Difference between revisions of "Symbiosis" - New World Encyclopedia

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[[image:Common_clownfish.jpg|thumb|250px|right|Common [[Clownfish]] (Amphiprion ocellaris) in their magnificent sea anemone (Heteractis magnifica) home.]]
 
[[image:Common_clownfish.jpg|thumb|250px|right|Common [[Clownfish]] (Amphiprion ocellaris) in their magnificent sea anemone (Heteractis magnifica) home.]]
'''Symbiosis''' (pl. symbioses) is the close, interactive association (living together) of members of two or more [[species]]. The term symbiosis was coined by the [[German]] [[botany|botanist]] Anton de Bary in 1879 from the Greek ''symbioun'' (to live together), from the prefix ''sym'' (sum, together, or together with) and the word ''bios'' (life). In the narrowest sense of the term, which is popularly used, symbiosis has been defined as interactive association of two species in a mutually beneficial relationship. However, biologists define it according to the original, comprehensive concept of de Barry, which is the living together of members of two or more species in all the various forms, whether the interaction is beneficial, harmful, or neutral to one or both organisms.
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'''Symbiosis''' (plural: "symbioses") is the close, interactive association (living together) of members of two or more [[species]]. The term symbiosis was coined by the [[German]] [[botany|botanist]] [[Anton de Bary]] in 1879 from the Greek ''symbioun'' (to live together), from the prefix ''sym'' (sum, together, or together with) and the word ''bios'' (life). In the narrowest sense of the term, as popularly used, symbiosis has been defined as the interactive association of two species in a mutually beneficial relationship. However, [[biology|biologists]] define it according to the original, comprehensive concept of de Bary: the living together of members of two or more species in all the various forms, whether the interaction is beneficial, harmful, or neutral to one or both organisms.
  
In a symbiosis, the larger partner (macro-species) is known as 'host' whereas the smaller partner (micro-species) is known as the '''symbiont''' (plural: '''symbionts''') or '''symbiote''' (plural: '''symbiotes'''). In the case of cattle and the cattle egret, large ruminants are the host and the cattle egret, a small white [[heron]], is the symbiont as it feeds on the ticks and other [[parasites|ectoparasites]] of the former, as well as [[insect]]s the cattle disturb as they feed. When a microscopic symbiont lives inside the cells of a host, it is referred to as an [[endosymbiont]].
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In a symbiosis, the larger partner (macrospecies) is known as the “'''host,'''” whereas the smaller partner (microspecies) is known as the '''symbiont''' (plural: "symbionts") or '''symbiote''' (plural "symbiotes"). In the case of cattle and the cattle egret (a small white [[heron]]), large ruminants are the host and the cattle egret is the symbiont, feeding on the ticks and other [[parasites|ectoparasites]] of the former, as well as [[insect]]s that the cattle disturb as they feed.  
  
In nature, one does not find organisms that do not relate with other organisms and the abiotic environment. It is not only that they interact for their own survival, but also they contribute to others. Each organism has value for the [[ecosystem]] in which it is living. One such inter-relationship is expressed in the form of a food web, in other words, the relationship of eating and being eaten. In this relationship, one finds a concept of dual purposes of each being: the individual purpose (eating) and whole purpose (providing value to others). The phenomena further explicates the inter-relationship of organisms. (See [[Biology#Interactions: Harmony and dual purposes|Interactions:Harmony and dual purposes]]).
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In nature, there are no organisms that do not interact at some level with both other organisms and the abiotic environment. While organisms tend to interact for their own survival, one or both of the interacting organisms often will also contribute to the benefit of other third parties—each organism has value for the larger [[ecosystem]] in which it is immersed. A typical interrelationship is expressed in the form of a food web showing the relationships of eating and of being eaten. In this relationship, one finds a concept of dual purposes for each being: the individual purpose (eating) and whole purpose (providing value to others). This principle of organisms having both individual and whole purposes is further explicated by the phenomena of symbiosis. (See [[Biology#Interactions: Harmony and dual purposes|Interactions:Harmony and dual purposes]]).
  
 
== Types of symbioses ==
 
== Types of symbioses ==
===A. Based on the occurrence of symbionts on the host ===
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===A. Classification based on location of symbionts relative to host ===
One manner of classifying symbiosis is according to the physical location of the symbionts. Two types of symbiosis are recognized:
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One manner of classifying symbioses is according to the physical location of the symbionts. Two types of symbioses are recognized:
  
====1. Ectosymbiosis ====
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====Ectosymbiosis ====
In ectosymbiosis, symbionts occur on the body surface of the hosts. The body surface can be an external surface such as the skin, but also includes internal surfaces, such as the linings of digestive canal and respiratory tract, the ducts of glands, and so forth. Examples include the attachment of a leech to the skin of avertebrate and the presence of ''Escherichia coli'' on the mucosal layer of a person's intestine.
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:In ectosymbiosis, symbionts occur on the body surface of the hosts. The body surface can be an external surface, such as the [[skin]], but also includes internal surfaces, such as the linings of [[digestive system|digestive canal]] and [[respiratory system|respiratory tract]], the ducts of glands, and so forth. Examples include the attachment of a [[leech]] to the skin of a [[vertebrate]] and the presence of ''[[Escherichia coli]]'' on the mucosal layer of a person's [[intestine]].
  
====2. Endosymbiosis ====
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====Endosymbiosis ====
It endosymbiosis, symbionts occur inside the cells or the tissue (intercellular space) of the host. A classical example of endosymbiosis is nitrogen fixing [[bacterium|bacteria]] (''Rhizobium sps.'') living in [[root nodules]] of leguminous plants. Other examples include reef-building [[corals]] containing single-celled [[algae]], and several [[insect]] species containing bacterial endosymbionts. Many other examples of endosymbiosis exist.
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:In endosymbiosis, symbionts occur inside the [[cell (biology)|cells]] or the tissue (intercellular space) of the host. A classical example of endosymbiosis is the presence of nitrogen fixing [[bacterium|bacteria]] (''Rhizobium sps.'') living in [[root nodules]] of leguminous plants. Other examples include reef-building [[corals]] containing single-celled [[algae]], and several [[insect]] species containing bacterial endosymbionts.  
  
Many instances of endosymbiosis are obligate, where neither the endosymbiont nor the host can survive without the other, such as gutless marine worms that get nutrition from their endosymbiotic bacteria. However, not all endosymbioses are obligate. Also, some endosymbioses can be harmful to either of the organisms involved.
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:Many instances of endosymbioses are obligate, where neither the endosymbiont nor the host can survive without the other, such as "gutless marine worms" that get nutrition from endosymbiotic bacteria. However, not all endosymbioses are obligate. Furthermore, some endosymbioses can be harmful to both organisms involved.  
  
It is generally agreed that certain [[organelle]]s of the [[Eukaryote|eukaryotic]] cell, especially [[mitochondria]] and [[chloroplast]]s, originated as bacterial endosymbionts. This theory is known as the [[endosymbiotic theory]], confirmed and popularized by [[Lynn Margulis]].
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:One theory is that certain [[organelle]]s of the [[Eukaryote|eukaryotic]] cell, especially [[mitochondria]] and [[chloroplast]]s, originated as bacterial endosymbionts. This theory is known as the [[endosymbiotic theory]], as postulated and popularized by Lynn Margulis.
  
===B. Based on impact on symbiotic partners===
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===B. Classification based on impact on symbiotic partners===
:Symbioses can be categorized into following types:
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Table 1 shows several types of symbioses based on the impact (positive, negative, or neutral) on the symbiotic partners.
  
 
{| border="0" cellpadding="5" width="1" style="float: right; border-collapse: collapse; margin: 1em" class="toccolours"
 
{| border="0" cellpadding="5" width="1" style="float: right; border-collapse: collapse; margin: 1em" class="toccolours"
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| '''Effect on Species A''' || '''Effect on Species B''' || '''Type of interaction'''
 
| '''Effect on Species A''' || '''Effect on Species B''' || '''Type of interaction'''
 
|-
 
|-
| 0 || 0 || [[Neutralism]]
+
| 0 || 0 || Neutralism
 
|-
 
|-
| - || 0 || [[Amensalism]]
+
| + || + || Mutualism
 
|-
 
|-
| 0 || - || [[Amensalism]]
+
| + || 0 || Commensalism
 
|-
 
|-
| + || 0 || [[Commensalism]]
+
| 0 || + || Commensalism
 
|-
 
|-
| 0 || + || [[Commensalism]]
+
| + || - || [[Parasitism]] or Predation
 
|-
 
|-
| - || - || [[Synnecrosis]]
+
| - || + || Parasitism or Predation
 
|-
 
|-
| + || + || [[Mutualism]]
+
| - || 0 || Amensalism
 
|-
 
|-
| + || - || [[Predation]] or [[Parasitism]]
+
| 0 || - || Amensalism
|-
 
| - || + || [[Predation]] or [[Parasitism]]
 
 
|}
 
|}
 
|-
 
|-
| align="center" |Some types of relationships listed by the effect they have on each partner. '0' is no effect, '-' is detrimental, and '+' is beneficial.
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| align="left" |Table 1. Some types of relationships listed by the effect they have on each partner. '0' is no effect, '-' is detrimental, and '+' is beneficial.  
 
|}
 
|}
  
It is important to note that these interactions are not always static.  In many cases, two species will interact differently under different conditions.  This is particularly true in, but not limited to, cases where species have multiple, drastically different life stages.
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From the table, we can derive five main categories of symbiotic relationships among organisms. They are:
 
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* (Type 0,0). '''Neutralism''' is a lack of benefit or detriment experienced by either members of the pair of interacting organisms.
:From the table, we can derived four categories of symbiotic relationships among organisms. They are:
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* (Type 0,+). '''Commensalism''' occurs when one member of the association benefits while the other is not affected. Type 0,+ includes phoresis, which is the transport of one species by another.
::# 0 0 Type (Neutralism)
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* (Type +,+). '''Mutualism''' occurs when the symbiotic association is advantageous to both members of the pair.
::# 0 + Type (Phoresis, Commensalism)
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* (Type -,+). '''Parasitism''' (as well as predation, competition, and allelopathic interference) is a situation in which the association is disadvantageous or destructive to one of the organisms and beneficial to the other.
::# + + Type (Mutualism)
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* (Type -,0). '''Amensalism''' is found when the association is disadvantageous to one member while the other is not affected.
::# - + Type (Parasitism, Predation, Amensalism, Competition, Allelopathic interference)
 
  
:Before discussing each of the categories in detail, it is necessary to point out the conclusion that we can draw from the table above. There is no symbioses known where there is harm to both partners or harm to one but without any benefits to another. This proves that the interactions among species are intentionally aimed at fulfilling dual purposes as mentioned above.
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There is one other theoretical category of biological interactions, but, if occurring, it would be rare and short-lived:
 +
* (Type -,-). '''Synnecrosis''' occurs when an interaction is detrimental to both species.
  
 +
It is important to note that these interactions are not always static. In many cases, two species will interact differently under different conditions. This is particularly true in, but not limited to, cases where [[species]] have multiple, drastically different life stages.
  
The various forms of symbiosis include: -
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==Descriptions of types of symbiosis==
* [[parasitism]], in which the association is disadvantageous or destructive to one of the organisms and beneficial to the other (+ -)
 
* [[mutualism]], in which the association is advantageous to both (+ +)
 
* [[commensalism]], in which one member of the association benefits while the other is not affected (+ 0)
 
* [[amensalism]], in which the association is disadvantageous to one member while the other is not affected (- 0)
 
  
In some cases, the term symbiosis is used only if the association is obligatory and benefits both organisms. Symbiosis as defined in this article does not restrict the term to only the mutually beneficial interactions.
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=== Type 0,0. Neutralism===
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Neutralism is the term used in [[ecology]] to signify a biological interaction in which the population density of two [[species]] appear to have no effect whatsoever on each other. Examples of this type of symbiosis include pelicans and cormorants feeding in the [[ocean]], [[spider]]s and mantis preying on [[insect]]s of the same bush, diverse songbirds feeding and nesting together in a woodland, and a large number of microorganisms acting as residents in different body organs of [[human being]]s, such as skin, nose, mouth, and so forth.
  
 +
A critical examination of each of the examples of neutralism leads to the conclusion that true neutralism is unlikely to exist and would be difficult, or impossible, to prove. It seems that living together—within the complex networks of interactions presented by [[ecosystem]]s—would likely involve at least some degree of positive or negative impact of one species on the other. If two species do not interact directly, links can be found via other species and shared resources. Since true neutralism is rare or nonexistent, the term is often extended to situations where interactions are merely insignificant or negligible.
  
* [[Neutralism]] is a lack of interaction. Since all species sharing an environment interact in some way, a complete lack of interaction is rarely seen in nature. However, the term can also signify a relationship in which each species derives neither benefit nor detriment to any measurable degree.
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=== Type 0,+. Phoresis, commensalism===
 +
In a Type 0,+ symbiosis, the host is said to experience neither benefit nor harm, whereas the symbiont receives benefit, which ranges from simple transport to protection, food, and shelter.
 +
====Phoresis (Phoresy)====
 +
:Phoresis (meaning "to carry") is the simplest form of type 0,+ symbiosis, occurring mainly among [[species]] sharing the same [[ecology|ecological]] niche. Phoresis involves transport of one organism by another. There is no physiological or biochemical dependence between the host and symbiont. Two organisms come in contact by chance, establish a fairly loose mechanical association, and the smaller one (known as the phoront) is carried by the transport host (mechanical vector). For example, food and water-borne disease organisms ([[virus]]es, [[bacteria]], [[protozoan]]s, and multicellular [[parasite]]s) are transported from their source to the destination by [[fly|flies]] and other [[insect]]s because of direct contacts. [[Fungi]] and free living [[nematode]]s are carried by dung [[beetle]]s, with which they share a common niche. The sucker fish Remora is equipped with a sucker disk on the dorsal side of its head. With the help of this disc, the [[fish]] attaches itself to the belly of larger fish so as to get benefit of faster movement of the latter. Because the Remora fish also can obtain some food scraps of the larger fish, this association can also be considered as commensalism.
  
* [[Mutualism]] benefits both populations.  It is often non obligatory or temporary.
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====Commensalism====
  
* [[Synnecrosis]] is detrimental to both species. It is a rare and necessarily short-lived condition as evolution selects against it.
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:Commensalism is a loose, usually facultative (neither dies if the relationship ends), and interspecific association between a larger "host" and a smaller "commensal," without any [[metabolism|metabolic]] dependency. Only the smaller commensal derives benefits, such as sharing space, substrate, defense, shelter, transport, and/or food, and does so without helping or harming the host. In some cases, the interaction is obligatory for the commensal. ''Escherichia coli'', and a variety other of [[bacteria]], are harbored comfortably in the [[large intestine]] of people to feed on nutrients that are outgoing at any rate. There is no obvious harm or benefit to the host. (Although some benefits have been recognized as contributed by certain bacteria that live within the human digestive system, such as aiding immunity, synthesizing certain vitamins, and fermenting complex indigestible [[carbohydrate]]s(O'Hara and Shanahan 2006)). ''Entamoeba gingivalis'' is also a commensal in the human mouth. Commensalism means "eating at the same table," graphically conveying a situation whereby the commensal is able to feed on food that is not being consumed by the host—offering benefit to the commensal without impacting the host.
  
* [[Amensalism]] is detrimental to one species and neutral to the other.
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:Examples of [[metazoan]] commensals include [[barnacle]]s on a [[whale]] and epiphytes on a [[tree]]. Sedentary adult barnacles attach to a whale, or other large moving objects, and get renewed sources of food and oxygen supply without giving causing obvious harm to the carrying host. Similarly, epiphytic [[plant]]s like orchids and bromeliads grow perched on sturdier trees for better exposure to sunlight and for absorbing moisture and nutrients from the air.
  
* [[Commensalism]] benefits one organism and the other organism is neither benefited nor harmed.
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:Even though innumerable examples of commensalism have been recognized, a closer inspection suggests that true commensal relationships tends to be rather rare. For example, the pinworm in the large intestine of [[human being]]s has been considered as a commensal relationship. In reality, at the time of the pinworms reproduction, it intentionally causes itching in the anal region. [[Shrimp]] has been considered to be commensal on the [[sea cucumber]], using it for passive transport. From time to time, the shrimp leaves the host for food. But it also has been pointed out that the commensal may feed on the parasites of the host body surface and also provide protection to the host because of its warning coloration. Thus, commensalism in many cases appears in reality to fall into the categories of either mutualism or parasitism.
  
* [[Predation]] is an interaction between organisms in which one organism captures [[biomass]] from another. It is often used as a synonym for [[carnivore | carnivory]] but formally also includes [[herbivory]], [[parasitism]], and [[parasitoid]]ism.
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===Type +,+. Mutualism===
 +
Mutualism is the species-specific interaction of two organisms for mutual benefit. At times, the term symbiosis is used specifically for this type +,+ association, using the term synonymously with mutualism. However, this type of interaction is more appropriately termed mutualism, given the diverse associations to which biologists apply the term symbiosis.  
  
 +
Mutualism is a type of biological interaction that may range from loose to intimate, and from facultative to obligatory with both involved partners being called "mutuals." When the relationship is intimate and obligatory, each mutual is [[physiology|physiologically]] (metabolically) dependent on another for survival and [[reproduction]]. This means that members in the association derive key benefits from the other with respect to development, stimuli, nutrition, digestive enzymes, and maturation factors, such that neither of the mutuals can survive and reproduce successfully in the absence of the other. Examples of mutualism can be categorized as follow:
 +
====Nutritional mutualism====
 +
:In "[[nutritional symbiosis|nutritional mutualism]]," both mutuals contribute to each other either simple organic nutrients, inorganic minerals, or digestive enzymes for the digestion process. Some important examples of nutritional mutualism are:
 +
:*In '''nitrogen fixation''', the [[bacteria]] ''Rhizobium sps.'' in the root and/or shoot nodules of [[legume]]s fix atmospheric [[nitrogen]] to make it available for [[plant]]s. Meanwhile, the plants provide [[glucose]] and other organic nutrients, as well as a biological environment to the bacteria.
 +
:*'''Mycorrhiza''' (fungus-root) has been defined as an apparent underground structure developed as a result of the mutualistic association between [[fungi]] (mainly basidiomycetes) and the roots of almost all plant species. There are different degrees of complexity in the association. Plants benefit by having enhanced root development and increased absorption of water and minerals. The fungus, in return, receives an organic food supply from the plants. Mycorrhiza can be used to enhance the growth of plants, even under poor soil conditions.
 +
:*'''Syntrophy''' is the obligatory association of two microorganisms for mutual production of biochemical substances that the partners need for physiological processes. For example, sulfur reducing bacteria ''Desulfuromonas acetoxidans'' need [[sulfur]] for oxidizing [[acetic acid]] to produce [[carbon dioxide]], hydrogen sulfide, and [[energy]]. Their partner organisms, green photosynthetic bacteria (''Chlorobium''), use the provided hydrogen sulfide for [[photosynthesis]], producing a continuous supply of sulfur. Based on this principle of syntrophy, effective microorganism (EM) preparation is in use for enhancing all sorts of microbial processes.
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:*About 50 percent of the food stuff of '''ruminants''' is composed of [[cellulose]], which can be digested by the ruminants only with the assistance of protozoans (ciliates like ''Diplodinium sp.'' and ''Entodinium sp.'') and obligate anaerobic bacteria (''Ruminococcus albus'', ''R. flavefaciens'', etc.) lodged in the rumen (a special compartment) of their stomach. As a result of the anaerobic milieu, the bacterial fermentation produces fatty acids, carbon dioxide, and hydrogen gas that are used by methanogenic bacteria (''Methanobacterium ruminantium'') present there to produce [[methane]]. Thus, ruminants are an important source of methane in the atmosphere. Like ruminants, '''termites''' harbor protozoan (flagellates) and bacterial flora in their gastrointestinal (GI) tract for digestion of cellulose.
  
 +
:*'''Lichen''' offers a common example of nutritional symbiosis. [[Lichens]] are symbiotic organisms made up by the association of microscopic [[green alga]]e or [[cyanobacteria]] and filamentous [[fungi]]. They co-exist in an obligate and intimate, but ectosymbiotic, association. The thallus (body proper) of lichen is mainly made of a close network of fungal mycelium (mycobiont) and is responsible for absorption of water, minerals, and gases, as well as forming a protective tissue structure for algal partner. Algal cells (phycobiont), which constitute about 10 percent of the thallus, are embedded in the compact mass of mycelium and are responsible for [[photosynthesis]], nitrogen fixation, and production of food for both partners.
  
 +
====Cleaning symbioses====
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:Cleaning symbioses are facultative and loose associations of mutuals based on the principle that one's parasites are someone else's food. In this case, the larger mutual cooperates with and allows the smaller mutual to remove its ectoparasites, like leeches, ticks, and so forth—even from gills, inside of mouth, and nostrils. Some of the cleaning partners include larger [[fish]] and [[crustacean]]s or fishes; [[shark]] and remora fish; buffalo and oxpecker; rhinocherus and tick bird; and cattle and cattle egret.
 +
:A famous land version of symbiosis is the relationship of the [[Egyptian Plover]] bird and the [[crocodile]]. In this relationship, the bird is well known for preying on [[parasite]]s that feed on crocodiles and that are potentially harmful for the animal. To that end, the crocodile openly invites the [[bird]] to hunt on its body, even going so far as to open the jaws to allow the bird to enter the mouth safely to hunt. For the bird's part, this relationship not only is a ready source of food, but a safe one considering that few predator species would dare strike at the bird at such close proximity to its host.
  
====1.  0 0 Type: 'Neutralism'====
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====Transport symbioses====
: The special term used for this 0 0 type category is neutralism. This term has been used in ecology to signify the biological interaction in which the population density of one species has absolutely no effect whatsoever on the other. Examples given for this symbiosis are pelicans and cormorants feeding in the ocean, spider and mantis preying insects of the same bush, numerous songs birds feeding and nesting together in a woodland, large number of microorganisms that normally act as residents of different body organs of human beings such as skin, nose, mouth, etc.
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:In transport symbiosis, one mutual takes advantage of the fast and secure movement of another mutual, whereas the latter receives mainly cleaning services. The Remora fish and shark, and the shrimp and sea cucumber, have already been described above. Another typical example of this association is the torsalo fly (''Dermatobia hominis'') making use of a small insect for transport of its eggs to the vertebrate host, including humans. The female parasitizes small flies and [[mosquito]]es and glues its eggs underneath their bodies, then frees them to reach to the host. Upon contact with the warm skin of the host, the larvae come out of the eggs, penetrate the skin and develop underneath the skin of the host. This association can also be considered as a type of parasitism.
:When you critically evaluate each of the examples of neutralism on the basis of ecological principles, then you will come to the conclusion that neutralism is unlikely to exist and is impossible to prove. Because, living together always involves at least some degree of positive or negative impact on the partner. Thus, living together is a purposeful interaction of different degree of intimacy.
 
  
====2.  0 + Type====
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====Pollination symbioses====
:In this symbiosis, the host is said to experience neither benefit nor harm, whereas the symbiont gets benefit ranging from a simple transport to protection, food and shelter.
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[[Image:Selasphorus rufus1.jpg|thumb|Rufous humming bird on a flower.]]
=====Phoresis (Phoresy)=====
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:Pollination symbioses refer to the mutual benefit exchanged between flower-visiting small [[bird]]s, and [[insect]]s like [[humming bird]]s, [[butterfly|butterflies]] and [[bee]]s, with flower bearing plants, with the former receiving nectar while pollinating the latter.
: It is the simplest form of 0 + type of symbiosis occurring mainly among the species sharing the same ecological niche. This relationship is not species specific. By chance, the two involved organisms come in contact, establish fairly loose mechanical association and the smaller one (specially known as phoront) is carried by the transport host (mechanical vector) indifferently. Food and water borne disease organisms (virues, bacteria, protozoans and multicellular parasites) are transported from their point of sources to the destination by flies and other insects because of direct contacts. Fungi and free living nematodes are carried by dung beetles as they share the same common niche. Remosa (sucker fish) is equipped with a sucker disk on the dorsal side of its head. With the help of the disc, the fish attaches itself to the belly of larger fish so as to get benefit of faster movement of the latter. Because it can also get some food scraps of the larger fish, this association can also be considered as commensalism.
 
  
=====Commensalism=====
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====Defense symbioses====
: This term was coined to mean "eating at the same table" type of living together for food. It is, therefore, a loose, usually facultative (neither dies if the relationship ends) and interspecific association between a larger 'host' and a smaller 'commensal' without any metabolic dependency. Here, only commensal derives benefit of sharing space, substrate, defense, shelter, transport and/or food without helping or harming the host. In some cases, the interaction is obligatory for the commensal. ''Escherichia coli'', and a variety other of bacteria are harbored [[Image:Oncidium-longipes.jpg|thumb|An orchid plant perched on a tree]] comfortably in the large intestine of man to feed on anyway out going food rest. There is no obvious harm or benefit to the host. ''Entamoeba gingivalis'' is also a  commensal in human mouth. Examples of metazoan commensals are barnacles on whale, epiphytes on tree, etc. Sedentary adult barnacles by attaching itself on whale or any other moving objects get renewed sources of food and oxygen supply without giving any trouble to the carrying host. Similarly, epiphytic plants like orchids and bromeliads grow perched on sturdier trees for better exposer to sunlight and for absorbing moisture and nutrients from air.
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:An example of defense symbiosis is exemplified by the relationship between [[clownfish]] of the genus ''Amphiprion'' (family: Pomacentridae) that dwell among the tentacles of tropical [[sea anemones]]. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the anemone fish from its predators. Special mucus on the clownfish protects it from the stinging tentacles.
:Even though enumerable examples of commensalism have been recognized, however, when you inspect them more closely, then you may conclude that pure commensal relationship tends to be rather rare. Pin worm in the large intestine of human being has been considered as a commensal, but at the time of its reproduction, it intentionally causes itching at the anal region. Shrimp is a commensal on sea cucumber for passive transport. From time to time, it leaves the host for food. But it has been pointed out that the commensal may feed on the parasites of the host body surface and also provide protection to the host because of its warning coloration. Thus, commensalism seems to be, in reality, either mutualism or parasitism.
 
  
====3. + + Type: 'Mutualism'====
+
[[Image:Goby fish with shrimp.jpg|thumb|Shrimp and goby fish.]]
: It is the species specific interaction of two organisms  for mutual support and help. Sometimes, we come across the instances where the term symbiosis has been used specifically for this + + type of association synonymously to mutualism. However, considering the overall interspecific associations, Mutualism is the most appropriate term for this type of symbiosis. It is a loose to intimate and facultative to obligatory interaction, in which both the involved partners are called 'mutuals'. When the relationship is intimate and obligatory, each mutual is physiologically (metabolically) dependent to another for the survival and reproduction, it means that one derives benefit of another with respect to developmental stimuli, nutrition, digestive enzyme and maturation factors, so neither can survive and reproduce successfully in absence of another partner. Examples of mutualism can be categorized as follow:
+
:Some goby fish species live in symbiosis with a [[shrimp]]. The shrimp digs and prepares a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind, leaving it vulnerable to predators when above ground. In case of danger, the goby fish touches the shrimp with its tail to warn it of imminent danger. When that happens, both the shrimp and goby fish quickly retract into the burrow.
=====Nutritional Symbioses=====
 
:Both mutuals contribute to each other either simple organic nutrients, inorganic minerals or digestive enzymes or digestion process. Some important nutritional symbioses are -
 
:In '''nitrogen fixation''', ''Rhizobium sps.'' of root and/or shoot nodules of legumes fix atmospheric nitrogen to make it available for plant, whereas the plant provides glucose and other organic nutrients as well as biological environment to the bacteria.
 
:'''Mycorrhiza''' (fungus-root) has been defined as an apparent underground structure developed as a result of mutualistic association between fungi (mainly basidiomycetes) and plant root of almost all plant species. There are different degrees of complexity in association. Plants benefit of having induced root development and increased absorption of water and minerals. Fungus partner in return gets organic food supply from the plant. So mycorrhiza can be used to grow plants in efficient way, even in poor soil condition
 
:'''Syntrophy''' is the obligatory association of two microorganisms for mutual production of biochemical substances what another partner need for its physiological processes. For example, sulfur reducing bacterium, ''Desulfuromonas acetoxidans'' need sulfur for oxidizing acetic acid so as to produce carbon dioxide, hydrogen sulfide and energy. Its partner, green photosynthetic bacterium (''Chlorobium''), in the same culture constantly use the provided hydrogen sulfide for its photosynthesis to make the first one continuous supply of sulfur. Based on the principle of syntrophy, effective microorganism (EM) preparation is in use for enhancing all sort of microbial processes.
 
:About 50 percent of the food stuff of '''ruminants''' is composed of cellulose, for the digestion of which, they have to take assistant from protozoans (ciliates like ''Diplodinium sp''. and ''Entodinium sp.'') and obligate anaerobic bacteria (''Ruminococcus albus'', ''R. flavefaciens'', etc.) lodged in rumen (a special compartment) of their stomach. Because of anaerobic milieu, the bacterial fermentation produces different fatty acids, carbon dioxide and hydrogen gas, which are in turn used by methanogenic bacteria (''Methanobacterium ruminantium'')present there to produce methane. Thus, ruminants are the important source of methane in the atmosphere. Like the ruminants, termites and others harbor protozoan (flagellates) and bacterial flora in gastrointestinal (GI) tract for digestion of cellulose.
 
:'''Lichen''' is the very common example of nutritional symbiosis and is the obligate [[Image:lichen02.jpg|thumb|Lichen over the rock]]and intimate but ectosymbiotic association of a fungus (especially ascomycetes and basidiomycetes) and an alga (green or blue green algae). The thallus (body proper) is mainly made of close network of fungal mycelium (mycobiont) and is responsible for absorption of water, minerals and gases and to form protective tissue structure for algal partner. Algal cells (phycobiont) constitute about 10 percent of the thallus, are embedded in the compact mass of mycelium and are responsible for photosynthesis, nitrogen fixation and production of food for both partners.
 
  
=====Cleaning Symbioses=====
+
From a survey of the biological world, it is clear that there are innumerable cases of viable mutualistic associations enabling organisms to explore diverse ecological niches. Mutualism even has enabled organisms to colonize stressful habitats and survive successfully even at self-insufficiency. Overall, this promotes the efficient and optimal use of biological features and natural resources.
:These are the facultative and loose associations of mutuals based on the principle that one's parasites are someone's food. Here, therefore, the larger mutual cooperates with and allows smaller mutual to remove its ectoparasites like leech, ticks, etc. even from gills, inside of mouth and nostrils. Some of the cleaning partners are crocodile and crocodile bird (Egyptian Plover), larger fishes and crustaceans or fishes, shark and remosa fish, buffalo and oxpecker, rhino and tick bird, cattle and cattle egret.
 
=====Transport Symbioses=====
 
:In the transport symbiosis, one mutual takes advantage of fast and secure movement of another mutual, whereas the latter get mainly the cleaning services. Remosa and shark, and shrimp and sea cucumber have already been described above. Typical example of this association is torsalo fly (''Dermatobia hominis'') of New World Tropics making use of small insect for transport of its eggs to the vertebrate host, including man. The female parasitizes small flies and mosquitoes and glues its eggs underneath their bodies and let them free to reach to the hosts. Upon contact with the warm skin of host, larva comes out of egg, penetrates the skin and develops underneath the skin of the host. This association can also be considered as a type of parasitism.
 
=====Pollination symbioses=====
 
[[Image:Selasphorus rufus1.jpg|thumb|Rufous humming bird on a flower]]
 
:Pollination symbiosis refer to the mutual benefit exchanged between flower visiting small birds, insects like butterfly, bee, etc. and flower bearing plants, as the former while getting nectar cause pollination in the latter.
 
  
=====Defense Symbioses=====
+
=== Type -,+. Parasitism, predation, competition, allelopathic interference ===
:An example of defense symbiosis is the relationship[[Image:common_clownfish.jpg|thumb|clownfish & sea anemone]] between clownfish of the genus ''Amphiprion'' (family: Pomacentridae) that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the anemone fish from its predators (a special mucus on the anemone fish protects it from the stinging tentacles). Some goby fish species live in symbiosis with a shrimp. The [[Image:Goby fish with shrimp.jpg|thumb|shrimp and goby fish]]shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind leaving it vulnerable to predators when above ground. In case of danger the goby fish touches the shrimp with its tail to warn it of imminent danger. When that happens both the shrimp and goby fish quickly retract into the burrow.
+
This association involves obvious harm to one partner for the benefit of another. This type of relationship occurs in many forms.
 +
====Parasitism====
 +
:Parasitism may be defined as an interspecific association in which one [[species]], the ''parasite'', lives on or in a second species, the ''host'', for a significant period of its life and exploits the host to obtain nourishment, shelter, and/or protection. This relationship can be loose or mostly intimate, and for the parasite, it is usually obligatory.
  
:From the survey of biological world, one can be convinced that there are enumerable cases of viable mutualistic associations enabling organisms to explore diverse ecological niches. Hence, the mutualism has enable organisms even to colonize stressful habitats and survive successfully even at self-insufficiency, thus promoting the efficient and optimal use of biological features as well as natural resources together.
+
:As a parasite has to break through the different layers of defense mechanisms of the host in order to localize and thrive on the host, the parasite must be equipped with specialized mechanisms. The relationship, therefore, is strongly species specific, in other words, a particular parasite can infect only a particular host, not any other. The parasite causes harm to the host in many ways, such as by depriving the host of digested food; erecting mechanical blockades of food, blood, lymph, and bile passages; and causing tissue damages by rupture, enzymatic digestion, induction of autolysis, or secretion of toxins.
  
====4.  -  + Type====
+
====='''''Types of Parasites'''''=====
:This association involves obvious harm in one partner for the benefit of another. This type of relationship occurs in many forms.
+
:Macroparasites are multicellular parasites that are visible to the naked eye, such as helminth parasites (parasitic worms, such as [[fluke]]s, [[tapeworm]]s, and [[roundworm]]s or nematodes). Mesoparasites are the ones that penetrate external openings, such as the buccal cavity, cloaca, external ear, and so forth. Microparasites are unicellular and invisible to the naked eye, such as [[protozoan]] parasites.  
=====Parasitism=====
 
:It may be defined as an interspecific association in which one species, the 'parasite', lives on or in a second species, the 'host', for a significant period of its life and exploits the host to obtain nourishment, shelter and/or protection. This relationship can be loose or mostly intimate, and for the parasite, it is usually obligatory. As the parasite has to break through the different layers of defense mechanism of the host for localizing on the host, establishing in the host as well as for growth and reproduction in the host, the parasite must be equipped with specific mechanism too. Therefore, the relationship is strongly species specific, i.e., particular parasite can infect only particular host, not any other. The parasite causes harm to the host in many ways, like by depriving of digested food, erecting mechanical blockade of food, blood, lymph, bile passages, causing tissue damages by rupture, enzymatic digestion, induction of autolysis or secretion of toxin.
 
:'''''Types of Parasites''''':- Macroparasites are multicellular parasites visible to naked eyes, e.g., helminth parasites. Mesoparasites are the ones which penetrate the external openings and infect in like buccal cavity, cloaca, external ear. Microparasites are unicellular and invisible to naked eyes, e.g., protozoan parasites. Ectoparasites, like leech, ticks, lice are macroparasites occurring on the body surface of the host for shorter or longer period of time. Endoparasites, on the other hand, enter the body of the host via different means and remain in specific site permanently or for longer period of time, e.g., intestinal parasites, blood parasites. Temporary parasites (leech, bed bug) visit its host only for a short period of time. Permanent parasites spent whole or a part of their life cycle in the host. Facultative parasites need association of host, but can also survive without parasitic mode of life. So, they are opportunistic. For example, parasitic nematode, ''Strongyloides stercoralis'', can also be free living. Majority of parasites are obligatory parasites and are totally dependent on the host for food, shelter and/or protection, so they cannot survive without host. Accidental parasites are those which happen to infect unusual host, other than normal definite host. Wandering or aberrant parasite instead of arriving at the site of infection in the definitive host reach unusual place as dead end and becoming unable to complete the life cycle. For example, tape worm (''Taenia solium'') reach to brain and remains there unless operated out. When a parasite is being parasitized by another organism, then the latter one is known as hyperpasasite or secondary parasite. It can be used for biological control of the pest and parasites. Monogenic parasites complete the whole life cycle in one host, for example, ''Entamoeba histolytica'', but the digenetic parasite needs, in addition to primary host, also secondary host to complete the whole life cycle. ''Plasmodium vivax'' (malaria parasite) complete its asexual part of life cycle in man  and the sexual part in female ''Anopheles'' mosquito.
 
:'''''Types of Hosts''''':- Definitive host is usually the main host. For digenetic parasites, it is the host for adult stage and for the completion of sexual part of life cycle. Intermediate or secondary host occurs only in the case of digenetic parasite for the completion of larval stage, asexual reproduction and for transmission to the definitive host. Accidental host is the one which can function as normal host but is infected only occasionally because of some reasons for example due to the lack of exposer or means of transmission. Permissive host is either definitive, intermediate or accidental host which allow parasite to complete its life cycle part or the whole. Non-permissive host, on the other hand, is host organism other than true definitive host but receiving the parasite in the position of definitive host so that the parasite find itself in a dead end. Paratenic host or transport host refer to host organism other than true intermediate host but receiving the parasite in the position of intermediate host so that the parasite are helped to go to the definitive host. For example ''Echinococcus granulosus'' normally passes to the dog through intermediate host like goat, sheep, etc. But the parasite instead of passing to intermediate host, may come to infect human being and remain in causing hydatiditis, as dog has no chance to get it from man. Reservoir host are permissive host alternative to definitive host so that infective stage can be passed from the host to the population of definitive host. Vector is usually the intermediate host playing active role in the transmission of the parasite.
 
  
:In parasitism, there is always exploitation and harm. However, such an association has been continuing and there has been no record of species extinction because of parasitism. There are also examples where the parasitism help the host in fulfilling its purpose of life: existence and species continuation. Further, some parasitism result in something which has higher value than the both host and parasite together. ''Cordyceps sinesis'' is a fungus and infects in the head a type of underground caterpillar, in such a way that it finally  projects itself out of the head giving the appearance of half fungi and half caterpillar. The whole body is now known as [http://www.visitnepal.com/nepal_information/yarsagumba.php yarsagumba] (gold rush) and is very much valuable in medicine as a vitalizing agent. Further, when we carefully analyze, we find that all the human parasitic diseases are the result of failure of human responsibility.
+
:Ectoparasites, like leeches, ticks, and lice, are macroparasites occurring on the body surface of the host. Endoparasites, on the other hand, enter the body of the host via different means and remain in specific sites for a long period of time, such as intestinal parasites and [[blood]] parasites.  
  
=====Predation=====
+
:Temporary parasites (leeches, bed bugs) visit their host only for a short period of time. Permanent parasites spend the whole or a part of their life cycle in the host.  
:It is a relationship between two species but not an association and represents the obvious example of eating and being eaten process. For example, a tiger which hunts is called 'predator' and a deer which is being hunted is known as 'prey'. Even though, it is included in - + type of relationship, this relationship contribute for the well being of both predator and prey species. The predators get food and the preys are protected from mass collapse. This is a good example of individual sacrifice for species' existence and development.
 
=====Amensalism=====
 
:In this interspecific interaction, one species of microorganism like ''Penicillium'', by the secretion of antibiotics or antimicrobial metabolites inhibits the growth and development of other microorganisms, like bacteria, without being self affected. Because the first species avoids competitors of its resources in this way, this interaction is a - + type.
 
=====Competition=====
 
:Interspecific competition is the relationship between the populations of two species resulting from utilization of same resources of the same place and at the same time. This interaction may not need the direct confrontation, but still seems that each is hampering the another. Because of this, some authors have catagorized competition in - - type. However, this is an interaction to set demarcation between species for coexistence or mutual survival, as every time at least one of the involved sides gets benefit in terms of species development in the dynamic environment.
 
  
=====Allelopathic Interference=====
+
:Facultative parasites can survive without the parasitic mode of life, but can adapt to it if placed in such a situation; they are opportunistic. For example, the parasitic nematode, ''Strongyloides stercoralis'', can also be free living. The majorities of parasites are obligatory parasites and are totally dependent on the host for food, shelter, and/or protection; they cannot survive without the host. Accidental parasites are those that happen to infect unusual hosts, other than the normal definite host. Wandering or aberrant parasites, instead of arriving at the site of infection in the definitive host, reach an unusual place as a dead end and are unable to complete the life cycle. For example, the [[tapeworm]] ''Taenia solium'' may migrate to the [[brain]] and remain there unless removed via an operation.  
:This is similar to amensalism and competition. For example, desert plant, because of scarcity of nutrients and water, secrete certain chemicals and inhibits the seed germination of other plants in the vicinity. The interference can be both intraspecific as well as interspecific. Pine tree exhibits allelopathic interference by making the surrounding soil acidic.
 
  
==Conclusion==
+
:When a parasite is being parasitized by another organism, then the latter one is known as hyperpasasite or secondary parasite. It can be used for biological control of the pest and parasites. Monogenic parasites complete the whole life cycle in one host, such as with ''Entamoeba histolytica''. A digenetic parasite needs, in addition to a primary host, also a secondary host to complete the entire life cycle. ''Plasmodium vivax'' ([[malaria]] parasite) completes its asexual part of life cycle in people and the sexual part in the female ''Anopheles'' [[mosquito]].
There is no such categorization of symbioses in nature as described above. In fact, it is a continuum of different degree of intimacy and cooperation between the involved species. Therefore, demarcation line between the categories are artificial and many examples surpass the boundary, e.g., remosa and shark.
 
  
As it has been explained in the introduction, different species interact not only for their own benefit, but also to contribute to others. This is obvious in phoresis, commensalism and in mutualism. Mutual sharing reduces interspecific competition, avoids competitive exclusion as suggested Gause principle, promotes efficient use of resources as well as contributes to the dynamicity and diversity of the natural ecosystem, as more niches are created and new interactions are generated.
+
====='''''Types of Hosts'''''=====
 +
:A definitive host is usually the main host. For digenetic parasites, it is the host for the adult stage and for the completion of the sexual part of the life cycle. An intermediate or secondary host is a temporary environment, but one that is essential for the completion of a particular parasite's life cycle. Such a host is found only in the case of digenetic parasites for the completion of larval stage, asexual reproduction, and for transmission to the definitive host. An accidental host may be one that can function as the normal host, but is infected only occasionally for some reason, for example due to the lack of exposure or means of transmission.  
  
Even the - + type of associations or relationships should be considered as contribution to the existence of others even at one's own expense or species development at the expense of its individual instead of simply saying exploitation. If the exploitation of one by another leads to something like yarsagumba, then we should consider it as dictated by the directive force of nature to maintain dynamicity and diversity and ultimately to serve the purpose of human being.
+
:A permissive host is either a definitive, intermediate, or accidental host that allows the parasite to complete its life cycle in part or in whole. A non-permissive host, on the other hand, is a host organism other than true definitive host, which receives the parasite but the parasite finds itself in a dead end.  
  
In case of human parasites, we can see that it is the result of coupling of human beings failure to fulfill their responsibility and organisms inherent tendency to colonize new niches. Person practicing good hygienic life style does not get intestinal parasites. People with sexual discipline need not worry about HIV infection.
+
:A paratenic host or transport host is a host organism other than true intermediate host that receives the parasite in the position of intermediate host so that the parasite is helped to go to the definitive host. For example ''Echinococcus granulosus'' normally passes to a [[dog]] through an intermediate host, such as a [[goat]] or [[sheep]]. But the parasite, instead of passing through the intermediate host, may come to infect a [[human being]] and remain, causing hydatiditis, and a dog has no chance to get it from a person. Reservoir hosts are permissive host alternatives to definitive hosts, such that the infective stage can be passed from the host to the population of the definitive host. A vector is usually the intermediate host playing an active role in the transmission of the parasite.
  
==Further references==
+
:In parasitism, there is always exploitation and harm. However, such an association has been continuing, and there are no known cases of species [[extinction]] traced to parasitism. There are examples where the parasitism would appear to help the host in fulfilling a life purpose: existence and species continuation. Furthermore, some parasitism appears to result in something that has higher value than the both host and parasite together. ''Cordyceps sinesis'' is a fungus that infects a type of underground caterpillar in the head in such a way that it ultimately projects itself out of the head, giving the appearance of half fungi and half caterpillar. The whole body is now known as yarsagumba (gold rush) and is very valuable in [[medicine]] as a vitalizing agent.
[http://www.path.cam.ac.uk/~schisto/General_Parasitology/Old.Gen.Parasitology.html General Information on Parasitology With Particular Reference to Parasitic Helminths]
 
  
[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Symbiosis.html Symbiosis]
+
====Predation====
 +
:Predation is a close relationship between two species, but one that represents the example of eating and being eaten. For example, a [[tiger]] that hunts is called a "predator" and a deer that is being hunted is known as the "prey." Even though predation is included as a type -,+ interaction, in the broad view this relationship actually contributes to the well being of both predator and prey species. Predators get food and prey species are protected from the mass collapse that could occur if the prey species proliferates excessively beyond the ecosystem's carrying capacity. This is an example of individual sacrifice for species' existence and development. If symbiosis is viewed as a prolonged, intimate relationship between species, then this does not qualify as symbiosis because it is short-term.
  
[http://www.cals.ncsu.edu/course/ent591k/symbiosis.html Symbiotic Relationships]
+
====Competition====
 +
:Interspecific competition is the relationship between populations of two species resulting from utilization of the same resources in the same place and at the same time. This interaction need not be direct confrontation, yet each is hampering the other. Because of this, some authors have categorized competition as type -,-. Again, however, this should not be looked at as only a harmful relationship. Rather, this interaction sets a demarcation between [[species]] for coexistence or mutual survival, as at least one of the involved sides receives benefit in terms of species development in the dynamic environment. Similar to predation, if symbiosis is considered to be a prolonged, intimate relationship, then competition is not viewed as symbiosis because it is not an intimate association.
  
 +
====Allelopathic interference====
 +
:The term "allelopathic" refers to effect of one plant species on another. "Allelopathic interference" is a harmful effect of one plant on another. It is similar to amensalism (see below) and competition. For example, [[desert]] [[plant]]s, because of scarcity of nutrients and [[water]], may secrete certain [[chemical]]s and inhibit the seed germination of other plants in the vicinity. The interference can be both intraspecific as well as interspecific. Pine trees exhibit allelopathic interference by making the surrounding soil acidic.
  
 +
===Type 0,-. Amensalism===
 +
'''Amensalism''' is a biological interaction, a type of symbiosis, between two [[species]] in which one impedes or restricts the success of the other without being affected, positively or negatively, by the presence of the other. Usually this occurs when one organism exudes a chemical compound as part of its normal [[metabolism]] that is detrimental to another organism.
  
 +
An example is the bread mold ''[[Penicillium]]'' that produces the penicillin antibiotic, a chemical that kills [[bacteria]]. A second example is the [[Walnut|black walnut]] tree (''Juglans nigra''). Its roots secrete juglone, a chemical that often kills neighboring plants. Plants in certain [[biome|biomes]], such as the [[chaparral]] or [[desert]], are very dependent on the effects of amensalism. It stabilizes the community by reducing competition for scarce [[nutrients]] in the water.
  
 +
The term amensalism also refers to biological interactions in which there is not an intimate relationship between species (thus, not a symbiosis), such as when cattle trample grass on the way to a water hole to drink. The grass is damaged, but the cows are unaffected.
  
 +
Although amensalism is listed as a type 0,- interaction, most examples given for symbiosis are more a type +,- interaction. For example, in the cases of excreting a substance, the first species is able to limit competition for resources.
  
 +
==Conclusion==
 +
The categorization of symbiosis types (mutualism, parasitism, commensalism, etc.) is an artificial construct. In reality, biological interactions do not always fit into such discrete categories. Rather, symbioses should be perceived as a continuum of interactions ranging from [[parasitism]] to mutualism. Even the direction of a symbiotic relationship can change during the lifetime of the symbionts due to developmental changes, as well as changes in the biotic/abiotic environment in which the interaction occurs. Demarcation lines between categories are not always so clear-cut, and many examples surpass the boundary, such as seen between a remora and its [[shark]] host.
  
 +
What is notable is that there are no known symbioses where there is harm to both partners, and cases of symbioses where one partner is harmed and other is unaffected seem rare. This supports the view that interactions among species are aligned with the principle of [[Biology#Interactions: Harmony and dual purposes|dual purposes]]—living for the individual and for higher purpose, as mentioned above. [[Species]] interact not only for their own benefit, but also contribute to others. In the case of phoresis, commensalism, and mutualism, this is obvious. Mutual cooperation reduces interspecific competition, avoids competitive exclusion as suggested by the Gause principle, and promotes efficient use of resources, as well as contributes to the diversity of the natural [[ecosystem]], as more niches are created and new interactions are generated.
  
 +
However, even type -,+ associations or relationships can be viewed as more than simply exploitation of one species by another, but rather as a member of one species contributing to the existence of others even at one's own expense, or advancing species development at the expense of the individual. If exploitation leads to something helpful to humans,like yarsagumba, then it might be perceived as serving the purpose of the [[human being]].
  
Symbiosis may be divided into two distinct categories: [[ectosymbiosis]] and [[endosymbiosis]]. In ectosymbiosis, the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of [[exocrine]] glands. In endosymbiosis, the symbiont lives either in the intracellular space of the host or extracellularly.
+
In the case of human parasites, some tie this phenomenon to failure of human beings themselves. Other organisms naturally have an inherent tendency to colonize new niches. However, a person practicing an hygienic lifestyle, such as exhibiting care about water and food consumed (cooking meat properly, etc.) can limit the possibility of getting intestinal parasites. There also is a larger social responsibility tied to such actions as educating the public how parasites are transmitted, proper treatment fo sewage, and so forth.
 
 
An example of mutual symbiosis is the relationship between [[clownfish]] of the genus [[Amphiprion]] (family, [[Pomacentridae]]) that dwell among the tentacles of tropical [[sea anemone]]s. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the anemone fish from its predators (a special mucus on the anemone fish protects it from the stinging tentacles).
 
 
 
[[image:Goby_fish_with_shrimp.jpg|thumb|left|200px|Some [[goby|goby fish]] species live in symbiosis with a [[shrimp]].]]
 
Another example is the [[goby|goby fish]], which sometimes lives together with a [[shrimp]]. The shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind leaving it vulnerable to predators when above ground. In case of danger the goby fish touches the shrimp with its tail to warn it of imminent danger. When that happens both the shrimp and goby fish quickly retract into the burrow.
 
 
 
A famous land version of symbiosis is the relationship of the [[Egyptian Plover]] bird and the [[crocodile]].  In this relationship, the bird is well known for preying on [[parasite]]s that feed on crocodiles which are potentially harmful for the animal.  To that end, the crocodile openly invites the bird to hunt on his body, even going so far as to open the jaws to allow the bird enter the mouth safely to hunt. For the bird's part, this relationship not only is a ready source of food, but a safe one considering that few predator species would dare strike at the bird at such close proximity to its host.
 
 
 
The biologist [[Lynn Margulis]], famous for the work on [[endosymbiosis]], contends that symbiosis is a major driving force behind [[evolution]]. She considers Darwin's notion of evolution, driven by [[competition]], as incomplete, and claims evolution is strongly based on [[co-operation]], [[interaction]], and [[mutual dependence]] among organisms. According to Margulis and Sagan (1986), "Life did not take over the globe by combat, but by networking".  As in humans, organisms that cooperate with others of their own or different species often outcompete those that don't.
 
 
 
However, mutualism, parasitism, and commensalism are often not discrete categories of interactions and should rather be perceived as a continuum of interaction ranging from parasitism to mutualism. In fact, the direction of a symbiotic interaction can change during the lifetime of the symbionts due to developmental changes as well as changes in the biotic/abiotic environment in which the interaction occurs.
 
 
 
== See also ==
 
* [[List of symbiotic relationships]]
 
  
{{Biological_interaction-footer}}
+
Biologist Lynn Margulis, famous for her work on endosymbiosis, contends that symbiosis is a major driving force behind [[evolution]]. She considers [[Darwin]]'s notion of evolution, driven by competition, as incomplete, and claims evolution is strongly based on cooperation, interaction, and mutual dependence among organisms. According to Margulis and Sagan (1986), "Life did not take over the globe by combat, but by networking." As in humans, organisms that cooperate with others of their own or different species often outcompete those that do not.
  
 
==References==
 
==References==
* Lynn Margulis and Dorion Sagan, ''Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors.'' Summit Books, New York, 1986. ISBN 0520210646
+
*Margulis, L., and D. Sagan. 1986. ''Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors.'' New York: Summit Books.
* Jan Sapp ''Evolution by Association'', Oxford University Press, 1994. ISBN 0195088212
+
*Nair, S. 2005. Bacterial associations: Antagonism to symbiosis. In ''Marine Microbiology: Facets and opportunities'' (Editor: Nagappa Ramaiah). Goa, India: National Institute of Oceanography, pp. 115-124.
 +
*O'Hara, A., and F. Shanahan. 2006. The gut flora as a forgotten organ. ''EMBO Rep'' 7(7): 688-93. PMID 16819463.
 +
*Sapp, J. 1994. ''Evolution by Association''. Oxford University Press.
  
  
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[[Category:Life sciences]]
 
[[Category:Life sciences]]

Latest revision as of 07:43, 24 October 2022


Common Clownfish (Amphiprion ocellaris) in their magnificent sea anemone (Heteractis magnifica) home.

Symbiosis (plural: "symbioses") is the close, interactive association (living together) of members of two or more species. The term symbiosis was coined by the German botanist Anton de Bary in 1879 from the Greek symbioun (to live together), from the prefix sym (sum, together, or together with) and the word bios (life). In the narrowest sense of the term, as popularly used, symbiosis has been defined as the interactive association of two species in a mutually beneficial relationship. However, biologists define it according to the original, comprehensive concept of de Bary: the living together of members of two or more species in all the various forms, whether the interaction is beneficial, harmful, or neutral to one or both organisms.

In a symbiosis, the larger partner (macrospecies) is known as the “host,” whereas the smaller partner (microspecies) is known as the symbiont (plural: "symbionts") or symbiote (plural "symbiotes"). In the case of cattle and the cattle egret (a small white heron), large ruminants are the host and the cattle egret is the symbiont, feeding on the ticks and other ectoparasites of the former, as well as insects that the cattle disturb as they feed.

In nature, there are no organisms that do not interact at some level with both other organisms and the abiotic environment. While organisms tend to interact for their own survival, one or both of the interacting organisms often will also contribute to the benefit of other third parties—each organism has value for the larger ecosystem in which it is immersed. A typical interrelationship is expressed in the form of a food web showing the relationships of eating and of being eaten. In this relationship, one finds a concept of dual purposes for each being: the individual purpose (eating) and whole purpose (providing value to others). This principle of organisms having both individual and whole purposes is further explicated by the phenomena of symbiosis. (See Interactions:Harmony and dual purposes).

Types of symbioses

A. Classification based on location of symbionts relative to host

One manner of classifying symbioses is according to the physical location of the symbionts. Two types of symbioses are recognized:

Ectosymbiosis

In ectosymbiosis, symbionts occur on the body surface of the hosts. The body surface can be an external surface, such as the skin, but also includes internal surfaces, such as the linings of digestive canal and respiratory tract, the ducts of glands, and so forth. Examples include the attachment of a leech to the skin of a vertebrate and the presence of Escherichia coli on the mucosal layer of a person's intestine.

Endosymbiosis

In endosymbiosis, symbionts occur inside the cells or the tissue (intercellular space) of the host. A classical example of endosymbiosis is the presence of nitrogen fixing bacteria (Rhizobium sps.) living in root nodules of leguminous plants. Other examples include reef-building corals containing single-celled algae, and several insect species containing bacterial endosymbionts.
Many instances of endosymbioses are obligate, where neither the endosymbiont nor the host can survive without the other, such as "gutless marine worms" that get nutrition from endosymbiotic bacteria. However, not all endosymbioses are obligate. Furthermore, some endosymbioses can be harmful to both organisms involved.
One theory is that certain organelles of the eukaryotic cell, especially mitochondria and chloroplasts, originated as bacterial endosymbionts. This theory is known as the endosymbiotic theory, as postulated and popularized by Lynn Margulis.

B. Classification based on impact on symbiotic partners

Table 1 shows several types of symbioses based on the impact (positive, negative, or neutral) on the symbiotic partners.

Effect on Species A Effect on Species B Type of interaction
0 0 Neutralism
+ + Mutualism
+ 0 Commensalism
0 + Commensalism
+ - Parasitism or Predation
- + Parasitism or Predation
- 0 Amensalism
0 - Amensalism
Table 1. Some types of relationships listed by the effect they have on each partner. '0' is no effect, '-' is detrimental, and '+' is beneficial.

From the table, we can derive five main categories of symbiotic relationships among organisms. They are:

  • (Type 0,0). Neutralism is a lack of benefit or detriment experienced by either members of the pair of interacting organisms.
  • (Type 0,+). Commensalism occurs when one member of the association benefits while the other is not affected. Type 0,+ includes phoresis, which is the transport of one species by another.
  • (Type +,+). Mutualism occurs when the symbiotic association is advantageous to both members of the pair.
  • (Type -,+). Parasitism (as well as predation, competition, and allelopathic interference) is a situation in which the association is disadvantageous or destructive to one of the organisms and beneficial to the other.
  • (Type -,0). Amensalism is found when the association is disadvantageous to one member while the other is not affected.

There is one other theoretical category of biological interactions, but, if occurring, it would be rare and short-lived:

  • (Type -,-). Synnecrosis occurs when an interaction is detrimental to both species.

It is important to note that these interactions are not always static. In many cases, two species will interact differently under different conditions. This is particularly true in, but not limited to, cases where species have multiple, drastically different life stages.

Descriptions of types of symbiosis

Type 0,0. Neutralism

Neutralism is the term used in ecology to signify a biological interaction in which the population density of two species appear to have no effect whatsoever on each other. Examples of this type of symbiosis include pelicans and cormorants feeding in the ocean, spiders and mantis preying on insects of the same bush, diverse songbirds feeding and nesting together in a woodland, and a large number of microorganisms acting as residents in different body organs of human beings, such as skin, nose, mouth, and so forth.

A critical examination of each of the examples of neutralism leads to the conclusion that true neutralism is unlikely to exist and would be difficult, or impossible, to prove. It seems that living together—within the complex networks of interactions presented by ecosystems—would likely involve at least some degree of positive or negative impact of one species on the other. If two species do not interact directly, links can be found via other species and shared resources. Since true neutralism is rare or nonexistent, the term is often extended to situations where interactions are merely insignificant or negligible.

Type 0,+. Phoresis, commensalism

In a Type 0,+ symbiosis, the host is said to experience neither benefit nor harm, whereas the symbiont receives benefit, which ranges from simple transport to protection, food, and shelter.

Phoresis (Phoresy)

Phoresis (meaning "to carry") is the simplest form of type 0,+ symbiosis, occurring mainly among species sharing the same ecological niche. Phoresis involves transport of one organism by another. There is no physiological or biochemical dependence between the host and symbiont. Two organisms come in contact by chance, establish a fairly loose mechanical association, and the smaller one (known as the phoront) is carried by the transport host (mechanical vector). For example, food and water-borne disease organisms (viruses, bacteria, protozoans, and multicellular parasites) are transported from their source to the destination by flies and other insects because of direct contacts. Fungi and free living nematodes are carried by dung beetles, with which they share a common niche. The sucker fish Remora is equipped with a sucker disk on the dorsal side of its head. With the help of this disc, the fish attaches itself to the belly of larger fish so as to get benefit of faster movement of the latter. Because the Remora fish also can obtain some food scraps of the larger fish, this association can also be considered as commensalism.

Commensalism

Commensalism is a loose, usually facultative (neither dies if the relationship ends), and interspecific association between a larger "host" and a smaller "commensal," without any metabolic dependency. Only the smaller commensal derives benefits, such as sharing space, substrate, defense, shelter, transport, and/or food, and does so without helping or harming the host. In some cases, the interaction is obligatory for the commensal. Escherichia coli, and a variety other of bacteria, are harbored comfortably in the large intestine of people to feed on nutrients that are outgoing at any rate. There is no obvious harm or benefit to the host. (Although some benefits have been recognized as contributed by certain bacteria that live within the human digestive system, such as aiding immunity, synthesizing certain vitamins, and fermenting complex indigestible carbohydrates(O'Hara and Shanahan 2006)). Entamoeba gingivalis is also a commensal in the human mouth. Commensalism means "eating at the same table," graphically conveying a situation whereby the commensal is able to feed on food that is not being consumed by the host—offering benefit to the commensal without impacting the host.
Examples of metazoan commensals include barnacles on a whale and epiphytes on a tree. Sedentary adult barnacles attach to a whale, or other large moving objects, and get renewed sources of food and oxygen supply without giving causing obvious harm to the carrying host. Similarly, epiphytic plants like orchids and bromeliads grow perched on sturdier trees for better exposure to sunlight and for absorbing moisture and nutrients from the air.
Even though innumerable examples of commensalism have been recognized, a closer inspection suggests that true commensal relationships tends to be rather rare. For example, the pinworm in the large intestine of human beings has been considered as a commensal relationship. In reality, at the time of the pinworms reproduction, it intentionally causes itching in the anal region. Shrimp has been considered to be commensal on the sea cucumber, using it for passive transport. From time to time, the shrimp leaves the host for food. But it also has been pointed out that the commensal may feed on the parasites of the host body surface and also provide protection to the host because of its warning coloration. Thus, commensalism in many cases appears in reality to fall into the categories of either mutualism or parasitism.

Type +,+. Mutualism

Mutualism is the species-specific interaction of two organisms for mutual benefit. At times, the term symbiosis is used specifically for this type +,+ association, using the term synonymously with mutualism. However, this type of interaction is more appropriately termed mutualism, given the diverse associations to which biologists apply the term symbiosis.

Mutualism is a type of biological interaction that may range from loose to intimate, and from facultative to obligatory with both involved partners being called "mutuals." When the relationship is intimate and obligatory, each mutual is physiologically (metabolically) dependent on another for survival and reproduction. This means that members in the association derive key benefits from the other with respect to development, stimuli, nutrition, digestive enzymes, and maturation factors, such that neither of the mutuals can survive and reproduce successfully in the absence of the other. Examples of mutualism can be categorized as follow:

Nutritional mutualism

In "nutritional mutualism," both mutuals contribute to each other either simple organic nutrients, inorganic minerals, or digestive enzymes for the digestion process. Some important examples of nutritional mutualism are:
  • In nitrogen fixation, the bacteria Rhizobium sps. in the root and/or shoot nodules of legumes fix atmospheric nitrogen to make it available for plants. Meanwhile, the plants provide glucose and other organic nutrients, as well as a biological environment to the bacteria.
  • Mycorrhiza (fungus-root) has been defined as an apparent underground structure developed as a result of the mutualistic association between fungi (mainly basidiomycetes) and the roots of almost all plant species. There are different degrees of complexity in the association. Plants benefit by having enhanced root development and increased absorption of water and minerals. The fungus, in return, receives an organic food supply from the plants. Mycorrhiza can be used to enhance the growth of plants, even under poor soil conditions.
  • Syntrophy is the obligatory association of two microorganisms for mutual production of biochemical substances that the partners need for physiological processes. For example, sulfur reducing bacteria Desulfuromonas acetoxidans need sulfur for oxidizing acetic acid to produce carbon dioxide, hydrogen sulfide, and energy. Their partner organisms, green photosynthetic bacteria (Chlorobium), use the provided hydrogen sulfide for photosynthesis, producing a continuous supply of sulfur. Based on this principle of syntrophy, effective microorganism (EM) preparation is in use for enhancing all sorts of microbial processes.
  • About 50 percent of the food stuff of ruminants is composed of cellulose, which can be digested by the ruminants only with the assistance of protozoans (ciliates like Diplodinium sp. and Entodinium sp.) and obligate anaerobic bacteria (Ruminococcus albus, R. flavefaciens, etc.) lodged in the rumen (a special compartment) of their stomach. As a result of the anaerobic milieu, the bacterial fermentation produces fatty acids, carbon dioxide, and hydrogen gas that are used by methanogenic bacteria (Methanobacterium ruminantium) present there to produce methane. Thus, ruminants are an important source of methane in the atmosphere. Like ruminants, termites harbor protozoan (flagellates) and bacterial flora in their gastrointestinal (GI) tract for digestion of cellulose.
  • Lichen offers a common example of nutritional symbiosis. Lichens are symbiotic organisms made up by the association of microscopic green algae or cyanobacteria and filamentous fungi. They co-exist in an obligate and intimate, but ectosymbiotic, association. The thallus (body proper) of lichen is mainly made of a close network of fungal mycelium (mycobiont) and is responsible for absorption of water, minerals, and gases, as well as forming a protective tissue structure for algal partner. Algal cells (phycobiont), which constitute about 10 percent of the thallus, are embedded in the compact mass of mycelium and are responsible for photosynthesis, nitrogen fixation, and production of food for both partners.

Cleaning symbioses

Cleaning symbioses are facultative and loose associations of mutuals based on the principle that one's parasites are someone else's food. In this case, the larger mutual cooperates with and allows the smaller mutual to remove its ectoparasites, like leeches, ticks, and so forth—even from gills, inside of mouth, and nostrils. Some of the cleaning partners include larger fish and crustaceans or fishes; shark and remora fish; buffalo and oxpecker; rhinocherus and tick bird; and cattle and cattle egret.
A famous land version of symbiosis is the relationship of the Egyptian Plover bird and the crocodile. In this relationship, the bird is well known for preying on parasites that feed on crocodiles and that are potentially harmful for the animal. To that end, the crocodile openly invites the bird to hunt on its body, even going so far as to open the jaws to allow the bird to enter the mouth safely to hunt. For the bird's part, this relationship not only is a ready source of food, but a safe one considering that few predator species would dare strike at the bird at such close proximity to its host.

Transport symbioses

In transport symbiosis, one mutual takes advantage of the fast and secure movement of another mutual, whereas the latter receives mainly cleaning services. The Remora fish and shark, and the shrimp and sea cucumber, have already been described above. Another typical example of this association is the torsalo fly (Dermatobia hominis) making use of a small insect for transport of its eggs to the vertebrate host, including humans. The female parasitizes small flies and mosquitoes and glues its eggs underneath their bodies, then frees them to reach to the host. Upon contact with the warm skin of the host, the larvae come out of the eggs, penetrate the skin and develop underneath the skin of the host. This association can also be considered as a type of parasitism.

Pollination symbioses

Rufous humming bird on a flower.
Pollination symbioses refer to the mutual benefit exchanged between flower-visiting small birds, and insects like humming birds, butterflies and bees, with flower bearing plants, with the former receiving nectar while pollinating the latter.

Defense symbioses

An example of defense symbiosis is exemplified by the relationship between clownfish of the genus Amphiprion (family: Pomacentridae) that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the anemone fish from its predators. Special mucus on the clownfish protects it from the stinging tentacles.
Shrimp and goby fish.
Some goby fish species live in symbiosis with a shrimp. The shrimp digs and prepares a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind, leaving it vulnerable to predators when above ground. In case of danger, the goby fish touches the shrimp with its tail to warn it of imminent danger. When that happens, both the shrimp and goby fish quickly retract into the burrow.

From a survey of the biological world, it is clear that there are innumerable cases of viable mutualistic associations enabling organisms to explore diverse ecological niches. Mutualism even has enabled organisms to colonize stressful habitats and survive successfully even at self-insufficiency. Overall, this promotes the efficient and optimal use of biological features and natural resources.

Type -,+. Parasitism, predation, competition, allelopathic interference

This association involves obvious harm to one partner for the benefit of another. This type of relationship occurs in many forms.

Parasitism

Parasitism may be defined as an interspecific association in which one species, the parasite, lives on or in a second species, the host, for a significant period of its life and exploits the host to obtain nourishment, shelter, and/or protection. This relationship can be loose or mostly intimate, and for the parasite, it is usually obligatory.
As a parasite has to break through the different layers of defense mechanisms of the host in order to localize and thrive on the host, the parasite must be equipped with specialized mechanisms. The relationship, therefore, is strongly species specific, in other words, a particular parasite can infect only a particular host, not any other. The parasite causes harm to the host in many ways, such as by depriving the host of digested food; erecting mechanical blockades of food, blood, lymph, and bile passages; and causing tissue damages by rupture, enzymatic digestion, induction of autolysis, or secretion of toxins.
Types of Parasites
Macroparasites are multicellular parasites that are visible to the naked eye, such as helminth parasites (parasitic worms, such as flukes, tapeworms, and roundworms or nematodes). Mesoparasites are the ones that penetrate external openings, such as the buccal cavity, cloaca, external ear, and so forth. Microparasites are unicellular and invisible to the naked eye, such as protozoan parasites.
Ectoparasites, like leeches, ticks, and lice, are macroparasites occurring on the body surface of the host. Endoparasites, on the other hand, enter the body of the host via different means and remain in specific sites for a long period of time, such as intestinal parasites and blood parasites.
Temporary parasites (leeches, bed bugs) visit their host only for a short period of time. Permanent parasites spend the whole or a part of their life cycle in the host.
Facultative parasites can survive without the parasitic mode of life, but can adapt to it if placed in such a situation; they are opportunistic. For example, the parasitic nematode, Strongyloides stercoralis, can also be free living. The majorities of parasites are obligatory parasites and are totally dependent on the host for food, shelter, and/or protection; they cannot survive without the host. Accidental parasites are those that happen to infect unusual hosts, other than the normal definite host. Wandering or aberrant parasites, instead of arriving at the site of infection in the definitive host, reach an unusual place as a dead end and are unable to complete the life cycle. For example, the tapeworm Taenia solium may migrate to the brain and remain there unless removed via an operation.
When a parasite is being parasitized by another organism, then the latter one is known as hyperpasasite or secondary parasite. It can be used for biological control of the pest and parasites. Monogenic parasites complete the whole life cycle in one host, such as with Entamoeba histolytica. A digenetic parasite needs, in addition to a primary host, also a secondary host to complete the entire life cycle. Plasmodium vivax (malaria parasite) completes its asexual part of life cycle in people and the sexual part in the female Anopheles mosquito.
Types of Hosts
A definitive host is usually the main host. For digenetic parasites, it is the host for the adult stage and for the completion of the sexual part of the life cycle. An intermediate or secondary host is a temporary environment, but one that is essential for the completion of a particular parasite's life cycle. Such a host is found only in the case of digenetic parasites for the completion of larval stage, asexual reproduction, and for transmission to the definitive host. An accidental host may be one that can function as the normal host, but is infected only occasionally for some reason, for example due to the lack of exposure or means of transmission.
A permissive host is either a definitive, intermediate, or accidental host that allows the parasite to complete its life cycle in part or in whole. A non-permissive host, on the other hand, is a host organism other than true definitive host, which receives the parasite but the parasite finds itself in a dead end.
A paratenic host or transport host is a host organism other than true intermediate host that receives the parasite in the position of intermediate host so that the parasite is helped to go to the definitive host. For example Echinococcus granulosus normally passes to a dog through an intermediate host, such as a goat or sheep. But the parasite, instead of passing through the intermediate host, may come to infect a human being and remain, causing hydatiditis, and a dog has no chance to get it from a person. Reservoir hosts are permissive host alternatives to definitive hosts, such that the infective stage can be passed from the host to the population of the definitive host. A vector is usually the intermediate host playing an active role in the transmission of the parasite.
In parasitism, there is always exploitation and harm. However, such an association has been continuing, and there are no known cases of species extinction traced to parasitism. There are examples where the parasitism would appear to help the host in fulfilling a life purpose: existence and species continuation. Furthermore, some parasitism appears to result in something that has higher value than the both host and parasite together. Cordyceps sinesis is a fungus that infects a type of underground caterpillar in the head in such a way that it ultimately projects itself out of the head, giving the appearance of half fungi and half caterpillar. The whole body is now known as yarsagumba (gold rush) and is very valuable in medicine as a vitalizing agent.

Predation

Predation is a close relationship between two species, but one that represents the example of eating and being eaten. For example, a tiger that hunts is called a "predator" and a deer that is being hunted is known as the "prey." Even though predation is included as a type -,+ interaction, in the broad view this relationship actually contributes to the well being of both predator and prey species. Predators get food and prey species are protected from the mass collapse that could occur if the prey species proliferates excessively beyond the ecosystem's carrying capacity. This is an example of individual sacrifice for species' existence and development. If symbiosis is viewed as a prolonged, intimate relationship between species, then this does not qualify as symbiosis because it is short-term.

Competition

Interspecific competition is the relationship between populations of two species resulting from utilization of the same resources in the same place and at the same time. This interaction need not be direct confrontation, yet each is hampering the other. Because of this, some authors have categorized competition as type -,-. Again, however, this should not be looked at as only a harmful relationship. Rather, this interaction sets a demarcation between species for coexistence or mutual survival, as at least one of the involved sides receives benefit in terms of species development in the dynamic environment. Similar to predation, if symbiosis is considered to be a prolonged, intimate relationship, then competition is not viewed as symbiosis because it is not an intimate association.

Allelopathic interference

The term "allelopathic" refers to effect of one plant species on another. "Allelopathic interference" is a harmful effect of one plant on another. It is similar to amensalism (see below) and competition. For example, desert plants, because of scarcity of nutrients and water, may secrete certain chemicals and inhibit the seed germination of other plants in the vicinity. The interference can be both intraspecific as well as interspecific. Pine trees exhibit allelopathic interference by making the surrounding soil acidic.

Type 0,-. Amensalism

Amensalism is a biological interaction, a type of symbiosis, between two species in which one impedes or restricts the success of the other without being affected, positively or negatively, by the presence of the other. Usually this occurs when one organism exudes a chemical compound as part of its normal metabolism that is detrimental to another organism.

An example is the bread mold Penicillium that produces the penicillin antibiotic, a chemical that kills bacteria. A second example is the black walnut tree (Juglans nigra). Its roots secrete juglone, a chemical that often kills neighboring plants. Plants in certain biomes, such as the chaparral or desert, are very dependent on the effects of amensalism. It stabilizes the community by reducing competition for scarce nutrients in the water.

The term amensalism also refers to biological interactions in which there is not an intimate relationship between species (thus, not a symbiosis), such as when cattle trample grass on the way to a water hole to drink. The grass is damaged, but the cows are unaffected.

Although amensalism is listed as a type 0,- interaction, most examples given for symbiosis are more a type +,- interaction. For example, in the cases of excreting a substance, the first species is able to limit competition for resources.

Conclusion

The categorization of symbiosis types (mutualism, parasitism, commensalism, etc.) is an artificial construct. In reality, biological interactions do not always fit into such discrete categories. Rather, symbioses should be perceived as a continuum of interactions ranging from parasitism to mutualism. Even the direction of a symbiotic relationship can change during the lifetime of the symbionts due to developmental changes, as well as changes in the biotic/abiotic environment in which the interaction occurs. Demarcation lines between categories are not always so clear-cut, and many examples surpass the boundary, such as seen between a remora and its shark host.

What is notable is that there are no known symbioses where there is harm to both partners, and cases of symbioses where one partner is harmed and other is unaffected seem rare. This supports the view that interactions among species are aligned with the principle of dual purposes—living for the individual and for higher purpose, as mentioned above. Species interact not only for their own benefit, but also contribute to others. In the case of phoresis, commensalism, and mutualism, this is obvious. Mutual cooperation reduces interspecific competition, avoids competitive exclusion as suggested by the Gause principle, and promotes efficient use of resources, as well as contributes to the diversity of the natural ecosystem, as more niches are created and new interactions are generated.

However, even type -,+ associations or relationships can be viewed as more than simply exploitation of one species by another, but rather as a member of one species contributing to the existence of others even at one's own expense, or advancing species development at the expense of the individual. If exploitation leads to something helpful to humans,like yarsagumba, then it might be perceived as serving the purpose of the human being.

In the case of human parasites, some tie this phenomenon to failure of human beings themselves. Other organisms naturally have an inherent tendency to colonize new niches. However, a person practicing an hygienic lifestyle, such as exhibiting care about water and food consumed (cooking meat properly, etc.) can limit the possibility of getting intestinal parasites. There also is a larger social responsibility tied to such actions as educating the public how parasites are transmitted, proper treatment fo sewage, and so forth.

Biologist Lynn Margulis, famous for her work on endosymbiosis, contends that symbiosis is a major driving force behind evolution. She considers Darwin's notion of evolution, driven by competition, as incomplete, and claims evolution is strongly based on cooperation, interaction, and mutual dependence among organisms. According to Margulis and Sagan (1986), "Life did not take over the globe by combat, but by networking." As in humans, organisms that cooperate with others of their own or different species often outcompete those that do not.

References
ISBN links support NWE through referral fees

  • Margulis, L., and D. Sagan. 1986. Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors. New York: Summit Books.
  • Nair, S. 2005. Bacterial associations: Antagonism to symbiosis. In Marine Microbiology: Facets and opportunities (Editor: Nagappa Ramaiah). Goa, India: National Institute of Oceanography, pp. 115-124.
  • O'Hara, A., and F. Shanahan. 2006. The gut flora as a forgotten organ. EMBO Rep 7(7): 688-93. PMID 16819463.
  • Sapp, J. 1994. Evolution by Association. Oxford University Press.


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