Oct 29 2013
The First Farmers-Case Study
The fungus-growing ants in the tribe Attini started cultivating fungi for food at least 50 million
years before the first human farmers.
The ant farmers nourish, protect, and eat the fungal species they grow, forming a relationship
that benefits both the farmer and the crop.
The ants cannot survive without their fungi; many of the fungi cannot survive without the ants.
A virgin queen leaves her mother's nest to begin a new colony, carrying fungi from her birth
colony. The fungi are cultivated in underground gardens. A colony may contain hundreds of
gardens, each the size of a football; they can feed 2–8 million ants.
Other species such as leaf cutter ants do not cultivate fungi found in the environment. Instead,
the fungi in their gardens comes only from propagules passed from a parent ant to each of its
descendant colonies. Leaf-cutter ants cut bits of leaves from plants and feed them to the fungi.
The ants chew the leaves to a pulp, fertilize them with their own droppings, and “weed” (pull
out bacteria and things that invade and compete with crop) the fungal gardens to help control
bacterial and fungal invaders. In response, the fungi produce specialized structures called
gongylidia, on which the ants feed.
Both ants and fungi benefit from the relationship (unholy alliance) Each partner helps the other
overcome the formidable defenses that protect plants from being eaten.
The ants scrape a waxy covering from the leaves that the fungi have difficulty
penetrating. (processing leaf material into something fungi can access/digest)
The fungus digests and detoxifies the chemicals that plants use to deter insect
herbivores. (allowing ants to feed)
Nonresident fungi, pathogens, and parasites can sometimes
invade the colonies. What prevents invaders from destroying the
gardens? Ants increase weeding rates & also enlist help from
In the 1990s, a parasitic fungus (Escovopsis) was
discovered that attacks the fungal gardens of leaf-cutter
ants. The parasite can be transmitted from one garden
to another, and rapidly destroys the gardens, leading to
death of the ant colony
Ants respond to Escovopsis by increasing garden
They also enlist the help of other species. The ants carry
a bacterium that makes chemicals that inhibit Escovopsis. The bacteria also secrete compounds that promote the growth of the
cultivated fungi. The bacteria also benefit. They get a place to live (in specialized
structures called crypts on the ant’s exoskeleton), and a source of food (glandular
secretions) from the ants. Thus, the bacterium is a third mutualist. (third species
benefitting/contributing to relationship)
Positive interactions occur when neither species is harmed and the benefits of the interaction
are greater than the costs for at least one species.
Facilitation is a synonym for positive interactions. (defines mutualism and commensalism as a
Mutualism: Mutually beneficial interaction between individuals of two species (+/+). (not always
Commensalism: Individuals of one species benefit, while individuals of the other species do not
benefit and are not harmed (+/0).
During a flood, a mouse hops onto a frog (frog has no benefit) More realistic
commensalism is birds hanging around herbivores. Herbivores kick insects up which
birds eat and benefit from. Herbivore is just a bystander
Symbiosis: A relationship in which the two species live in close physiological contact with each
other, such as corals and algae or humans and bacteria (bacteria in our gut which is largely
Symbioses can include parasitism (+/–), commensalism (+/0), and mutualism (+/+).
The benefits of positive interactions can take many forms. In mutualism and commensalism, the
growth, survival or reproduction of one or both species is increased by their interaction with the
species may provide its partner with food/shelter or a substrate to grow on
reduce heat/water stress
reduce negative effects of competitors/herbivores/predators/parasites
Sometimes there is a cost to one or both partners, but the net effect is positive.
(cost benefit analysis) For each species, the benefits are greater than the costs.
Mutualism and Commensalism are Ubiquitous
Mutualistic associations are everywhere.
Most plants form mycorrhizae: Symbiotic associations between the roots and various fungi.
These are usually mutualistic. (fungal filaments help roots extend in soil helping take up water
and nutrients from soil; fungi may protect from pathogens; fungus in turn gets carbs) The fungi increase the surface area for the plant to take up water and soil nutrients (over 3 m of
fungal hyphae may extend from
1 cm of plant root).
There are two major types of
fungus grows between
root cells and forms a
the root. The hyphae
extend short distances
into the soil.
Arbuscular mycorrhizae: The fungus grows into the soil, extending away from the root;
and also penetrates into some of the plant root cells. (tree like structure) It grows
between some root cells while penetrating cell walls of others. Most angiosperms and
gymnosperms form these associations.
Mutualistic associations are found in many other organisms and habitats. Corals form a
mutualism with symbiotic algae.
The coral provides the alga with a home, nutrients (nitrogen and phosphorus), and
access to sunlight. The alga provides the coral with carbohydrates produced by
All of the numerous invertebrate and vertebrate species that live in and around coral
reefs depend directly or indirectly on this mutualism.
Herbivores such as cattle and sheep depend on bacteria and protists that live in their guts to
help metabolize cellulose. (eat woody materials full of cellulose. They can't actually digest
cellulose, but mutualists in gut help with metabolism). Wood-eating insects also have gut
protists that can digest cellulose. (wood eating cockroach has protist in gut helping it digest
cellulose so it can eat wood)
Commensalism is also everywhere.
Millions of species form +/0 relationships with organisms that provide habitat. A species
that depends on the habitat often has little effect on the species that provides the
Lichens that grow on trees or bacteria on your skin. (no consequence for tree).
In forests, many species depend on the trees for habitat, and do no harm to the
Evolution of Mutualism and Commensalism
Different types of ecological interactions can evolve into commensalism or mutualism.
For example, a lichen that grows on a trees leaves may initially harm the tree by
reducing its access to light. However, over time this relationship may evolve towards a
commensalism if the tree gains the ability to tolerate the lichens presence.
Mutualism can arise from a host–parasite interaction. In a strain of Amoeba proteus that was infected by a bacterium, the bacterium initially
caused the host to be smaller, grow slowly, and often killed it.
However, parasites and hosts often coevolve. Five years later, the bacterium had
evolved to be harmless to the amoeba; the amoeba had evolved to be dependent on
the bacterium for metabolic functions.
Neither species could survive without the other! (anecdotal example of how this
relationship forms mutualism)
Mutualism and Commensalism-Broad Set of Interactions
Some positive interactions are highly species-
specific, and required/obligate (not optional
for either species).
The leaf cutter ants and fungus cannot
survive without each other, and both
have evolved unique features that
benefit the other species.
Tropical figs are pollinated by fig
wasps. Neither species can reproduce
without the other. The wasps and the
figs have coevolved with regards to
reproductive strategies. Fig flowers are
contained within the receptacle. In
monoecious figs (separate male and
female flowers) the male and female flowers are located in different parts of the
receptacle, and male flowers mature after the female flowers. Some of the female
flowers have short styles and some have long styles. The wasps have complex
reproductive behaviors in the fig receptacle that ensures pollination. A female fig wasp
enters the receptacle carrying pollen from male flowers in another receptacle. Once
inside, the wasp inserts her ovipositor through the styles to lay her eggs in the ovary,
but the wasp can't effectively lay eggs in long style. Therefore, larvae are typically only
formed within short styled flowers. The wasp deposits pollen on the stigmas of the short
and long stem flowers. After seeds are produced, females get pollen from male flowers
before leaving the receptacle and start the process again.
Many mutualisms and commensalisms are facultative (not obligate) and show few signs of
coevolution. In a commensalism, the relationship is always facultative for the species that does
not benefit. In addition, coevolution does not occur as natural selection does not act on a
species that does not benefit from the relationship.
In deserts, the shade of adult plants creates cooler, moister conditions (compared to soil
of an adjacent open area). Seeds of many plants can only germinate in this shade. The
adult is called a nurse plant.(provides microclimate for other plants to establish). One
species of nurse plant may protect the seedlings of many other species. Desert ironwood serves as a nurse plant for 165 different species. Most of the
species it nurses can also germinate and grow under other plant species. This is
typical of facultative interactions. A species that requires nursing may be found
under a variety of nurse plant species. The nurse plant and the beneficiary
species may evolve little in response to one another.
Facultative interactions are also seen in forests. Large herbivores such as deer or moose
may accidentally consume seeds of herbaceous plants as they eat the leaves. Many
seeds pass through unharmed, and are deposited with feces. Thus, it becomes a
dispersal mechanism. Such interactions are sporadic and facultative; there is little
evidence to suggest that the species have coevolved.
The below diagram shows how far deer can travel compared to ants (red line
shows max distance) with regards to Trillium seed dispersal. The graph shows
where seeds are dropped by deer. The trillium benefits greatly by deer due to
dispersal (carries seeds far in gut) The deer benefits because it is eating the