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Ecology is the branch of science that studies the distribution and abundance of living organisms, and the interactions between organisms and their environment. The environment of an organism includes both its physical habitat, which can be described as the sum of local abiotic factors like climate and geology, as well as the other organisms which share its habitat. The term was coined in 1866 by the German biologist Ernst Haeckel from the Greek oikos meaning "household" and logos meaning "science:" the "study of the household of nature." Ecological systems are studied at several different levels from individuals and populations to ecosystems and biosphere level. Ecology is a multi-disciplinary science, drawing on many other branches of science. Applied ecology is the practice of employing ecological principles and understanding to solve real world problems. Examples include measuring the economic worth of ecosystems, calculating fishing quotas, measuring environmental impact from construction or logging, building a case for the conservation of a species, and determining the most effective way to protect a species. In a broader sense, ecology can also mean:
For example, human ecology looks at humans and their interactions with the natural environment. Political ecology takes on both alternative meanings, and may use ecology's methods in a new context by looking at interactions of societies and states rather than species or populations, but may also mean politics related to environmental issues. Ecology, as a scientific discipline, does not itself dictate what is right or wrong. However maintaining biodiversity within ecosystems and related ecological goals (such as preventing species extinctions) have become scientific ways to express the goals of environmentalism and has given scientific methodology, measure, and terminology to environmental issues, making the two heavily linked. A holistic view is also stressed in both ecology and environmentalism. ScopeEcology is usually considered a branch of biology, the general science that studies living beings. These can be studied at several levels, from proteins and nucleic acids (in biochemistry and molecular biology), cells (in cellular biology), organisms (in botany, zoology, and other similar disciplines), and finally at the level of populations, communities, and ecosystems — which are the subjects of ecology. Conversely, some people consider ecology to be the parent discipline and biology to be the sub-disciple, since ecology includes not only the interactions between organisms, but also their interactions with their surroundings. Because of its focus on the broadest level of life and on the interrelations between living beings and their environment, ecology draws heavily on other branches of science, such as geology and geography, meteorology, pedology, chemistry, and physics. For this reason, ecology is often said to be a holistic science.
Disciplines of ecologyEcology is a broad science which can be subdivided into major and minor sub-discipline. The major sub-disciplines include (in a nested series from the smallest to the largest):
Ecology can also be sub-divided on the basis of target groups: or from the perspective of the studied biomes: (temperate zone ecology could also exist as a distinct sub-field, but ecology as a whole has an overwhelmingly temperate bias, so the sub-field is redundant). Other specialized branches of ecology include:
Ecology also plays important roles in many inter-disciplinary fields:
Finally, ecology has also inspired (and lent its name to) other non-biological disciplines such as Fundamental principles of ecologyBiosphere and biodiversityMain articles: Biosphere, Biodiversity, Unified neutral theory of 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, and 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 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. 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 ecosystemMain 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, such as 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 stabilityMain articles: ecological factor, biogeochemistry, Homeostasis, Population dynamics The biotope is a region environmentally uniform, characterized by a whole set of geological, geographical and climatological parameters, which are called abiotic ecological factors:
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 consists of 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 co-operation 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 shown 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 landEcosystems 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 productivityIn 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 crisisGenerally, 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). 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 individuals 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, this increasing responsibility of humanity in some ecological crises has been clearly observed. Due to the increases in technology and a rapidly increasing population, humans have more influence on their own environment than any other ecosystem engineer. Some usually quoted examples as ecological crises are
History of ecologyOne 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 Carolus Linnaeus, whose work is usually considered the origin of modern ecology. The botanical geography and Alexander von HumboldtThroughout the 18th and the beginning of the 19th century, the great maritime powers such as Britain, Spain, and Portugal 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 century, 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 WallaceTowards 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 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 VernadskyBy 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: Arthur TansleyOver the 19th century, botanical geography and zoogeography combined to form the basis of biogeography. This science, which deals with habitats of species, 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. Tansley's concept of the ecosystem was adopted by the energetic and influential biology educator Eugene Odum. Along with his brother, Howard Odum, Eugene P. Odum wrote a textbook which (starting in 1953) educated more than one generation of biologists and ecologists in North America. Human ecologyHuman 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 exploitation activities such as logging and fishing, and as side effects of agriculture, mining, and industry. Besides ecology and biology, this discipline involved many other natural and social sciences, such as anthropology and ethnology, economics, demography, architecture and urban planning, medicine and psychology, and many more. The development of human ecology led to the increasing role of ecological science in the design and management of cities. In recent years human ecology has been a topic that has interested organizational researchers. Hannan and Freeman (Population Ecology of Organizations (1977), American Journal of Sociology) argue that organizations do not only adapt to an environment. Instead it is also the environment that selects or rejects populations of organizations. In any given environment (in equilibrium) there will only be one form of organization (isomorphism). Organizational ecology has been a prominent theory in accounting for diversities of organizations and their changing composition over time. James Lovelock and the Gaia hypothesisThe Gaia theory, proposed by James Lovelock, in his work Gaia: A New Look at Life on Earth, advanced the view that the Earth should be regarded as a single living macro-organism. In particular, it argued that the ensemble of living organisms has jointly evolved an ability to control the global environment — by influencing major physical parameters as the composition of the atmosphere, the evaporation rate, the chemistry of soils and oceans — so as to maintain conditions favorable to life. This vision was largely a sign of the times, in particular the growing perception after the Second World War that human activities such as nuclear energy, industrialization, pollution, and overexploitation of natural resources, fueled by exponential population growth, were threatening to create catastrophes on a planetary scale. Thus Lovelock's Gaia hypothesis, while controversial among scientists, was embraced by many environmental movements as an inspiring view: their Earth-mother, Gaia, was "becoming sick from humans and their activities". Conservation and environmental movementsSince the 19th century, ecology has been relevant to social and philosophical movements related to protection of the natural environment, such as conservationism and environmentalism. Today ecology is a major political topic, and a source of ideology for major political organizations such as the Green Party and Greenpeace. Ecology and global policyEcology became a central part of the World's politics as early as 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 Rene 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 conference leading to the Kyoto Protocol. 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. See also
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