüneburg Heath in Germany are impressive examples. Without regular grazing by a species of German moorland sheep called Heidschnucken, these areas would soon be replaced by birch-pine-forests.
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