ecology principles

Ecology

see also Ecology History of Ecology

Fundamental principles of ecology

Biosphere and 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

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:

how could be carried out the colonization of an arid area?

What are the ecosystems dynamics and changes

how does an ecosystem interact at local, regional and global scale

is the current state stable?

what is the value of an ecosystem? How does the interaction of ecological systems provide benefit to humans, especially in the provision of healthy water?

Ecosystems are often classified by reference to the biotopes concerned. The following ecosystems may be defined :

as continental ecosystems (or terrestrial), such as forest ecosystems, meadow ecosystems (meadows, steppes, savannas), or agro-ecosystems (agricultural systems).

as ecosystems of inland waters, such as lentic ecosystems (lakes, ponds) or lotic ecosystems (rivers)

as oceanic ecosystems (seas, oceans).

Another classification can be done by reference to its communities (for example a human ecosystem).

Dynamics and stability

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

water, is at the same time, an essential element to life, as well as a milieu

air, which provides oxygen and carbon dioxide to living species, and allows the dissemination of pollen and spores

soil, at the same time source of nutriment and support of development

temperature, which should not exceed certain extremes, even if tolerance to heat is significant for some species

light, allowing photosynthesis.

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 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)
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Spatial relationships and subdivisions of land

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:

producers -- plants which are capable of photosynthesis

consumers -- animals, which can be primary consumers (herbivorous), or secondary or tertiary consumers (carnivorous).

decomposers -- bacteria, mushrooms which degrade organic matter of all categories, and restore minerals to the environment.

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:

the forests (1/3 of the Earth's land area) contain dense biomasses and are very productive. The total production of the world's forests corresponds to half of the primary production.

savannas, meadows, and marshes (1/3 of the Earth's land area) contain less dense biomasses, but are productive. These ecosystems represent the major part of what humans depend on for food.

extreme ecosystems in the areas with more extreme climates -- deserts and semi-deserts, tundra, alpine meadows, and steppes -- (1/3 of the Earth's surface) have very sparse biomasses and low productivity

finally, the marine and fresh water ecosystems (3/4 of Earth's surface) contain very sparse biomasses (apart from the coastal zones).

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 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

Permian-Triassic extinction event 250 million of years ago

Cretaceous-Tertiary extinction event 65 million years ago

global warming related to the greenhouse effect. Warming could involve flooding of the Asian deltas (see also ecorefugees), multiplication of extreme weather phenomena and changes in the nature and quantity of the food resources (see Global warming and agriculture)

Ozone layer hole issue

Deforestation and desertification, with disappearance of many species.

The nuclear meltdown at Chernobyl in 1986 caused the death of many people and animals from cancer, and caused mutations in a large number of animals and people. The area around the plant is now abandoned because of the large amount of radiation generated by the meltdown.