Mutualism:

Mutualism is a positive reciprocal relationship between two species. Through this relationship both species enhance their survival, growth or fitness. To a certain extent the relationship is more a reciprocal exploitation rather than a cooperative effort on the part of the individuals involved. (Smith, Ecology & Field Biology).

Mutualism can take on many forms:

Symbiosis: in which both organisms live together in closely proximity, and in which both generally derive benefit. The relationship is obligate, meaning at least one of the species must be involved in the relationship to survive.

Non-symbiotic mutualism: the species do not live together, nor are dependent on each other; the relationship is facultative or opportunistic but does profit the organisms when together.

Many mutualistic relationships have been documented.

The wood termite-protozoa relationship, the yucca-moth relationship & ant acacia described below courtesy of the web sites are common examples given in textbooks.

How can mutualism evolve?

Most agree that mutualistic relationships evolved from negative associations ( predator prey, parasitism etc.).Basically the organism being negatively impacted had two options: escape the relationship or adapt to it, and in the process make the relationship more advantageous to itself.

A potential example is (fungal) mycorrhizae- initially they may have been parasitic on the roots they inhabited. -However in those couplings where mineral nutrients leached from the fungal tissue to the plant host resulting in better survival of the plant, more carbohydrate were then available for the fungus. Eventually a truly mutually beneficial association evolved. 

Birds, bats & insects who visited plants for a number of reasons and in the process picked up pollen, allowed those plants hosted a greater opportunity for genetic diversity. If enhanced outcrossing lead to higher reproductive success, those plants who encouraged visitors with enticements of nectar, pollen or pseudo-mating opportunities naturally increased in frequency over time.

Mutualism may also be defined by a functional approach:
Nutritional Symbiosis:
* Termite fungus gardens
* Cockroach endosymbionts
Shelter Symbiosis:
* Ant mimics (inquilines)
* Slavemaker ants
* Gall insects
Transport Symbiosis:
* Torsalo (Human bot flies): think back to my parsite problem
* Scelionid wasps
Pollination Symbiosis:
* Nectar guides
* Yucca moths
* Bumblebees and scotch broom
* Fig wasps
* Pseudocopulation in orchids
Defensive Symbiosis
* Ants and acacias
* Aphid farmers

Theory of mutualism
1. This is a relatively poorly studied ecological interaction



Alternative way to present this is as:

dN1/dt = r1N1[(K1-N1+a12N2)/K1]
dN2/dt = r2N2[(K2-N2+a21N1)/K2];

where all variables are same as in logistic model, except for a21 is mutualistic per capita effect of species 1 on species 2, and a21 is effect of species 1 on species 2.

Behavior of model? Very simplistic, this leads to an "orgy" of spiraling upwards populations of both species involved in mutualism .Such spiraling population abundances do not actually occur in nature, so this model must be substantially unrealistic, and inapplicable to nature.

Better models not yet developed.
What generalizations can be made about mutualism, to give us a more realistic picture?

The need for mutualism (and thus the benefit) decreases with increased resource availability.
i. Examples: Leguminous plants like alders dominate in nitrogen-poor environments, because legumes frequently have nitrogen-fixing bacteria as mutualists; mycorrhizal fungi in nutrient poor soils (phosphorus limiting)


Theory of mutualisms must incorporate resource-use dynamics
Mutualisms are most frequent in stressful habitats
i. E.g., tropical dry forests, severely stressed seasonally by water shortages)
ii. Thus, theory of mutualism must increase life-history characteristics, and how these provide negative feedback against simple population expansion of both participants in mutualistic relationship


Penalties accrue to mutualists that provide more resources to partner than necessary; one would expect natural selection to favor just enough contribution by mutualists to other species involved to maintain mutualism, and no more--i.e., selection for some "optimum level" of participation (e.g., plants that produce nectar just sweet enough to attract pollinator, but no sweeter so as not to waste energy & metabolic products)

Mutualisms are more complicated than just positive feedback, cooperation, or altruism.
Also mutualisms, alone, do not necessarily stabilize interaction of two species

Here are examples of symbiotic relationships:

II. Examples of mutualisms: obligate nonsymbiotic relationship

Ant-acacia mutualism:

"In this relationship found most commonly in Central America savannas, the ant hollows out the large thorns of the plant for nests, feed on sweet secretions from the four nectaries at the base of each petiole and on the protein rich Beltian bodies found on the tips of the leaves, which together provide an almost complete diet for the ant. The ants in return protect these trees from invertebrate as well as vertebrate herbivores. With any movement of the branch, the ants emerge releasing a nasty odor as well as physically attacking the surprised herbivore. They are quite effective.


African ants and acacia trees get along great: The ants live in the acacia's special swollen thorns and pay the tree "rent" by attacking leaf-eating insects. But the ants steer clear of bees and other insects that pollinate the acacia's flowers, allowing the tree to reproduce, which in turn keeps alive the symbiotic relationship. Now scientists know why the ants turn up their feelers at pollinators: The tree exudes a chemical that tells ants to keep away. The findings, reported in Nature, show how a plant has evolved a way to thwart a potential conflict with a symbiotic insect. Studying acacia trees in Tanzania, ecologists Pat Willmer of theUniversity of St. Andrews in Fife, the United Kingdom, and Graham Stone of the University of Oxford observed that Crematogaster ants seem to avoid crawling over young, fresh flowers but not older ones that had already been pollinated. They were puzzled until they realized that on rainy days, "the effect seemed to disappear," Willmer recalls, and the ants would patrol new flowers as well. Thinking the young flowers might be making a water-soluble repellent, Willmer rubbed a young flower on an old one. The ants avoided that older flower. The researchers are still trying to identify the warning compound, although they speculate that pollen from the acacia blossom might be it. The bottom line, says Willmer, is that "the plants can manipulate the insects to do what they want."

The temporary repellent is particularly ingenious because it ends up maximizing the number of seeds the acacia can produce. After pollination, when the repellent wears off, the renewed presence of the ants protects the developing seeds from being eaten, says Ted Schultz, an entomologist at the Smithsonian Institution's National Museum of Natural History inWashington, D.C. This work is among the first to demonstrate conflict resolution in plant-animal interactions, he adds. "But there are probably all sorts of conflicts and controls [in such symbiotic relationships]. This is probably just the tip of the iceberg."

Backward evolution?

From ENN: Parasitic ants ; In a study that may help define the line between a mutualistic interaction and a parasitic one researchers at the University of California, Davis, have been studying a species of African ants that are killing the acacia trees that host them. Many specialized plant-ant species live cooperatively with their hosts; the plants house and feed the ant colony, while the ants protect their hosts from herbivores, pathogens and competitors. Not so with the African ant C. nigriceps. Maureen Stanton, a professor of evolution and ecology, says their results suggest that the selfish pruning behavior has evolved because it increases the life span of C. nigriceps colonies, even though it removes all the host tree's flowers and stops the tree from reproducing. The study was published in the Oct. 6 issue of the journal Nature.

Termites protozoa:

From a web site on termites.......

"Termites eat wood, a lot of wood. But this dietary preference for cellulose is most unusual because cellulose, the macromolecules forming the cell walls of green plants, is a tough, insoluble carbohydrate,a potential sources of energy but indigestible by all but a few animals. Termites, cockroaches, cows and other grazers can use it only because their guts contain tens of thousands of microorganisms which convert cellulose to sugars, usable by both microorganism and host. Termites are much more efficient than cows and other grazers; they remove undigested cellulose from cow pies.

Termites eat dead plant material and animal dung, thereby removing this litter from the surface of the land, permitting sunlight and moisture to reach new growth. On its own, dung and other organic plant material decomposes slowly in a dry environment. Without subterranean termites to break it down, the dry litter would cover the land.

When dead plant material is broken down inside a termite's gut, carbon and minerals (N, P, S) are released. These nutrients are used by the insect and its gut flora, or returned to the soil, where they can again be recycled. In these ways subterranean termites are responsible for most of the cycling of carbon and other nutrients in a desert or desert grassland.

As subterranean termites build their nests and foraging galleries, they greatly improve the fertility and productivity of the soil. In plots of soil from which they had chemically excluded termites, scientists found that water infiltrated much more slowly, and that the soil was more dense and stored less water than in plots which contained termites. Foraging galleries around dead grass stems and other food items are made with material brought up from deep in the soil. These galleries eventually erode and are added to the surface soil - at a rate of 44 kilograms per hectare (about 40 pounds per acre) per day, according to one study. Over time, the turnover of soil significantly affects the content and even the creation of soil. "

Lichens:an obligate symbiotic relationship & mycorrhizae

The basic structure of a lichen is a mass of fungal hyphae; imbedded in this mass is a zone of algae .

  • 25+ different algal species are involved in associations, with the majority of them green algae (although some species are cyanobacteria ( blue-greens)).
  • The fungus partner itself is generally an ascomycete, although again many different species of fungi can form this relationship.
  • The fungi gain nutrition from the photosynthetic algae while the fungi house and supposedly protect the algae from the elements providing moisture, perhaps protection from the sun and a source of minerals.
  • There is some dispute how mutualistic the relationship is. There is a fine line between the role of protector and hostage holder. It may be, that as the algae can do well on their own that the relationship may be less obligate, though certainly intimate. Nutrients may be simply leaking out of the algae; it may be that the fungi is benignly parasitizing the algae.

    SEM of lichen: the linear fungal hyphae and the roundball-like algal groupings.


    Mycorrhizae is the relationship between a fungus and a higher plant's root system. In this relationship, the plant feeds the fungus, while the fungus supplies the plant with mineral nutrients ( especially phosphorous) and according to some sources additional moisture.

    (Note red inclusions in root cells - these are the endomycorrhizae living in parenchyma cells)

     

  • In endomycorrhizae, the fungus actually penetrates the root cells, forming a network in the root itself. In ectomycorrhizae, the fungus develops a mantle about the root that extends into the soil and internally about the cells. The relationship is critical in nutrient deficient soil, with the fungi aiding in the absorption of the nutrients as well as the breakdown of decomposing materials. The fungi also aid the plant in defending it against pathogen invasion by preventing carbohydrates from leaching out through the root thus attracting potential invaders.
  • This relationship is so important, that some researchers believe the the association formed early in evolution, allowing the first land plants to survive on a soiless, nutrient poor landscape.
  • When reestablishing forests in areas decimated by intense logging or forest death due to pollution ( from copper smelting for example) seedlings are first inoculated with spores of symbiotic fungal species to aid in successful reintroduction.