Ecology

Ecology (from Greek oîkos, "house"; -λογία, -logos, study of) is the interdisciplinary scientific study of the distribution and abundance of organisms and their interactions with their environment. The environment of an organism includes all external factors, including abiotic ones such as climate and geology, and biotic factors, including members of the same species (conspecifics) and other species that share a habitat. If the general life science of biology is viewed as a hierarchy of levels of organization, from molecular processes, to cells, tissues and organs, and finally to the individual, the population and the ecosystem, then the study of the latter three levels belongs within the purview of ecology.

Examples of objects of ecological study include: Population processes, including reproductive behavior, mortality, bioenergetics and migrations, interspecific interations such as predation, competition, parasitism and mutualism, plant and animal community structures and their function and resilience, and biogeochemical cycling. Because of its vast scope, ecological science is often closely related to other disciplines. Thus, molecular ecology addresses ecological questions using tools from genetics, paleoecology uses tools from archeology, and theoretical ecologists use often highly complex mathematical models to explore how ecosystems and their elements function.

Aside from pure scientific inquiry, ecology is also a highly applied science. Much of natural resource management, such as forestry, fisheries, wildlife management and habitat conservation is directly related to ecological sciences and many problems in agriculture, urban development and public health are informed by ecological considerations.

The term "ecology" has also been appropriated for philosophical ideologies like social ecology and deep ecology and is sometimes used as a synonym for the natural environment or environmentalism. Likewise "ecological" is often taken in the sense of environmentally friendly.

Historical roots of ecology

Ecology as a scientific discipline is relatively young, reaching prominence mostly in the second half of the 20th century. However, systematic ecological studies can trace roots to ancient times, with Aristotle and Theophrastus, for example, making early observations on animal migrations and plant biogeography respectively. Several notable 19th century scientists such as Alexander Humboldt (1769 – 1859), Charles Darwin (1809 – 1882), Alfred Russel Wallace (1823 – 1913) and Karl Möbius (1825 – 1908) made many important contributions, from laying down the foundation of biogeography to identifying an interacting groups of organisms as a functionally connected community (biocoenosis).

The term "ecology" itself (German: Oekologie) was first coined by the German biologist Ernst Haeckel in 1866, who defined it as "the comprehensive science of the relationship of the organism to the environment." The first significant textbook on the subject (together with the first university course) was written by the Danish botanist, Eugenius Warming. For this early work, Warming is sometimes identified as the founder of ecology.

Scope

Ecology is usually considered as a branch of biology, the general science that studies living organisms. It is associated with the highest levels of biological organization, including the individual organism, the population, the ecological community, the ecosystem and the biosphere as a whole. When referring to the study of a single species, a distinction is often made between its "ecology" and its "biology". For example, "polar bear biology" might include the study of the polar bear's physiology, morphology, pathology and ontogeny, whereas "polar bear ecology" would include a study of its prey species, its population and metapopulation status, distribution, dependence on environmental conditions, etc.

Because of its focus on the interrelations between organisms and their environment, ecology is a multidisciplinary science that draws on many other branches, including geology and geography, meteorology, soil science, genetics, chemistry, physics, mathematics and statistics. Due to its breadth of scope, ecology is considered by some to be a holistic science, one that over-arches older disciplines such as biology which in this view become sub-disciplines contributing to ecological knowledge. It has been argued that the mechanistic models which have driven the development of most other sciences are inappropriate for unraveling the complex interactions in most ecosystems, and that progress in ecology is better served by a central paradigm driven by information theory and complexity theory.

Ecology is also a highly applied science, especially with respect to issues of natural resource management. Efforts related to wildlife conservation, habitat management, mitigation of ecological impacts of environmental pollution, ecosystem restoration, species reintroductions, fisheries, forestry and game management are often the direct domain of applied ecology. Urban development, agricultural and public health issues are also often informed by ecological perspectives and analysis.

Fundamental principles

Levels of organization
 
Some of the biodiversity of a coral reef
Ecology can be studied at a wide range of levels, from large to small scale. These levels of ecological organization, as well as an example of a question ecologists would ask at each level, include:
Biosphere: " What role does concentration of atmospheric carbon dioxide play in the regulation of global temperature?"
Region: "How has geological history influenced regional diversity within certain groups of organisms?"

Landscape: "How do vegetated corridors affect the rate of movement by mammals among isolated fragments?"
Ecosystem: "How does fire affect nutrient availability in grassland ecosystems?"
Community: "How does disturbance influence the number of mammal species in African grasslands?"
Interactions: "What evolutionary benefit do zebras gain by allowing birds to remove parasites?"
Population: "What factors control zebra populations?"
Individual Organism: "How do zebras regulate internal water balance?" 
These levels range from broadest to most specific.

Biosphere

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

The outer layer of the planet Earth can be divided into 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 (or sphere of life), sometimes described as "the fourth envelope," is all living matter on the planet or that portion of the planet occupied by life. It reaches well into the other three spheres, although there are no permanent inhabitants of the atmosphere. Relative to the volume of the Earth, the biosphere is only the very thin surface layer that extends from 11,000 meters below sea level to 15,000 meters above.

It is thought that life first developed in the hydrosphere, at shallow depths, in the photic zone. (Recently, though, a competing theory has emerged, that life originated around hydrothermal vents in the deeper ocean. See Origin of life.) Multicellular organisms then appeared and colonized benthic zones. Photosynthetic organisms gradually produced the chemically unstable oxygen-rich atmosphere that characterizes our planet. Terrestrial life developed later, protected from UV rays by the ozone layer. Diversification of terrestrial species is thought to be increased by the continents drifting apart, or alternately, colliding. Biodiversity is expressed at the ecological level (ecosystem), population level (intraspecific diversity), species level (specific diversity), and genetic level.

The biosphere contains great quantities of elements such as carbon, nitrogen, hydrogen, and oxygen. Other elements, such as phosphorus, calcium, and potassium, are also essential to life, yet are present in smaller amounts. At the ecosystem and biosphere levels, there is a continual recycling of all these elements, which alternate between the mineral and organic states.

Although there is a slight input of geothermal energy, the bulk of the functioning of the ecosystem is based on the input of solar energy. Plants and photosynthetic microorganisms 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 that drives the ecosystem. Some of this glucose is used directly by other organisms for energy. Other sugar molecules can be converted to 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) break the glucose back down into its constituents, water and carbon dioxide, thus regaining the stored energy the sun originally gave to the plants. The proportion of photosynthetic activity of plants and other photosynthesizers 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 of elements in areas of intense biological activity and areas of slight biological activity.

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

For a better understanding of how the biosphere works, and various dysfunctions related to human activity, American scientists attempted to simulate the biosphere in a small-scale model, called Biosphere II.

Ecosystem

A central principle of ecology is that each living organism has an ongoing and continual relationship with every other element that makes up its environment. The sum total of interacting living organisms (the biocoenosis) and their non-living environment (the biotope) in an area is termed an ecosystem. Studies of ecosystems usually focus on the movement of energy and matter through the system.

Almost all ecosystems run on energy captured from the sun by primary producers via photosynthesis. This energy then flows through the food chains to primary consumers (herbivores who eat and digest the plants), and on to secondary and tertiary consumers (either carnivores or omnivores). Energy is lost to living organisms when it is used by the organisms to do work, or is lost as waste heat.

Matter is incorporated into living organisms by the primary producers. Photosynthetic plants fix carbon from carbon dioxide and nitrogen from atmospheric nitrogen or nitrates present in the soil to produce amino acids. Much of the carbon and nitrogen contained in ecosystems is created by such plants, and is then consumed by secondary and tertiary consumers and incorporated into themselves. Nutrients are usually returned to the ecosystem via decomposition. The entire movement of chemicals in an ecosystem is termed a biogeochemical cycle, and includes the carbon and nitrogen cycle.

Ecosystems of any size can be studied; for example, a rock and the plant life growing on it might be considered an ecosystem. This rock might be within a plain, with many such rocks, small grass, and grazing animals -- also an ecosystem. This plain might be in the tundra, which is also an ecosystem (although once they are of this size, they are generally termed ecozones or biomes). In fact, the entire terrestrial surface of the earth, all the matter which composes it, the air that is directly above it, and all the living organisms living within it can be considered as one, large ecosystem.

Ecosystems can be roughly divided into terrestrial ecosystems (including forest ecosystems, steppes, savannas, and so on), freshwater ecosystems (lakes, ponds and rivers), and marine ecosystems, depending on the dominant biotope.

Dynamics and stability

Ecological factors that affect dynamic change in a population or species in a given ecology or environment are usually divided into two groups: abiotic and biotic.

Abiotic factors are geological, geographical, hydrological, and climatological parameters. A biotope is an environmentally uniform region characterized by a particular set of abiotic ecological factors. Specific abiotic factors include:
Water, which is at the same time an essential element to life and a milieu
Air, which provides oxygen, nitrogen, and carbon dioxide to living species and allows the dissemination of pollen and spores
Soil, at the same time a source of nutriment and physical support 
Soil pH, salinity, nitrogen and phosphorus content, ability to retain water, and density are all influential
Temperature, which should not exceed certain extremes, even if tolerance to heat is significant for some species
Light, which provides energy to the ecosystem through photosynthesis
Natural disasters can also be considered abiotic

Biocenose, or community, is a group of populations of plants, animals, microorganisms. Each population is the result of procreations between individuals of the same species and cohabitation in a given place and for a given time. When a population consists of an insufficient number of individuals, that population is threatened with extinction; the extinction of a species can approach when all biocenoses composed of individuals of the species are in decline. In small populations, consanguinity (inbreeding) can result in reduced genetic diversity, which can further weaken the biocenose.

Biotic ecological factors also influence biocenose viability; these factors are considered as either intraspecific or interspecific relations.
Intraspecific relations are those that 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.
 
An antlion lies in wait under its pit trap, built in dry dust under a building, awaiting unwary insects that fall in. Many pest insects are partly or wholly controlled by other insect predators.
Interspecific relations—interactions between different species—are numerous, and usually described according to their beneficial, detrimental, or neutral effect (for example, mutualism (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). A high predator to prey ratio can have a negative influence on both the predator and prey biocenoses in that low availability of food and high death rate prior to sexual maturity can decrease (or prevent the increase of) populations of each, respectively. Selective hunting of species by humans that leads to population decline is one example of a high predator to prey ratio in action. Other interspecific relations include parasitism, infectious disease, and competition for limited resources, which can occur when two species share the same ecological niche.

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 recycling 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 unchecked human influence and extreme weather or geological 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, called the climax, which is reached after a succession of events (for example a pond can become a peat bog).

Spatial relationships and subdivisions of land

Ecosystems are not isolated from each other, but are interrelated. For example, water may circulate between ecosystems by 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 large 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 the physical environment (for example, oceans or mountain ranges) and the climate. Their variation is generally related to the distribution of species according to their ability to tolerate temperature, dryness, or both. For example, one may find photosynthetic algae only in the photic part of the ocean (where light penetrates), whereas conifers are mostly found in mountains.

Though this is a simplification of a 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 as for plant species) is decreasing most rapidly near the equator and less rapidly as one approach the poles.

The biosphere may also be divided into ecozones, 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.