In your Textbook

Begon et al. Chap. 12 and 13, pp. 429-521 and 212-213 Pianka Chap. 11, pp. 228-239 and 322-336 Stiling Chap. 8 and 12, pp. 228-239 and 315-328

An important part of an organism's environment is other organisms. The following table summarises the possible pair-wise interactions between species. The +, -- and o represent the change in fitness of one species as a result of the interaction with the other.

Interactions Between Species
		S p e c i e s   A
		Benefit (+)	Harm (-)	No Effect (o)
S p e c i e s   B	Benefit (+)	Mutualism/ Protocooperation	Parasitism/Predation	Commensalism
	Harm (-)	Parasitism/Predation	Competition	Amensalism
	No Effect (o)	Commensalism	Amensalism	Neutralism

Symbiosis

The concept and term symbiosis are attributed to the German mycologist Anton de Bary who, in 1879, defined it to cover any close interspecific association. Symbiosis literally means 'living together' and de Bary explicitly included commensalism, mutualism and parasitism. (see Ahmadjian and Paracar, Chap. 1, 1986; Harper, 1985)

For a long time, symbiosis was used to refer to mutualism exclusively. However, over the last thirty years the broader meaning has been re-established and that is used here.

Harper (1985) provided a figure in which he plotted a number of specific interspecific relationships on two axes. The x-axis ran from benefit to harm for the larger organism of the pair (the examples having been selected as ones where the smaller organism always benefits). The y-axis was a measure of increased intimacy or closeness of the association. The obviously non-symbiotic relationships are restricted to the area beneath a diagonal line from the center of the x-axis to half-way up the right y-axis.

This figure shows that there is continuus variation of both variables and that the benefit-harm axis is not the only one that describes a symbiotic relationship. On the graph, intimacy ranges from the lack of contact of a pair of competitors (not symbiosis) to the extreme intimacy of mitochondria and chloroplasts in eukaryotic cells (ancient mutualisms) and the Epstein-Barr virus in humans (parasitism). These extremely intimate relationships are also obligate to the beneficiary.

This introduces a third variable whether the relationship is opportunistic or facultative or obligate for one or both partners. Is the benefit essential for the completion of the life cycle of the organism? Does it enhance fitness? Can one species successfully complete its life cycle in the absence of the other? The answers to these questions are useful in determining the nature of the relationship. As are the other two variables. 

Neutralism

Neutralism is the situation where in a community there are two species populations which have absolutely no impact on the survival, fitness or any other aspect of the members of each other. It is unlikely that there are many, or any, such species pairs. To be true, this would require minimal interaction between the two and therefore can not be symbiosis.

Amensalism

In a relationship of this type, one species suffers and the other is unaffected. For example, an animal goes to a pond to drink. In its approach to the pond, it tramples the pond side vegetation. Although the plants may not be killed, their fitness will be affected. The animal gains no benefit and suffers no harm. This relationship is casual, transient and, most definitely, not intimate and, therefore, it is not symbiosis.

Something to think about

Man's actions routinely harm other life forms but have no effect on us.

Mutualism 

Mutualistic relationships may be subdivided based on whether the relationship is obligate or not. Both partners benefit, i.e. their fitness is enhanced, in all mutualistic relationships but the relationship is only obligate if one or both partners cannot complete their lifecycle in the absence of the other.

A few authors continue to restrict the term symbiosis to refer to obligate mutualism. If the relationship is faculatitive or opportunistic, i.e. neither species requires the other to complete its lifecycle, these authors use the term non-symbiotic mutualism. More generally, these cases are termed protocooperation with the implied suggestion that these are evolving relationships.

Another way to classify mutualisms, is to use three functional categories:  trophic, defensive, and dispersive

Trophic mutualisms involve partners specialised in complementary ways to obtain energy and mineral ions E.g. nitrogen-fixing bacteria and leguminous plants, mycorrhizas and lichens. Defensive mutualisms involve species that receive "food" or shelter from their partners in return for defending those partners against herbivory, predation, or parasites. E.g. Bull's horn Acacia and its ants. .

Dispersive mutualisms include animals transporting pollen for a nectar or other reward and animals that disperse seeds in the process of eating or digesting the nutritional fruits that contain those seeds.

However, not all mutualisms fit conviently into one of these categories.

Examples

There are many examples of plant-fungus mutualisms (see  Ahmadjian and Paracar, Chap. 7, 1986). These include lichens and mycorrhizas. Selosse and Le Tacon (1998) argued that these mutualisms provide various adaptations to terrestrial constraints and may have played a crucial role during terrestrialization and evolution of land phototrophs.
. Try this for another lichen view also see Hale (1974), Hawksworth and Hill (1984).   For more on mycorrhizas see: Allen (1991), Allen (1992), Harley and Smith (1983), Jackson and Mason (1984), Read et al. (1992), Smith and Read (1997).

Central American Swollen-Thorn Acacias

Other examples of ant-plant relationships which may be mutualistic include: The Cecropia-Azteca association in Costa Rica (Longino, 1997). Nick Plummer's Ant plants - Rubiaceae, Asclepiadaceae, Nepenthaceae, Bromeliaceae and Ferns.

Keeler (1985) provides a cost:benefit analysis of mutualism.

Commensalism

The term commensal was introduced by Pierre-Joseph van Beneden in 1876. He used it for associations in which one animal shared food obtained by another animal. The two animals ate from the "same table".

Parasitism

Yan et al. (1998)

 

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