Osmosis is the net movement of water across a partially permeable membrane from a region of high solvent potential to an area of low solvent potential, up a solute concentration gradient. It is a physical process in which a solvent moves, without input of energy, across a semipermeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations.[1] Osmosis releases energy, and can be made to do work, as when a growing tree-root splits a stone.
Computer simulation of the process of osmosis
Net movement of solvent is from the less-concentrated (hypotonic), to the more-concentrated (hypertonic) solution, which tends to reduce the difference in concentrations. This effect can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic. The osmotic pressure is defined to be the pressure required to maintain an equilibrium, with no net movement of solvent. Osmotic pressure is a colligative property, meaning that the property depends on the molar concentration of the solute but not on its identity. Osmosis is the result of diffusion across a semi-permeable membrane.
Osmosis is important in biological systems as many biological membranes are semipermeable. In general, these membranes are impermeable to organic solutes with large molecules, such as polysaccharides, while permeable to water and small, uncharged solutes. Permeability may depend on solubility properties, charge, or chemistry as well as solute size. Water molecules travel through the plasma cell membrane, tonoplast (vacuole) or protoplast in two ways. Either by diffusing across the phospholipid bilayer directly, or via aquaporins (small transmembrane proteins similar to those in faciliated diffusion and in creating ion channels). Osmosis provides the primary means by which water is transported into and out of cells. The turgor pressure of a cell is largely maintained by osmosis, across the cell membrane, between the cell interior and its relatively hypotonic environment.
Contents[hide]
1 Basic explanation of Osmosis
2 Examples of osmosis
3 Osmotic pressure
4 Reverse osmosis
5 Forward osmosis
6 See also
7 Reference
8 External links
//
[edit] Basic explanation of Osmosis
Consider a permeable membrane, such as visking tubing, with apertures small enough to allow water (solvent) molecules, but not larger solute molecules, to pass through. When this membrane is immersed in liquid it is constantly hit by molecules of the liquid, in motion due to their thermal kinetic energy. In this respect solute and solvent molecules are indistinguishable. At a molecular scale, every time a molecule hits the membrane it has a defined likelihood of passing through. Here, there is a difference: for water molecules this probability is non-zero; for solute molecules it is zero.
Suppose the membrane is in a volume of pure water. In this case, since the circumstances on both sides of the membrane are equivalent, water molecules pass in each direction at the same rate; there is no net flow of water through the membrane.
If there is a solution on one side, and pure water on the other, the membrane is still hit by molecules from both sides at the same rate. However, some of the molecules hitting the membrane from the solution side will be solute molecules, and these will not pass through the membrane. So water molecules pass through the membrane from this side at a slower rate. This will result in a net flow of water to the side with the solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the solution side becomes such that the movement in each direction is equal: dynamic equilibrium. This could either be due to the water potential on both sides of the membrane being the same, or due to osmosis being inhibited by factors such as pressure potential or Osmotic pressure.
Osmosis can also be explained via the notion of entropy, from statistical mechanics. As above, suppose a permeable membrane separates equal amounts of pure solvent and a solution. Since a solution possesses more entropy than pure solvent, the second law of thermodynamics states that solvent molecules will flow into the solution until the entropy of the combined system is maximized. Notice that, as this happens, the solvent loses entropy while the solution gains entropy. Equilibrium, hence maximum entropy, is achieved when the entropy gradient becomes zero.
[edit] Examples of osmosis
Effect of different solutions on blood cells
Plant cell under different enviroments
Osmotic pressure is the main cause of support in many plants. The osmotic entry of water raises the turgor pressure exerted against the cell wall, until it equals the osmotic pressure, creating a steady state.
When a plant cell is placed in a hypertonic solution, the water in the cells moves to an area higher in solute concentration, and the cell shrinks and so becomes flaccid [pron. flassid]. (This means the cell has become plasmolysed - the cell membrane has completely left the cell wall due to lack of water pressure on it; the opposite of turgid.)
Also, osmosis is responsible for the ability of plant roots to suck up water from the soil. Since there are many fine roots, they have a large surface area, water enters the roots by osmosis, and generates the pressure required for the water to travel up the plant.
Osmosis can also be seen very effectively when potato slices are added to a high concentration of salt solution. The water from inside the potato moves to the salt solution, causing the potato to shrink and to lose its 'turgor pressure'. The more concentrated the salt solution, the bigger the difference in size and weight of the potato slice.
In unusual environments, osmosis can be very harmful to organisms. For example, freshwater and saltwater aquarium fish placed in water of a different salinity (than they are adapted to) will die quickly, and in the case of saltwater fish rather dramatically. Additionally, note the use of table salt to kill leeches and slugs.
Suppose we place an animal or a plant cell in a solution of sugar or salt in water.
If the medium is hypotonic — a dilute solution, with a higher water concentration than the cell — the cell will gain water through osmosis.
If the medium is isotonic — a solution with exactly the same water concentration as the cell — there will be no net movement of water across the cell membrane.
If the medium is hypertonic — a concentrated solution, with a lower water concentration than the cell — the cell will lose water by osmosis.[2]
[edit] Osmotic pressure
As mentioned before, osmosis may be opposed by increasing the pressure in the region of high solute concentration with respect to that in the low solute concentration region. The force per unit area, or pressure, required to prevent the passage of water through a selectively-permeable membrane and into a solution of greater concentration is equivalent to the osmotic pressure of the solution, or turgor. Osmotic pressure is a colligative property, meaning that the property depends on the concentration of the solute but not on its identity.
Increasing the pressure increases the chemical potential of the system in proportion to the molar volume (δμ = δPV). Therefore, osmosis stops when the increase in potential due to pressure equals the potential decrease from Equation 1, i.e.:
Where δP is the osmotic pressure and V is the molar volume of the solvent.
For the case of very low solute concentrations, -ln(1-x2) ≈ x2 and Equation 2 can be rearranged into the following expression for osmotic pressure:
[edit] Reverse osmosis
Main article: Reverse osmosis
The osmosis process can be driven in reverse, with solvent moving from a region of high solute concentration to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. Recent advances in pressure exchange and the ongoing development of low pressure membranes have significantly reduced the costs of water purified by reverse osmosis. The reverse osmosis technique is commonly applied in desalination, water purification, water treatment, and food processing.
[edit] Forward osmosis
Main article: Forward osmosis
Osmosis may be used directly to achieve separation of water from a "feed" solution containing unwanted solutes. A "draw" solution of higher osmotic pressure than the feed solution is used to induce a net flow of water through a semi-permeable membrane, such that the feed solution becomes concentrated as the draw solution becomes dilute. The diluted draw solution may then be used directly (as with an ingestible solute like glucose), or sent to a secondary separation process for the removal of the draw solute. This secondary separation can be more efficient than a reverse osmosis process would be alone, depending on the draw solute used and the feedwater treated. Forward osmosis is an area of ongoing research, focusing on applications in desalination, water purification, water treatment, food processing, etc.
[edit] See also
Wikibooks has more about this subject:
School science/Osmosis demonstration
Diffusion
Plasmolysis
Water potential
Tuesday, June 12, 2007
Asexual Reproduction
Asexual reproduction is a form of reproduction which does not involve meiosis, ploidy reduction, or fertilization. Asexual reproduction only takes one parent. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea, bacteria, and protists. Many plants and fungi reproduce mostly asexually as well. While all prokaryotes reproduce asexually (without the formation and fusion of gametes), mechanism for lateral gene transfer such as conjugation, transformation and transduction are sometimes likened to sexual reproduction.[1]
Contents[hide]
1 Asexual reproduction vs. sexual reproduction
2 Types of asexual reproduction
2.1 Binary fission
2.2 Budding
2.3 Spore formation
2.4 Vegetative reproduction
2.5 Fragmentation
2.6 Parthenogenesis
2.7 Apomixis and nucellar embryony
3 Alternation between sexual and asexual reproduction
4 Examples in animals
5 Notes and references
6 Further reading
7 See also
8 External links
//
[edit] Asexual reproduction vs. sexual reproduction
Asexual reproduction is relatively rare among multicellular organisms, for reasons that are not completely understood. Current hypotheses suggest that, while asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, over a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.
Because asexual reproduction does not require the formation of gametes (often in separate individuals) and bringing them together for fertilization, it occurs much faster than sexual reproduction and requires less energy. Asexual lineages can increase their numbers rapidly because all members can reproduce viable offspring. In sexual populations with two genders, some of the individuals are male and cannot themselves produce offspring. This means that an asexual lineage will have roughly double the rate of population growth under ideal conditions when compared with a sexual population half composed of males. This is known as the two-fold cost of sex. Other advantages include the ability to reproduce without a partner in situations where the population density is low (such as for some desert lizards), reducing the chance of finding a mate, or during colonisation of isolated habitats such as oceanic islands, where a single (female) member of the species is enough to start a population.
Another consequence of asexual reproduction, which may have both benefits and costs, is that offspring are typically genetically similar to their parent, with as broad a range as that individual receives from one parent. The lack of genetic recombination results in fewer genetic alternatives than with sexual reproduction. Many forms of asexual reproduction, for example budding or fragmentation, produce an exact replica of the parent. This genetic similarity may be beneficial if the genotype is well-suited to a stable environment, but disadvantageous if the environment is changing. For example, if a new predator or pathogen appears and a genotype is particularly defenseless against it, an asexual lineage is more likely to be completely wiped out by it. In contrast, a lineage that reproduces sexually has a higher probability of having more members survive due to the genetic recombination that produces a novel genotype in each individual. Similar arguments apply to changes in the physical environment. From an evolutionary standpoint, one could thus argue that asexual reproduction is inferior because it stifles the potential for change. However, there is also a significantly reduced chance of mutation or other complications that can result from the mixing of genes. A 2004 article in the journal Nature reported that the modern arbuscular mycorrhizas fungi, which reproduces asexually, is identical to fossil records dating back to the Ordovician period, 460 million years ago.[2]
[edit] Types of asexual reproduction
[edit] Binary fission
Main article: Binary fission
Many, but not all, single-celled organisms, such as archaea, bacteria, and protists, reproduce asexually through binary fission. An exception to the rule are unicellular fungi such as fission yeast, unicellular algae such as Chlamydomonas, and ciliates and some other protists, which reproduce both sexually and asexually. Some single-celled organisms rely on one or more host organisms in order to reproduce, but most literally divide into two organisms.
[edit] Budding
Main article: Budding
Some cells split via budding (for example baker's yeast), resulting in a smaller 'daughter' cell and larger 'mother' cell. Budding is also known on a multicellular level. An animal example is hydra, which reproduces by budding. The buds grow into fully mature individuals which eventually break away from the parent animal.
[edit] Spore formation
Main article: Sporogenesis
Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, who undergo gametic meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization event. This haploid individual gives rise to gametes through mitosis. Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy. However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle.
Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red alga Polysiphonia, and involves sporogenesis without meiosis. Thus the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants and most fungi and algae, are produced by meiosis.
[edit] Vegetative reproduction
Main article: Vegetative reproduction
Vegetative reproduction is a type of asexual reproduction found in plants where new independent individuals are formed without the production of seeds or spores. Examples for vegetative reproduction include the formation of plantlets on specialized leaves (for example in kalanchoe), the growth of new plants out of rhizomes or stolons (for example in strawberry), or the formation of new bulbs (for example in tulips). The resulting plants form a clonal colony.
[edit] Fragmentation
Main article: Fragmentation (biology)
Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms such as animals (some annelid worms and starfish), fungi, and plants. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in liverworts. Most lichens, which are a symbiotic union of a fungus and photosynthetic algae or bacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia, dust-like particles consisting of fungal hyphae wrapped around photobiont cells.
[edit] Parthenogenesis
Main article: Parthenogenesis
Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles, amphibians, fish, and, very rarely, birds). In plants, parthenogenesis is called apomixis, and involves the asexual production of seeds (which grow into clones of the mother plant).
[edit] Apomixis and nucellar embryony
Main article: Apomixis
Main article: Nucellar embryony
Apomixis is the plant equivalent of parthenogenesis and leads to the formation of seeds in flowering plants without fertilization. An example for an apomictic plant would be the European dandelion. Apomixis mainly occurs in two forms: In agamogenesis, the embryo arises from the unfertilized egg via a modified meiosis. In agamospermy, the embryo is formed from the diploid nucellus tissue surrounding the embryo sac and the seeds produced are genetically identical to the parent plant. An example is nucellar embryony in citrus seeds. Male apomixis can occur in rare cases, such as the Saharan Cypress where the seeds are derived entirely from pollen.
[edit] Alternation between sexual and asexual reproduction
Some species alternate between the sexual and asexual strategies, an ability known as heterogamy, depending on conditions. For example, the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Many protists and fungi alternate between sexual and asexual reproduction. For example, the slime mold Dictyostelium undergoes binary fission as single-celled amboebae under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and switch to sexual reproduction leading to the formation of spores. The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores. Many algae similarly switch between sexual and asexual reproduction. Asexual reproduction is far less complicated than sexual reproduction. In sexual reproduction one must find a mate.
[edit] Examples in animals
A number of invertebrates and some less advanced vertebrates are known to alternate between sexual and asexual reproduction, or be exclusively asexual. Alternation is observed in a few types of insects, such as aphids (which will, under favourable conditions, produce eggs that have not gone through meiosis, essentially cloning themselves) and the cape bee Apis mellifera capensis (which can reproduce asexually through a process called thelytoky). A few species of amphibians and reptiles have the same ability (see parthenogenesis for concrete examples). A very unusual case among more advanced vertebrates is the female turkeys' ability to produce fertile eggs in the absence of a male. The individual produced is often sickly, and nearly always male. This behaviour can interfere with the incubation of eggs in turkey farming.[3]
Contents[hide]
1 Asexual reproduction vs. sexual reproduction
2 Types of asexual reproduction
2.1 Binary fission
2.2 Budding
2.3 Spore formation
2.4 Vegetative reproduction
2.5 Fragmentation
2.6 Parthenogenesis
2.7 Apomixis and nucellar embryony
3 Alternation between sexual and asexual reproduction
4 Examples in animals
5 Notes and references
6 Further reading
7 See also
8 External links
//
[edit] Asexual reproduction vs. sexual reproduction
Asexual reproduction is relatively rare among multicellular organisms, for reasons that are not completely understood. Current hypotheses suggest that, while asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, over a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.
Because asexual reproduction does not require the formation of gametes (often in separate individuals) and bringing them together for fertilization, it occurs much faster than sexual reproduction and requires less energy. Asexual lineages can increase their numbers rapidly because all members can reproduce viable offspring. In sexual populations with two genders, some of the individuals are male and cannot themselves produce offspring. This means that an asexual lineage will have roughly double the rate of population growth under ideal conditions when compared with a sexual population half composed of males. This is known as the two-fold cost of sex. Other advantages include the ability to reproduce without a partner in situations where the population density is low (such as for some desert lizards), reducing the chance of finding a mate, or during colonisation of isolated habitats such as oceanic islands, where a single (female) member of the species is enough to start a population.
Another consequence of asexual reproduction, which may have both benefits and costs, is that offspring are typically genetically similar to their parent, with as broad a range as that individual receives from one parent. The lack of genetic recombination results in fewer genetic alternatives than with sexual reproduction. Many forms of asexual reproduction, for example budding or fragmentation, produce an exact replica of the parent. This genetic similarity may be beneficial if the genotype is well-suited to a stable environment, but disadvantageous if the environment is changing. For example, if a new predator or pathogen appears and a genotype is particularly defenseless against it, an asexual lineage is more likely to be completely wiped out by it. In contrast, a lineage that reproduces sexually has a higher probability of having more members survive due to the genetic recombination that produces a novel genotype in each individual. Similar arguments apply to changes in the physical environment. From an evolutionary standpoint, one could thus argue that asexual reproduction is inferior because it stifles the potential for change. However, there is also a significantly reduced chance of mutation or other complications that can result from the mixing of genes. A 2004 article in the journal Nature reported that the modern arbuscular mycorrhizas fungi, which reproduces asexually, is identical to fossil records dating back to the Ordovician period, 460 million years ago.[2]
[edit] Types of asexual reproduction
[edit] Binary fission
Main article: Binary fission
Many, but not all, single-celled organisms, such as archaea, bacteria, and protists, reproduce asexually through binary fission. An exception to the rule are unicellular fungi such as fission yeast, unicellular algae such as Chlamydomonas, and ciliates and some other protists, which reproduce both sexually and asexually. Some single-celled organisms rely on one or more host organisms in order to reproduce, but most literally divide into two organisms.
[edit] Budding
Main article: Budding
Some cells split via budding (for example baker's yeast), resulting in a smaller 'daughter' cell and larger 'mother' cell. Budding is also known on a multicellular level. An animal example is hydra, which reproduces by budding. The buds grow into fully mature individuals which eventually break away from the parent animal.
[edit] Spore formation
Main article: Sporogenesis
Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, who undergo gametic meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization event. This haploid individual gives rise to gametes through mitosis. Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy. However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle.
Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red alga Polysiphonia, and involves sporogenesis without meiosis. Thus the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants and most fungi and algae, are produced by meiosis.
[edit] Vegetative reproduction
Main article: Vegetative reproduction
Vegetative reproduction is a type of asexual reproduction found in plants where new independent individuals are formed without the production of seeds or spores. Examples for vegetative reproduction include the formation of plantlets on specialized leaves (for example in kalanchoe), the growth of new plants out of rhizomes or stolons (for example in strawberry), or the formation of new bulbs (for example in tulips). The resulting plants form a clonal colony.
[edit] Fragmentation
Main article: Fragmentation (biology)
Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms such as animals (some annelid worms and starfish), fungi, and plants. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in liverworts. Most lichens, which are a symbiotic union of a fungus and photosynthetic algae or bacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia, dust-like particles consisting of fungal hyphae wrapped around photobiont cells.
[edit] Parthenogenesis
Main article: Parthenogenesis
Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles, amphibians, fish, and, very rarely, birds). In plants, parthenogenesis is called apomixis, and involves the asexual production of seeds (which grow into clones of the mother plant).
[edit] Apomixis and nucellar embryony
Main article: Apomixis
Main article: Nucellar embryony
Apomixis is the plant equivalent of parthenogenesis and leads to the formation of seeds in flowering plants without fertilization. An example for an apomictic plant would be the European dandelion. Apomixis mainly occurs in two forms: In agamogenesis, the embryo arises from the unfertilized egg via a modified meiosis. In agamospermy, the embryo is formed from the diploid nucellus tissue surrounding the embryo sac and the seeds produced are genetically identical to the parent plant. An example is nucellar embryony in citrus seeds. Male apomixis can occur in rare cases, such as the Saharan Cypress where the seeds are derived entirely from pollen.
[edit] Alternation between sexual and asexual reproduction
Some species alternate between the sexual and asexual strategies, an ability known as heterogamy, depending on conditions. For example, the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Many protists and fungi alternate between sexual and asexual reproduction. For example, the slime mold Dictyostelium undergoes binary fission as single-celled amboebae under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and switch to sexual reproduction leading to the formation of spores. The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores. Many algae similarly switch between sexual and asexual reproduction. Asexual reproduction is far less complicated than sexual reproduction. In sexual reproduction one must find a mate.
[edit] Examples in animals
A number of invertebrates and some less advanced vertebrates are known to alternate between sexual and asexual reproduction, or be exclusively asexual. Alternation is observed in a few types of insects, such as aphids (which will, under favourable conditions, produce eggs that have not gone through meiosis, essentially cloning themselves) and the cape bee Apis mellifera capensis (which can reproduce asexually through a process called thelytoky). A few species of amphibians and reptiles have the same ability (see parthenogenesis for concrete examples). A very unusual case among more advanced vertebrates is the female turkeys' ability to produce fertile eggs in the absence of a male. The individual produced is often sickly, and nearly always male. This behaviour can interfere with the incubation of eggs in turkey farming.[3]
Sexual Reproduction
Sexual reproduction is a union that results in increasing genetic diversity of the offspring. It is characterized by two processes: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve genetic recombination.
The evolution of sex is a major puzzle. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years ago with DNA forming 3.5 - 4.6 billion years.[1]Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.
A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the clade itself—the ability for a population to radiate more rapidly due to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the 'ratcheting' of evolutionary speed as one clade competes with another for a limited resource.
In the first stage of sexual reproduction, 'meiosis', the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During 'fertilization', haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored.
Contents[hide]
1 Reproduction in plants
2 Reproduction in archosaurs (reptiles and birds)
3 Reproduction in mammals
3.1 The mammalian male
3.2 The mammalian female
3.3 Gestation
3.4 Birth
3.5 Monotremes
3.6 Marsupials
4 See also
5 References
//
[edit] Reproduction in plants
Main article: Plant sexuality
In flowering plants, the anther produces male gametophytes called pollen grains, which attach to the stigma on top of a carpel, in which the female gametophytes (inside ovules) are located. After the pollen tube grows through the carpel's style, the sperm cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate.
[edit] Reproduction in archosaurs (reptiles and birds)
Male and female birds and reptiles both have cloacae, an opening through which eggs, sperm, and wastes pass. Intercourse is performed by pressing the lips of the cloacae together, during which time the male transfers his sperm to the female. The female lays amniotic eggs in which the young gestate. Nevertheless, a few species, including most waterfowl and ostriches, have a phallus shaped organ analogous to the mammals' penis.
[edit] Reproduction in mammals
In placental mammals, offspring are born as juveniles: complete animals with the sex organs present although non-functional. After several months or years, the sex organs develop further to maturity and the animal becomes sexually mature. Most female mammals are only fertile during certain periods and during those times, they are said to be "in heat". At this point, the animal is ready to mate. Individual male and female mammals meet and carry out copulation. For most mammals, males and females exchange sexual partners throughout their adult lives.
[edit] The mammalian male
For more details on this topic, see Male reproductive system (human).
The male reproductive system contains two main divisions: the penis, and the testes, the latter of which is where sperm are produced. In humans, both of these organs are outside the abdominal cavity, but they can be primarily housed within the abdomen in other animals (for instance, in dogs, the penis is internal except when mating). Having the testes outside the abdomen best facilitates temperature regulation of the sperm, which require specific temperatures to survive. Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of sexual maturity until death. Prior to ejaculation the produced sperm are stored in the seminal vesicle, a small gland that is located just behind the bladder.Sperm are motile and swim via chemotaxis, using its mitochondria to propell towards the ovum.
[edit] The mammalian female
For more details on this topic, see Female reproductive system (human).
The female reproductive system likewise contains two main divisions: the vagina and uterus, which act as the receptacle for the sperm, and the ovaries, which produce the female's ova. All of these parts are always internal. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the Fallopian tubes. At certain intervals, the ovaries release an ovum (the singular of ova), which passes through the fallopian tube into the uterus.
If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg, fertilizing it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina.
The ova are larger than sperm and are generally all created by birth. They are for the most part stationary, aside from their transit to the uterus, and contain nutrients for the later zygote and embryo. Over a regular interval, a process of oogenesis matures one ovum to be sent down the Fallopian tube attached to its ovary in anticipation of fertilization. If not fertilized, this egg is flushed out of the system through menstruation in humans and great apes and reabsorbed in all other mammals in the estrus cycle.
[edit] Gestation
Main articles: Pregnancy (mammals) and Pregnancy
Gestation, called pregnancy in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the female, who is required to ingest significantly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the gestation period, varies greatly from species to species; it is 38 weeks in humans, 56–60 in giraffes and 16 days in hamsters.
[edit] Birth
Main article: Childbirth
Once the fetus is sufficiently developed, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an infant in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta is passed as well. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young. The end of the umbilical cord attached to the young’s abdomen eventually falls off on its own.
[edit] Monotremes
Monotremes, only five species of which exist, all from Australia and New Guinea, lay eggs. They have one opening for excretion and reproduction called the cloaca. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like birds. After less than two weeks the young hatches and crawls into its mother’s pouch, much like marsupials, where it nurses for several weeks as it grows.
[edit] Marsupials
Marsupials reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals. After birth, marsupial joeys crawl into their mother’s pouch and attach to a teat, where they receive nourishment and finish developing into self-sufficient animals.
The evolution of sex is a major puzzle. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years ago with DNA forming 3.5 - 4.6 billion years.[1]Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.
A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the clade itself—the ability for a population to radiate more rapidly due to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the 'ratcheting' of evolutionary speed as one clade competes with another for a limited resource.
In the first stage of sexual reproduction, 'meiosis', the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During 'fertilization', haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored.
Contents[hide]
1 Reproduction in plants
2 Reproduction in archosaurs (reptiles and birds)
3 Reproduction in mammals
3.1 The mammalian male
3.2 The mammalian female
3.3 Gestation
3.4 Birth
3.5 Monotremes
3.6 Marsupials
4 See also
5 References
//
[edit] Reproduction in plants
Main article: Plant sexuality
In flowering plants, the anther produces male gametophytes called pollen grains, which attach to the stigma on top of a carpel, in which the female gametophytes (inside ovules) are located. After the pollen tube grows through the carpel's style, the sperm cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate.
[edit] Reproduction in archosaurs (reptiles and birds)
Male and female birds and reptiles both have cloacae, an opening through which eggs, sperm, and wastes pass. Intercourse is performed by pressing the lips of the cloacae together, during which time the male transfers his sperm to the female. The female lays amniotic eggs in which the young gestate. Nevertheless, a few species, including most waterfowl and ostriches, have a phallus shaped organ analogous to the mammals' penis.
[edit] Reproduction in mammals
In placental mammals, offspring are born as juveniles: complete animals with the sex organs present although non-functional. After several months or years, the sex organs develop further to maturity and the animal becomes sexually mature. Most female mammals are only fertile during certain periods and during those times, they are said to be "in heat". At this point, the animal is ready to mate. Individual male and female mammals meet and carry out copulation. For most mammals, males and females exchange sexual partners throughout their adult lives.
[edit] The mammalian male
For more details on this topic, see Male reproductive system (human).
The male reproductive system contains two main divisions: the penis, and the testes, the latter of which is where sperm are produced. In humans, both of these organs are outside the abdominal cavity, but they can be primarily housed within the abdomen in other animals (for instance, in dogs, the penis is internal except when mating). Having the testes outside the abdomen best facilitates temperature regulation of the sperm, which require specific temperatures to survive. Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of sexual maturity until death. Prior to ejaculation the produced sperm are stored in the seminal vesicle, a small gland that is located just behind the bladder.Sperm are motile and swim via chemotaxis, using its mitochondria to propell towards the ovum.
[edit] The mammalian female
For more details on this topic, see Female reproductive system (human).
The female reproductive system likewise contains two main divisions: the vagina and uterus, which act as the receptacle for the sperm, and the ovaries, which produce the female's ova. All of these parts are always internal. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the Fallopian tubes. At certain intervals, the ovaries release an ovum (the singular of ova), which passes through the fallopian tube into the uterus.
If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg, fertilizing it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina.
The ova are larger than sperm and are generally all created by birth. They are for the most part stationary, aside from their transit to the uterus, and contain nutrients for the later zygote and embryo. Over a regular interval, a process of oogenesis matures one ovum to be sent down the Fallopian tube attached to its ovary in anticipation of fertilization. If not fertilized, this egg is flushed out of the system through menstruation in humans and great apes and reabsorbed in all other mammals in the estrus cycle.
[edit] Gestation
Main articles: Pregnancy (mammals) and Pregnancy
Gestation, called pregnancy in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the female, who is required to ingest significantly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the gestation period, varies greatly from species to species; it is 38 weeks in humans, 56–60 in giraffes and 16 days in hamsters.
[edit] Birth
Main article: Childbirth
Once the fetus is sufficiently developed, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an infant in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta is passed as well. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young. The end of the umbilical cord attached to the young’s abdomen eventually falls off on its own.
[edit] Monotremes
Monotremes, only five species of which exist, all from Australia and New Guinea, lay eggs. They have one opening for excretion and reproduction called the cloaca. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like birds. After less than two weeks the young hatches and crawls into its mother’s pouch, much like marsupials, where it nurses for several weeks as it grows.
[edit] Marsupials
Marsupials reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals. After birth, marsupial joeys crawl into their mother’s pouch and attach to a teat, where they receive nourishment and finish developing into self-sufficient animals.
Ecology (from Greek: οίκος, oikos, "household"; and λόγος, logos, "knowledge") is the scientific study of the distribution and abundance of living organisms and how the distribution and abundance are affected by interactions between the organisms and their environment. The environment of an organism includes both 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.
The term oekologie was coined in 1866 by the German biologist Ernst Haeckel.
Contents[hide]
1 Scope
1.1 Disciplines of ecology
2 History of ecology
3 Fundamental principles of ecology
3.1 Biosphere
3.2 The ecosystem concept
3.3 Dynamics and stability
3.4 Spatial relationships and subdivisions of land
3.5 Ecosystem productivity
3.6 Ecological crisis
4 Footnotes
5 See also
5.1 Lists
5.2 Related topics
6 External links
//
[edit] Scope
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.
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.
As a scientific discipline, ecology does not dictate what is "right" or "wrong". However, 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:
The behavioral relationship between individuals of a species is behavioral ecology — for example, the study of the queen bee, and how she relates to the worker bees and the drones.
The organized activity of a species is community ecology; for example, the activity of bees assures the pollination of flowering plants. Bee hives additionally produce honey which is consumed by still other species, such as bears.
The relationship between the environment and a species is environmental ecology — for example, the consequences of environmental change on bee activity. Bees may die out due to environmental changes (see pollinator decline). The environment simultaneously affects and is a consequence of this activity and is thus intertwined with the survival of the species.
[edit] Disciplines of ecology
Main article: Ecology (disciplines)
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:
Ecophysiology and Behavioral ecology examine adaptations of the individual to its environment.
Autecology studies the dynamics of populations of a single species.
Community ecology (or synecology) focuses on the interactions between species within an ecological community.
Ecosystem ecology studies the flows of energy and matter through the biotic and abiotic components of ecosystems.
Landscape ecology examines processes and relationship across multiple ecosystems or very large geographic areas.
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; one could be a theoretical plant community ecologist, or a polar ecologist interested in animal genetics. Animals can be reproduced by plants.
[edit] History of ecology
Main article: History of ecology
[edit] Fundamental principles of ecology
[edit] Biosphere
Main articles: Biosphere, Biodiversity, and Unified neutral theory of biodiversity
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. (Although recently 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 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.
[edit] The ecosystem concept
Main article: Ecosystem
The first principle of ecology is that each living organism has an ongoing and continual relationship with every other element that makes up its environment. An ecosystem can be defined as any situation where there is interaction between organisms and their environment.
The ecosystem is composed of two entities, the entirety of life, 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:
Whether the colonization of a barren area could be carried out
Investigation the ecosystem's dynamics and changes
The methods of which an ecosystem interacts at local, regional and global scale
Whether the current state is stable
Investigating the value of an ecosystem and the ways and means that interaction of ecological systems provides benefits 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, such as forest ecosystems, meadow ecosystems such as steppes or savannas, or agro-ecosystems
As ecosystems of inland waters, such as lentic ecosystems such as lakes or ponds; or lotic ecosystems such as rivers
As oceanic ecosystems.
Another classification can be done by reference to its communities, such as in the case of an human ecosystem.
[edit] Dynamics and stability
Main articles: biogeochemistry, Homeostasis, and Population dynamics
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:
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 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, 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.
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.
An ant lion 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 which 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 limiting 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 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).
[edit] Spatial relationships and subdivisions of land
Main articles: Biome and ecozone
Ecosystems are not isolated from each other, but are interrelated. For example, water may circulate between ecosystems by the means of a river or ocean current. Water itself, as a liquid medium, even defines ecosystems. Some species, such as salmon or freshwater eels move between marine systems and fresh-water systems. These relationships between the ecosystems lead to the concept of a biome.
A biome is a homogeneous ecological formation that exists over a 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.
[edit] 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 -- usually plants which are capable of photosynthesis but could be other organisms such as bacteria around ocean vents that are capable of chemosynthesis.
Consumers -- animals, which can be primary consumers (herbivorous), or secondary or tertiary consumers (carnivorous and omnivores).
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. 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:
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 land area) 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).
[edit] Ecological crisis
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 may be more or less brutal (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 (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 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.
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:
The Exxon Valdez Oil Spill off the coast of Alaska in 1989
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 eco refugees), multiplication of extreme weather phenomena and changes in the nature and quantity of the food resources (see Global warming and agriculture). See also international Kyoto Protocol.
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 by humans because of the large amount of radiation generated by the meltdown. Twenty years after the accident, the animals have returned.
[edit] Footnotes
Levels of Organization'
Species
Population
Community
Ecosystem
Biome
Biosphere
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.
The term oekologie was coined in 1866 by the German biologist Ernst Haeckel.
Contents[hide]
1 Scope
1.1 Disciplines of ecology
2 History of ecology
3 Fundamental principles of ecology
3.1 Biosphere
3.2 The ecosystem concept
3.3 Dynamics and stability
3.4 Spatial relationships and subdivisions of land
3.5 Ecosystem productivity
3.6 Ecological crisis
4 Footnotes
5 See also
5.1 Lists
5.2 Related topics
6 External links
//
[edit] Scope
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.
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.
As a scientific discipline, ecology does not dictate what is "right" or "wrong". However, 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:
The behavioral relationship between individuals of a species is behavioral ecology — for example, the study of the queen bee, and how she relates to the worker bees and the drones.
The organized activity of a species is community ecology; for example, the activity of bees assures the pollination of flowering plants. Bee hives additionally produce honey which is consumed by still other species, such as bears.
The relationship between the environment and a species is environmental ecology — for example, the consequences of environmental change on bee activity. Bees may die out due to environmental changes (see pollinator decline). The environment simultaneously affects and is a consequence of this activity and is thus intertwined with the survival of the species.
[edit] Disciplines of ecology
Main article: Ecology (disciplines)
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:
Ecophysiology and Behavioral ecology examine adaptations of the individual to its environment.
Autecology studies the dynamics of populations of a single species.
Community ecology (or synecology) focuses on the interactions between species within an ecological community.
Ecosystem ecology studies the flows of energy and matter through the biotic and abiotic components of ecosystems.
Landscape ecology examines processes and relationship across multiple ecosystems or very large geographic areas.
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; one could be a theoretical plant community ecologist, or a polar ecologist interested in animal genetics. Animals can be reproduced by plants.
[edit] History of ecology
Main article: History of ecology
[edit] Fundamental principles of ecology
[edit] Biosphere
Main articles: Biosphere, Biodiversity, and Unified neutral theory of biodiversity
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. (Although recently 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 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.
[edit] The ecosystem concept
Main article: Ecosystem
The first principle of ecology is that each living organism has an ongoing and continual relationship with every other element that makes up its environment. An ecosystem can be defined as any situation where there is interaction between organisms and their environment.
The ecosystem is composed of two entities, the entirety of life, 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:
Whether the colonization of a barren area could be carried out
Investigation the ecosystem's dynamics and changes
The methods of which an ecosystem interacts at local, regional and global scale
Whether the current state is stable
Investigating the value of an ecosystem and the ways and means that interaction of ecological systems provides benefits 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, such as forest ecosystems, meadow ecosystems such as steppes or savannas, or agro-ecosystems
As ecosystems of inland waters, such as lentic ecosystems such as lakes or ponds; or lotic ecosystems such as rivers
As oceanic ecosystems.
Another classification can be done by reference to its communities, such as in the case of an human ecosystem.
[edit] Dynamics and stability
Main articles: biogeochemistry, Homeostasis, and Population dynamics
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:
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 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, 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.
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.
An ant lion 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 which 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 limiting 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 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).
[edit] Spatial relationships and subdivisions of land
Main articles: Biome and ecozone
Ecosystems are not isolated from each other, but are interrelated. For example, water may circulate between ecosystems by the means of a river or ocean current. Water itself, as a liquid medium, even defines ecosystems. Some species, such as salmon or freshwater eels move between marine systems and fresh-water systems. These relationships between the ecosystems lead to the concept of a biome.
A biome is a homogeneous ecological formation that exists over a 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.
[edit] 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 -- usually plants which are capable of photosynthesis but could be other organisms such as bacteria around ocean vents that are capable of chemosynthesis.
Consumers -- animals, which can be primary consumers (herbivorous), or secondary or tertiary consumers (carnivorous and omnivores).
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. 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:
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 land area) 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).
[edit] Ecological crisis
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 may be more or less brutal (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 (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 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.
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:
The Exxon Valdez Oil Spill off the coast of Alaska in 1989
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 eco refugees), multiplication of extreme weather phenomena and changes in the nature and quantity of the food resources (see Global warming and agriculture). See also international Kyoto Protocol.
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 by humans because of the large amount of radiation generated by the meltdown. Twenty years after the accident, the animals have returned.
[edit] Footnotes
Levels of Organization'
Species
Population
Community
Ecosystem
Biome
Biosphere
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