Bringing together significant work on all aspects of the subject, Behavioral Ecology is broad-based and covers both empirical and theoretical approaches. Studies on the whole range of behaving organisms, including plants, invertebrates, vertebrates, and humans, are included.
Alcohol and Alcoholism publishes papers on biomedical, psychological and sociological aspects of alcoholism and alcohol research, provided that they make a new and significant contribution to knowledge in the field. Papers include new results obtained experimentally, descriptions of new experimental (including clinical) methods of importance to the field of alcohol research and treatment, or new interpretations of existing results. Theoretical contributions are considered equally with papers dealing with experimental work provided that such theoretical contribution are not of a largely speculative or philosophical nature. Alcohol and Alcoholism is the official journal of the Medical Council on Alcohol.
study of the relationships of organisms to their physical environment and to one another. The study of an individual organism or a single species is termed autecology; the study of groups of organisms is called synecology. The Ecosystem Within the biosphere—the total expanse of water, land, and atmosphere able to sustain life—the basic ecological unit is the ecosystem. An ecosystem may be as small as a tidal pool or a rotting log or as large as an ocean or a continent-spanning forest. Each ecosystem consists of a community of plants and animals in an environment that supplies them with raw materials for life, i.e., chemical elements and water. The ecosystem is delimited by the climate, altitude, water and soil characteristics, and other physical conditions of the environment. The Food Web and Other Vital Cycles The energy necessary for all life processes reaches the earth in the form of sunlight. By photosynthesis green plants convert the light energy into chemical energy, and carbon dioxide and water are transformed into sugar and stored in the plant. Herbivorous animals acquire some of the stored energy by eating the plants; those animals in turn serve as food for, and so pass the energy to, predatory animals. Such sequences, called food chains, overlap at many points, forming so-called food webs. For example, insects are food for reptiles, which are food for hawks. But hawks also feed directly on insects and on other birds that feed on insects, while some reptiles prey on birds. Since a severe loss of the original energy occurs with each transfer from species to species, the ecologist views the food (energy) structure as a pyramid: Each level supports a smaller number and mass of organisms. Thus in a year's time it would take millions of plants weighing tons to feed the several steer weighing a few tons that could support one or two people. The ecological conclusion is that if human beings would eat more plants and fewer animals, food resources would stretch much further. Once the energy for life is spent, it cannot be replenished except by the further exposure of green plants to sunlight. The chemical materials extracted from the environment and elaborated into living tissue by plants and animals are continually recycled within the ecosystem by such processes as photosynthesis, respiration, nitrogen fixation, and nitrification. These natural processes of withdrawing and returning materials are variously called the carbon cycle, the oxygen cycle, and the nitrogen cycle. Water is also cycled. Evaporation from lakes and oceans forms clouds; the clouds release rain that is taken up by the soil, absorbed by plants, and passed on to feeding animals—which also drink directly from pools and lakes that catch the rain. The water in plant and animal wastes and dead tissue then evaporates and can be recycled. Interference with these vital cycles by disturbance of the environment—for example, by pollution of the air and water—may disrupt the workings of the entire ecosystem. The cycles are facilitated when an ecosystem has a sufficient biological diversity of species to fill its so-called ecological niches, the different functional sites in the environment where organisms can act as producers of energy, consumers of energy, or decomposers of wastes. Such diversity tends to make a community stable and self-perpetuating. Climax Communities A climax community is one that has reached the stable stage. When extensive and well defined, the climax community is called a biome. Examples are tundra, grassland, desert, and the deciduous, coniferous, and tropical rain forests. Stability is attained through a process known as succession, whereby relatively simple communities are replaced by those more complex. Thus, on a lakefront, grass may invade a build-up of sand. Humus formed by the grass then gives root to oaks and pines and lesser vegetation, which displaces the grass and forms a further altered humus. That soil eventually nourishes maple and beech trees, which gradually crowd out the pines and oaks and form a climax community. In addition to trees, each successive community harbors many other life forms, with the greatest diversity populating the climax community. Similar ecological zonings occur among marine flora and fauna, dependent on such environmental factors as bottom composition, availability of light, and degree of salinity. In other respects, the capture by aquatic plants of solar energy and inorganic materials, as well as their transfer through food chains and cycling by means of microorganisms, parallels those processes on land. The early 20th-century belief that the climax community could endure indefinitely is now rejected because climatic stability cannot be assumed over long periods of time. In addition nonclimatic factors, such as soil limitation, can influence the rate of development. It is clear that stable climax communities in most areas can coexist with human pressures on the ecosystem, such as deforestation, grazing, and urbanization. Polyclimax theories stress that plant development does not follow predictable outlines and that the evolution of ecosystems is subject to many variables. Bibliography See E. P. Odum, Fundamentals of Ecology (3d ed. 1971); R. L. Smith, ed., The Ecology of Man: An Ecosystem Approach (1971); P. A. Colinvaux, Introduction to Ecology (1973); R. M. Darnell, Ecology and Man (1973); T. C. Emmel, An Introduction to Ecology and Population Biology (1973); D. B. Sutton and N. P. Harman, Ecology: Selected Concepts (1973); K. E. F. Watt, Principles of Environmental Science (1973); D. Worster, Nature's Economy (1977); R. Brewer, The Science of Ecology (1988). ____________________The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved. |
Rather than looking at our galaxy and the universe as extensions of Earth, this new ecology.com feature lets us look at our planet a little bit differently, more like Earth as a part of the grand design and evolution of the universe. It's the world we live in, the place we all call home. As you start reading through the information and numerous links, you will have at least a better understanding of what and where "here" is and how we are all connected.
Earth at a Glance: Fast Facts
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| Earth is located in the outer edge of our galaxy, called the Milky Way, about 28,000 light years from the galactic center. It is part of a Solar System that includes eight other known planets, and the only known planet with the ability to sustain life as we know it. The Milky Way is one of billions of spiral galaxies in the universe. (Image: NASA, Galaxy M83, similar size and shape to the Milky Way) |
Age: About 4.5 Billion Years Old
Location: In the Solar System, on the outer edge of the Milky Way, about 28,000 light years from the galactic center (Source: European Space Agency). It takes the solar system 225 million years to make one full trip around the Milky Way.
Closest Major Galaxy: Andromeda, about 2.3 million light years away.
Age of the Milky Way: 16 billion years.
Home System: Solar System (One Sun)
Earth's Sun: A medium sized, yellow star. Scientists call it a G2 star. It is the largest object in the solar system and contains 99.8 percent of the solar system's mass. It is located in the center of the solar system.
Distance from the Sun (average): About 93.1 million miles (also one Astronomical Unit or AU). It is the third planet from the Sun.
Farthest Distance from the Sun: 94.5 million miles.
Closest Distance to the Sun: 91.4 million miles.
Speed through Space (around the sun): 18.4 miles per second or about 67,000 miles per hour.
Solar Orbit: It takes Earth 365.2422 days to orbit the sun. This is the basis for the year.
Rotational Speed: About 1,070 miles per hour at the equator.
Rotational Time: It takes the Earth 23 hours, 56 minutes and four (4) seconds to make one complete 360° rotation.
Rotational Tilt: 23.5° on its axis, a straight line through the planet from the North Pole to the South Pole. The tilt is in relation to Earth's near circular orbit around the sun.
Gravitational Pull: One Earth Unit. This measurement is relative to other objects in the universe. Earth's gravity is the force that pulls objects toward the center of the Earth and is measured by the Earth's mass. Gravity is what gives objects their weight. Without gravity, the Earth's spin would fling everything on the planet out into space! See Earth's Weight (Mass), below.
Atmospheric Pressure: 14.7 pounds per square inch at sea level. This is the measure of force exerted on objects by the weight of the air. Many times gravity and atmospheric pressure are considered one and the same thing. Actually, they are not! In fact, it is the Earth's gravitational pull on the atmosphere that gives weight to the atmosphere. The atmospheric pressure decreases above sea level, and it increases below sea level.
Earth's Weight (Mass): 5.972 sextillion (1,000 trillion) metric tons. That's 5,972,000,000,000,000,000,000 tons! Actually, scientists prefer to refer to this measurement as the Earth's mass instead of weight since weight is the result of Earth's gravitational pull on another object. And the Earth cannot pull on itself! As the Earth orbits the Sun, it is weightless. If the Earth were placed on the Sun, it would weigh more than if it were placed on Jupiter, the largest planet in the solar system but much smaller than the sun. Yet, Earth (or any other object for that matter) would have the same mass regardless of where it is located.
Earth's Size, Distances & Surface Features
Equatorial Diameter: 7,928 miles
Polar Diameter: 7,901 miles
Circumference: 24,907 miles
Surface Area: 197 million square miles
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| Mt. Everest |
Highest Point: 29,028 feet above sea level, Mount Everest, formed 60 million years ago, located on the border of Tibet and Nepal in the Central Himalayas in southeast Asia.
Lowest Point (on Land): 1,320 feet below sea level, Dead Sea, located on the border between Israel and the West Bank to the west and Jordan to the east. It is so salty -- the saltiest on Earth -- that it is unable to support any type of life.
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| Artist's depiction of the Chicxulub impact crater. (NASA) |
Deepest Point on Earth: 35,802 feet, Mariana Trench in the Pacific Ocean. The water pressure there is over eight tons per square inch.
Largest Impact Crater: Chicxulub crater, about 125 miles wide and 7.5 miles deep, buried under several hundred meters of sediment. It is located off of Chicxulub, Mexico, on the northern coast of the Yucatan peninsula. It was created about 65 million years ago by an asteroid that collided with the Earth and subsequently caused the extinction of 70% of the world's species, including the dinosaurs. Only two other such craters in Canada (Sudbury) and South Africa (Vredefort) may be larger.
Earth's Layers: Earth is made up of four layers which comprise its surface, its interior and its atmosphere. They are:
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~ Outer Core - So hot that it is molten, with about 10% sulfur, about 1,410 miles thick. It is 98% responsible for producing Earth's magnetic field as it spins around the solid inner core.
~ Inner Core - Under such pressure that it remains solid, about 756 miles thick.
Earth's Vital Systems & Elements
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| Atlantic Ocean |
~ Oceans: 317 million cubic miles
~ Ice Caps, glaciers: Seven (7) million cubic miles
~ Ground water: Two (2) million cubic miles.
~ Fresh Water Lakes: 30,000 cubic miles
~ Inland Seas: 25,000 cubic miles
~ Soil Moisture: 16,000 cubic miles
~ Atmosphere: 3,100 cubic miles
~ Rivers: 300 cubic miles (Almost all of humans' drinking water comes from rivers.)
~ Oxygen: 46.6 %
~ Silicon: 27.7%
~ Aluminum: 8.1%
~ Iron: 5%
~ Calcium: 3.6%
~ Sodium: 2.8%
~ Potassium: 2.6%
~ Magnesium: 2.1%
~ Other: 1.5%
(Source: US Geologic Survey)
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| Vostok, Antarctica |
~ Classified to date: 2.1 million species.
~ Most Unclassified Species: Invertebrates (animals without a backbone), such as insects, worms, sponges, crustaceans, mollusks, spiders, etc.
~ Total Endangered or Threatened: 12,259 species of plants and animals are known to be endangered or threatened and face a high risk of extinction in the near future, according to The World Conservation Union. IUCN documented its findings in its Red List of Threatened Species published in November 2003.
.The study was assembled by over 8,000 species experts throughout the world. Among their findings: A total of 762 extinctions have been recorded since the 1500s, with a number of others surviving only in artificial habitats or other special settings such as zoos. Biologists agree, however, that there are certainly hundreds if not thousands of species that have become extinct without having been discovered.
For a summary listing of these threatened species by groups, click here …The US Fish & Wildlife Service (USFW) lists 1,849 species of plants and animals as endangered or threatened under its Endangered Species Act which was signed into law by President Richard Nixon in 1973. Before a plant or animal species can receive protection under the Endangered Species Act, it must first be placed on the Federal list of endangered and threatened wildlife and plants. See Species Information: Threatened and Endangered Animals and Plants
In addition to its 1,286 total listings, the U.S. recognizes 558 non-U.S. species, including 268 mammals, 181 birds, 79 reptiles, 11 fishes, nine amphibians, two clams, four insects, three plants and one snail. The U.S. also has 555 specific approved recovery programs, some of which cover more than one species.
To review the current US Fish & Wildlife Service's Summary of Listed Species by groups, click here…
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| Amazon Rainforest |
Extinct Species: 99.9 percent of all Earth's species living at one time or another have become extinct. Without extinctions, we -- humans -- would not be here. There is no known estimate of how many species of living things have become extinct today or since animals first began to appear on Earth in the Cambrian Period, some 600 million years ago. According to the World Resources Institute, 100 species become extinct every day due to tropical deforestation. It is the rain forests that contain more than half of all living things, and there are many species that we never discovered which have succumbed to extinction.
~ Most Massive Known Extinction: There are five periods of mass extinctions recognized by scientists. The most massive extinction occurred 250 million years ago at the end of the Permian Period when between 75% and 97% of Earth's species are estimated to have died out. Perhaps the most recognized mass extinction occurred 65 million years in the late Cretaceous Period when an asteroid slammed into Earth's surface, resulting in the ultimate loss of 70% of the world species, including the dinosaurs. The current period, called the Holocene, may see the greatest mass destruction of species ever due to anthropogenic or human causes.
~ Extinction Cycles: Scientists have determined that mass extinctions are a part of Earth evolution of life forms and are indicative of changes taking place within the planet. They estimate that mass extinctions take place about every 26-28 million years. At the same times, existing and new species evolve and replace or add to the current surviving species.
~ Last Known Extinction: Species are becoming extinct every day.
Leading Environmental Issues: Global warming, deforestation (rain forests) and habitat loss, water pollution and potable fresh water, air pollution, energy and energy resources, human population growth, species protection and diversity (plant and animal), land use, food production.
Solar System CompanionsMoons: One (1), circles the Earth in about 27 days and eight hours. No known life. All other planets, except for Mercury and Venus, have moons. At least 91 other moons are attached to other plants in the Solar System.
Sister Planets (in order of distance from the Sun):
Inner Planets: The inner zone of planets which are comprised mostly of rock and metal.
Outer Planets: The gas giants, formed from icy particles and leftover gas.
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- Saturn: The second largest planet, with its famous rings that stretch over 600 million miles from edge to edge.
- Uranus: The third largest planet, the only one of which rotates on its side.
- Neptune: Found in 1846, it is the fourth largest of the planets and the outer most of the gas giants.
- Pluto. Not a gas giant. The smallest of the planets -- smaller than Earth's moon -- it is composed of nitrogen ice and rock, and is sometimes closer to the sun than Neptune.
Ecology (from Greek: οίκος, oikos, "household"; and λόγος, logos, "knowledge") is the scientific study of the distribution and abundance of life and the interactions between organisms and their environment. The environment of an organism includes physical properties, which can be described as the sum of local abiotic factors such as insolation (sunlight), climate, and geology, and biotic factors, which are other organisms that share its habitat.
The word "ecology" is often used more loosely in such terms as social ecology and deep ecology and in common parlance as a synonym for the natural environment or environmentalism. Likewise "ecologic" or "ecological" is often taken in the sense of environmentally friendly.
Ecology is usually considered a branch of biology, the general science that studies living organisms. Organisms can be studied at many different levels, from proteins and nucleic acids (in biochemistry and molecular biology), to cells (in cellular biology), to individuals (in botany, zoology, and other similar disciplines), and finally at the level of populations, communities, and ecosystems, to the biosphere as a whole; these latter strata are the primary subjects of ecological inquiry. Ecology is a multi-disciplinary science. Because of its focus on the higher levels of the organization of life on earth and on the interrelations between organisms and their environment, ecology draws heavily on many other branches of science, especially geology and geography, meteorology, pedology, genetics, chemistry, and physics. Thus, 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. In support of viewing ecology as a subject in its own right as opposed to a sub-discipline of biology, Robert Ulanowicz stated that "The emerging picture of ecosystem behavior does not resemble the worldview imparted by an extrapolation of conceptual trends established in other sciences."[3]
Agriculture, fisheries, forestry, medicine and urban development are among human activities that would fall within Krebs' (1972: 4) explanation of his definition of ecology: where organisms are found, how many occur there, and why.
Ecological knowledge such as the quantification of biodiversity and population dynamics have provided a scientific basis for expressing the aims of environmentalism and evaluating its goals and policies. Additionally, a holistic view of nature is stressed in both ecology and environmentalism.
Consider the ways an ecologist might approach studying the life of honeybees:
Ecology is a broad discipline comprising many sub-disciplines. A common, broad classification, moving from lowest to highest complexity, where complexity is defined as the number of entities and processes in the system under study, is:
Ecology can also be sub-divided according to the species of interest into fields such as animal ecology, plant ecology, insect ecology, and so on. Another frequent method of subdivision is by biome studied, e.g., Arctic ecology (or polar ecology), tropical ecology, desert ecology, etc. The primary technique used for investigation is often used to subdivide the discipline into groups such as chemical ecology, genetic ecology, field ecology, statistical ecology, theoretical ecology, and so forth. These fields are not mutually exclusive.
Ecology can be studied at a wide range of levels, from a large to small scale. These levels of ecological organization, as well as an example of a question ecologists would ask at each level, include:
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 which 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, 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. Biodiversity is expressed at the ecological level (ecosystem), population level (intraspecific diversity), species level (specific diversity), and genetic level. Recently technology has allowed the discovery of the deep ocean vent communities. This remarkable ecological system is not dependent on sunlight but bacteria, utilising the chemistry of the hot volcanic vents, are at the base of its food chain.
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.
While 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 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) 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, which store water, ensure thermal and climatic stability, as well as 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 simulated the biosphere in a small-scale model, called Biosphere II.
 | To meet Wikipedia's quality standards, this article or section may require cleanup because it is in a list format that may be better presented using prose. You can help by converting this section to prose, if appropriate. Editing help is available. (August 2007) |
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 of two entities, the entirety of life, the biocoenosis, and the medium that life exists in, the biotope. Within the ecosystem, species are connected by food chains or food webs. Energy from the sun, captured by primary producers via photosynthesis, flows upward through the chain to primary consumers (herbivores), and then to secondary and tertiary consumers (carnivores and omnivores), before ultimately being lost to the system as waste heat. In the process, matter is incorporated into living organisms, which return their nutrients to the system via decomposition, forming biogeochemical cycles such as the carbon and nitrogen cycles.
The concept of an ecosystem can apply to units of variable size, such as a pond, a field, or a piece of dead wood. An ecosystem within another ecosystem is called a micro ecosystem. For example, an ecosystem can be a stone and all the life under it. A meso ecosystem could be a forest, and a macro ecosystem a whole eco region, with its drainage basin.
The main questions when studying an ecosystem 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, such as in the case of an human ecosystem.
Ecological factors which 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:
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 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 that can further weaken the biocenose.
Biotic ecological factors also influence biocenose viability; these factors are considered as either intraspecific and interspecific relations.
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 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, reached after a succession of events, the climax (for example a pond can become a peat bog).
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 large region 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 and less rapidly as one approaches 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.
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. And the higher one goes up the chain, the more energy and resources is lost and consumed. Thus, from an energy—and environmental—point of view, it is more efficient for humans to be primary consumers (to subsist from vegetables, grains, legumes, fruit, etc.) than as secondary consumers (from eating herbivores, omnivores, or their products, such as milk, chickens, cattle, sheep, etc.) and still more so than as a tertiary consumer (from consuming carnivores, omnivores, or their products, such as fur, pigs, snakes, alligators, etc.). An ecosystem(s) is unstable when the load capacity is overrun and is especially unstable when a population doesn't have an ecological niche and overconsumers.
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).
Generally, an ecological crisis occurs with the loss of adaptive capacity when the resilience of an environment or of a species or a population evolves in a way unfavourable to coping with perturbations that interfere with that ecosystem, landscape or species survival. It may be that the environment quality degrades compared to the species needs, after a change in an 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 increased 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 a rise in the number of individuals (overpopulation).
Ecological crises vary in length and severity, occurring within a few months or taking as long as a few million years. They can also be of natural or anthropic origin. They may relate to one unique species or to many species, as in an Extinction event. Lastly, an ecological crisis may be local (as an oil spill) or global (a rise in the sea level due 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.
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 common examples of ecological crises are:
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