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:
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.
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.
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
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.
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.
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:
* Termite fungus gardens
* Cockroach endosymbionts
* Ant mimics (inquilines)
* Slavemaker ants
* Gall insects
* Torsalo (Human bot flies): think back to my parsite problem
* Scelionid wasps
* Nectar guides
* Yucca moths
* Bumblebees and scotch broom
* Fig wasps
* Pseudocopulation in orchids
* 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
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
What generalizations can be made about mutualism, to give us a more
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
Theory of mutualisms must incorporate resource-use dynamics
are most frequent in stressful habitats
i. E.g., tropical dry forests, severely stressed seasonally by water
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)
more complicated than just positive feedback, cooperation, or altruism.
Also mutualisms, alone, do not necessarily stabilize interaction of
Here are examples
of symbiotic relationships:
of mutualisms: obligate
"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."
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.
From a web site
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.
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.
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
SEM of lichen:
the linear fungal hyphae and the roundball-like algal groupings.
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
- 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