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Ecology is the branch of science that studies habitats and the interactions between living things and the environment. The term was coined in 1866 by the Darwinist and German biologist Ernst Haeckel from the Greek oikos meaning "house" and logos meaning "science"). The environment includes both the abiotic environment -- non-living things like climate and geology -- and the biotic environment -- living things like plants and animals. Much of ecological research is concerned with the distribution and abundance of organisms and how these are influenced by characteristics and properties of the environment: organisms influence their environment and the environment influences organisms.

Table of contents
1 Usage
2 History of ecology
3 Fundamental principles of ecology
4 Ecological crisis
5 Political ecology
6 Related fields
7 See also


Ecology is frequently defined, particularly in human ecology, as the study of the following triangular relationship:

The term ecology means different things depending on who is using it. For many scientists, ecology belongs to the basic biological sciences. However, most ecologists argue that ecology is a scientific field of its own. For most non-scientists, ecology is first and foremost the protection of nature and the environment from humans and our activities; some argue that this position confuses ecology with environmentalism. Others view ecology as more than a science -- to them it is a certain vision of the world, which would consist in people living in harmony with the other living beings, and in not seeing the other organisms which surround us as mere objects to be used, but rather as belonging to a larger coherent system.

History of ecology

One of the first ecologists may have been Aristotle who had interest in many species of animals. He was followed by numerous naturalists such as Buffon and Linné, whose work is usually considered the origin of modern ecology.

Botanical geography and Alexander von Humboldt

Throughout the 18th and the beginning of the 19th century, the great maritime powers such as France and Germany, launched many world exploratory expeditions to develop maritime commerce with other countries, and to discover new natural resources, as well as to catalog them. At the beginning of the 18th century, about twenty thousand plant species were known, versus forty thousand at the beginning of the 19th, and almost 400,000 today.

These expeditions were joined by many scientists, including botanists, such as the German explorer Alexander von Humboldt. Humboldt is often considered the true father of ecology. He was the first to take on the study of the relationship between organisms and their environment. He exposed the existing relationships between observed plant species and climate, and described vegetation zones using latitude and altitude, a discipline now known as geobotany.

In 1804, for example, he reported an impressive number of species, particularly plants, for which he sought to explain their geographic distribution with respect to geological data. One of Humboldt's famous works was "Idea for a Plant Geography" (1805).

Other important botanists include Aimé Bonpland and Eugenius Warming.

The notion of biocenose: Charles Darwin and Alfred Wallace

Towards 1850 there was a breakthrough in the field with the publishing of the work of Charles Darwin on The Origin of Species: Ecology passed from a repetitive, mechanical model to a biological, organic, and hence [[evolution| evolutionary]] model.

Alfred Russel Wallace, contemporary and competitor to Darwin, was first to propose a "geography" of animal species. Several authors recognized at the time that species were not independent of each other, and grouped them into plant species, animal species, and later into communities of living beings or biocenose. This term was coined in 1877 by Karl Möbius.

The biosphere - Eduard Suess and Vladimir Vernadsky

By the 19th century, ecology blossomed due to new discoveries in chemistry by Lavoisier and de Saussure, notably the nitrogen cycle. After observing the fact that life developed only within strict limits of each compartment that makes up the atmosphere, hydrosphere, and lithosphere, the Austrian geologist Eduard Suess proposed the term biosphere in 1875. Suess proposed the name biosphere for the conditions promoting life, such as those found on Earth, which includes flora, fauna, minerals, matter cycles, et cetera.

In the 1920s Vladimir I. Vernadsky, a Russian geologist who had defected to France, detailed the idea of the biosphere in his work "The biosphere" (1926), and described the fundamental principles of the biogeochemical cycles. He thus redefined the biosphere as the sum of all ecosystems.

First ecological damages were reported in the 18th century, as the multiplication of colonies caused deforestation. Since the 19th century, with the industrial revolution, more and more pressing concerns have grown about the impact of human activity on the environment. The term ecologist has been in use since the end of the 19th century.

The ecosystem concept and Arthur Tansley

Over the 19th century, botanical geography and zoogeography combined to form the basis of biogeography.
Biogeography, which deals with habitats of species, is often confused with ecology; it seeks to explain the reasons for the presence of certain species in a given location.

It was in 1935 that Arthur Tansley, the british ecologist, coined the term ecosystem, the interactive system established between the biocenose (the group of living creatures), and their biotope, the environment in which they live. Ecology thus became the science of ecosystems.

James Lovelock and the Gaia hypothesis

Main article: Gaia theory

Since the Second World War, the subdiscipline of human ecology, dealing with the place and role of humanity on the Earth, has dealt with the new challenges of the dangers and problems associated with nuclear energy, industrialization, the consequences of pollution, the exhaustion of natural resources by industrialized nations and the exponential population growth of many third world countries.

The vision of "Gaďa" is a sign of the times, proposed by James Lovelock, in his work The Earth is Alive, which compares the Earth to a single living macro-organism. Though controversial, the Gaia Hypothesis increased concern about ecology to the general public. Some of the public adopted the view that their Earth-mother, Gaia, was "becoming sick from humans and their activities". From a scientific point of view, this hypothesis is consistent with the new notion of the ecology as being a global vision of the biosphere and of biodiversity.

Human ecology

Human ecology began in the 1920s, through the study of changes in vegetation succession in the city of Chicago. It became a distinct field of study in the 1970s. This marked the first recognition that humans, who had colonized all of the Earth's continents, were a major ecological factor. Humans greatly modify the environment through the development of the habitat (in particular urban planning), by intensive fishing, and through agricultural and industrial activities. Human ecology began as a field of study with the participation of anthropologists, architects, biologists, demographers, ecologists, ergonomicists, ethnologists, urban planners and doctors.

Human ecology is the branch of ecology which studies humankind, the organized activity of this species, and its environment. In addition, environmentalism -- a philosophy resulting from ecology and applicable to human societies -- developed. Finally, political ecology also appeared in the 1920s. It consists of applying ecological science to the policy and the management of the city (see also List of planned cities).

Global ecology

In 1971, UNESCO launched a research program called Man and Biosphere, with the objective of increasing knowledge about the mutual relationship between humans and nature. A few years later it defined the concept of Biosphere Reserve.

In 1972, the United Nations held the first international conference on the human environment in Stockholm, prepared by René Dubos and other experts. This conference was the origin of the phrase "Think Globally, Act Locally". The next major events in ecology were the development of the concept of biosphere and the appearance of terms "biological diversity" -- or now more commonly biodiversity -- in the 1980s. These terms were developed during the Earth Summit in Rio de Janeiro in 1992, where the concept of the biosphere was recognized by the major international organizations, and risks associated with reductions in biodiversity were publicly acknowledged.

Then, in 1997, the dangers the biosphere was facing were recognized from an international point of view at the Kyoto conference. In particular, this conference highlighted the increasing dangers of the greenhouse effect -- related to the increasing concentration of greenhouse gases in the atmosphere, leading to global changes in climate. In Kyoto, most of the world's nations recognized the importance of looking at ecology from a global point of view, on a worldwide scale, and to take into account the impact of humans on the Earth's environment.

Fundamental principles of ecology

Disciplines of ecology

For many ecology belongs to the science of biology which relates to living beings. In biology, various levels of organization exist, such as molecular biology (which includes nucleic acids), cellular biology, the biology of organisms (on the individual level and organism level), the study of populations, the study of communities, and the study of ecosystems and the biosphere.

The field of ecology would include the last categories of these. Indeed, it is a holistic science which studies not only each element's relationship with other elements, but also the evolution of these reports according to modifications' which undergo it in the animal and plants populations. These types are described from the smallest to the largest level. Some of these other disciplines are:

What is called ecology is thus actually a unit of science, known as ecological sciences. Disciplines within the ecological sciences include geology, biochemistry, geography, pedology, and physical science.

Biosphere and biodiversity

Main articles: Biosphere, Biodiversity

For modern ecologists, ecology can be studied at several levels: population level (individuals of the same species), biocenose level (or community of species), ecosystem level, and biosphere level.

The Earth, from an ecological point of view, consists of several compartments, the hydrosphere (or sphere of water), the lithosphere (or sphere of soils and rocks) and the atmosphere (or sphere of the air).
The biosphere, sometimes described as fourth envelope, is the part of the planet on which the life developed. It is a very thin surface layer, which goes down to 11000 meters of depth to rising up to 15000 meters of altitude, although the majority of life live in the zone located between -100 meters and +100 meters.

Life first developed in the hydrosphere, at low depth, in the photic zone. Multicellular organisms then appeared and colonized benthic zones. Terrestrial life developed later, after the ozone layer protecting living beings from UV rays formed. Diversification of terrestrial species is thought to be increased by the continents drifting apart, or alternately, colliding.
Biosphere and biodiversity are inseparable characteristics of the Earth. Biosphere is defined as being the sphere of life, whereas biodiversity is its diversity. The sphere is the container, whereas diversity is the contents. This diversity is expressed at the same time at the ecological level (ecosystem), population level (intraspecific diversity) and species level (specific diversity).

The biosphere contains great quantities of elements such as carbon, nitrogen and oxygen. Other elements, such as phosphorus, calcium, potassium are also essential to life. At the ecosystem and biosphere level, there is a permanent recycling of all these elements, which alternate between the mineral state and the organic state.

While there is a slight input of geothermal energy, the bulk of the functioning of the ecosystem is primarily based on the input of solar energy. Plants convert light into into chemical energy by the process of photosynthesis, which creates glucose (a simple sugar) and releases free oxygen. Glucose thus becomes the secondary energy source which drives the ecosystem. Some of this glucose is used directly by other organisms for energy. Other sugar molecules can be converted to other molecules such as amino acids. Plants use some of this sugar, concentrated in nectar to entice pollinators to aid them in reproduction. (Honeybees concentrate the sugar still further as honey, which can be said to be "stored summer sunshine").

Cellular respiration is the process by which organisms (like mammals) breakdown the glucose back to its constituents, water and carbon dioxide, gaining back the stored energy the sun originally gave to the plants. The proportion of photosynthetic activity of plants to the respiration of other organisms determines the specific composition of the Earth's atmosphere, particularly its oxygen level. Global air currents mix the atmosphere and maintain nearly the same balance in areas of intense biological activity and areas of slight biological activity.

Water is also exchanged between the hydrosphere, the lithosphere, the atmosphere and the biosphere in regular cycles. The oceans are large tanks, which store water, ensure a thermal and climatic stability, as well as the transport of the chemical elements thanks to large oceanic currents.

For better understanding of how the biosphere works, and the dysfunctions related to human activity, American scientists carried out, under greenhouses, a small-scale model of the biosphere, called Biosphere II.

The concept of an ecosystem

Main article: Ecosystem

The first principle of ecology is that each living organism has an ongoing and continual relationship with every other element that makes up its environment. An ecosystem can be defined as any situation where there is interaction between organisms and their environment.

The ecosystem is composed of two entities, the entirety of life (called the biocenose) and the medium that life exists in (the biotope). Within the ecosystem, species are connected and dependent upon one another in the food chain, and exchange energy and matter between themselves and with their environment.

The concept of an ecosystem can apply to units of variable size, a pond, a field, or a piece of deadwood. A unit of smaller size is called a microecosystem. For example, an ecosystem can be a stone and all the life under it. A mesoecosystem could be a forest, and a macroecosystem a whole ecoregion, with its watershed.

The main questions when studying an ecosystems are:

Ecosystems are often classified by reference to the biotopes concerned. The following ecosystems may be defined : Another classification can be done by reference to its communities (for example a human ecosystem).

Dynamics and stability

Main articles: ecological factor, geobiochemical cycle, Homeostasis

The biotope is a region environmentally uniform, characterized by a whole set of geological, geographical and climatological parameters, which are called abiotic ecological factorss:

Biocenose, or community, is a group of populations of plants, animals, micro-organisms. Each population is the result of procreations between individuals of same species and cohabiting in a given place and at a given time. When a population is constitued by an insufficient number of individuals, the species is threatened with extinction, either by underpopulation, or by because of consanguinity. A population can be reduced for several reasons, for example, disappearance of its habitat (destruction of a forest) or by excessive predation (such as the hunting of a given species).

Biocenose is characterized by biotic ecological factors of two types: intraspecific and interspecific relations.

Intraspecific relations are those which are established between individuals of the same species, forming a population. They are relations of cooperation or competition, with division of the territory, and sometimes organization in hierarchical societies.

Interspecific relations, i.e. those existing between different species, are numerous, and usually described according to their beneficial, detrimental or neutral effect (for example, symbiosis (relation ++) or competition (relation --)). The most significant relation is the relation of predation (to eat or to be eaten), which leads to the essential concepts in ecology of food chains (for example, the grass is consumed by the herbivore, itself consumed by a carnivore, itself consumed by a carnivore of larger size). Ecological niche is the area shared by two species when they live at the same place with the same type of diet.

The existing interactions between the various living beings go along with a permanent mixing of mineral and organic substances, absorbed by organisms for their growth, their maintenance and their reproduction, to be finally rejected as waste. These permanent recyclings of the elements (in particular carbon, oxygen and nitrogen) as well as the water are called biogeochemical cycles. They guarantee a durable stability of the biosphere (at least when human influence and extreme weather phenomena are left aside). This self-regulation, supported by negative feedback controls, ensures the perenniality of the ecosystems. It is showed by the very stable concentrations of most elements of each compartment. This is referred to as homeostasis. The ecosystem also tends to evolve to a state of ideal balance, reached after a succession of events, the climax (for example a pond can become a peat bog).

Spatial relationships and subdivisions of land

Main articles: Biome, ecozone

Ecosystems are not isolated from each other, but are interrelated. For example, water may circulate between ecosystems by the means of a river or ocean current. Water itself, as a liquid medium, even defines ecosystems. Some species, such as salmon or freshwater eels move between marine systems and fresh-water systems. These relationships between the ecosystems lead to the concept of a biome.

A biome is a homogeneous ecological formation that exists over a vast region, such as tundra or steppes. The biosphere comprises all of the Earth's biomes -- the entirety of places where life is possible -- from the highest mountains to the depths of the oceans.

Biomes correspond rather well to subdivisions distributed along the latitudes, from the equator towards the poles, with differences based on to the physical environment (for example, oceans or mountain ranges) and to the climate. Their variation is generally related to the distribution of species according to their ability to tolerate temperature and/or dryness. For example, one may find photosynthetic algae only in the photic part of the ocean (where light penetrates), while conifers are mostly found in mountains.

Though this is a simplification of more complicated scheme, latitude and altitude approximate a good representation of the distribution of biodiversity within the biosphere. Very generally, the richness of biodiversity (as well for animal than plant species) is decreasing most rapidly near the equator (as in Brazil) and less rapidly as one approaches the poles.

The biosphere may also be divided into ecozone, which are very well defined today and primarily follow the continental borders. The ecozones are themselves divided into ecoregions, though there is not agreement on their limits.

Ecosystem productivity

In an ecosystem, the connections between species are generally related to food and their role in the food chain. There are three categories of organisms:

These relations form sequences, in which each individual consumes the preceding one and is consumed by the one following, in what are called food chains or food network. In a food network, there will be fewer organisms at each level as one follows the links of the network up the chain.

These concepts lead to the idea of biomass (the total living matter in a given place), of primary productivity (the increase in the mass of plants during a given time) and of secondary productivity (the living matter produced by consumers and the decomposers in a given time).

These two last ideas are key, since they make it possible to evaluate the load capacity -- the number of organisms which can be supported by a given ecosystem. In any food network, the energy contained in the level of the producers is not completely transferred to the consumers. Thus, from an energy point of view, it is more efficient for humans to be primary consumers (to get nourishment from grains and vegetables) than as secondary consumers (from herbivores such as beef and veal), and more still than as a tertiary consumer (from eating carnivores).

The productivity of ecosystems is sometimes estimated by comparing three types of land-based ecosystems and the total of aquatic ecosystems:

Humanity's actions over the last few centuries have seriously reduced the amount of the Earth covered by forests (deforestation), and have increased agro-ecosystems (agriculture). In recent decades, an increase in the areas occupied by extreme ecosystems has occurred (desertification).

Ecological crisis

Generally, an ecological crisis is what occurs when the environment of life of a species or a population evolves in an unfavourable way to its survival.

It may be that the environment quality degrades compared to the species needs, after a change of abiotic ecological factor (for example, an increase of temperature, less significant rainfalls).
It may be that the environment becomes unfavourable for the survival of a species (or a population) due to an increase pressure of predation (for example overfishing).
Lastly, it may be that the situation becomes unfavourable to the quality of life of the species (or the population) due to raise in the number of individuals (overpopulation).

Ecological crises may be more or less brutal (occurring between a few months to a few million years). They can also be of natural or anthropic origin. They may relate to one unique species or on the contrary, to a high number of species (see the article on Extinction event).

Lastly, an ecological crisis may be local (as an oil spill) or global (a rise in the sea level related to global warming).

According to its degree of endemism, a local crisis will have more or less significant consequences, from the death of many indididuals to the total extinction of a species. Whatever its origin, disappearance of one or several species often will involve a rupture in the food chain, further impacting the survival of other species.

In the case of a global crisis, the consequences can be much more significant; some extinction events showed the disappearance of more than 90% of existing species at that time. However, it should be noted that the disappearance of certain species, such as the dinosaurs, by freeing an ecological niche, allowed the development and the diversification of the mammals. An ecological crisis thus paradoxically favored biodiversity.

Sometimes, an ecological crisis can be a specific and reversible phenomenon at the ecosystem scale. But more generally, the crises impact will last. Indeed, it rather is a connected series of events, that occur till a final point. From this stage, no return to the previous stable state is possible, and a new stable state will be set up gradually (see homeorhesy).

Lastly, if an ecological crisis can cause extinction, it can also more simply reduce the quality of life of the remaining individuals. Thus, even if the diversity of the human population is sometimes considered threatened (see in particular indigenous people), few people envision human disappearance at short span. However, epidemic diseases, famines, impact on health of reduction of air quality, food crises, reduction of living space, accumulation of toxic or non degradable wastes, threats on keystone species (great apes, panda, whales) are also factors influencing the well-being of people.

During the past decades, it was observed an increasing responsibility of human in some ecological crises. Due to his technological acquisitions and to a strong increase in population, man is the only species whose activity has a major influence on his environment of life.

Some usually quoted examples as ecological crises are

Political ecology

Main articles: Political ecology, environmentalism

In recent years, as a result of the deteriorating environment, there has been an emergence of ecological movements such as the Green Party. It is important to understand the fundamental differences between politics, ideology, and the science of ecology.

Related fields

Ecology includes many sub-disciplines including:

Agroecology - Applied ecology - Animal ecology - Aquatic ecology - Behavioral ecology - Chemical ecology - Community ecology - Conservation ecology - Ecoevolution - Ecological anthropology - Ecosystems ecology - Ecotoxicology - Human ecology - Industrial ecology - Landscape ecology - Macroecology - Microbial ecology - Molecular ecology - Paleoecology - Plant ecology - Population ecology - Restoration ecology - Social ecology - Soil ecology - Systems ecology - Theoretical ecology - Tropical ecology - Urban ecology

Ecology also plays important roles in the inter-disciplinary fields of ecological economics, ecological health, ecological design and ecological engineering.

See also