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Mutualisms Part 1
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Includes the introduction to mutualism, commensalism, mutualism and resources, how it provides protection, and sanctions to prevent parasitic behaviour.
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EcologyLecture 5 Mutualisms 1 01/03/21
Mutualism
- An association between two species that benefits both.
- For example, a cleaner shrimp cleaning a zebra moray eel, or an oxpecker and an impala.
Commensalism
- One species benefits and the other is unaffected.
- For example, clownfish obtain protection from anemones, but the anemone gets nothing
out of the relationship.
Mutualism and resources
- Leaf cutter ants have a mutualism with a fungus.
- Ants collect bits of a leaf and chew it to a pulp. This is stored underground, where that
fungus grows.
- In turn, the fungus produces gongylidia – specialised structures that the ants eat. Ants can
digest these nutrients.
- Thus circumvent the chemical defences of the leaves which are digested by the fungus.
Other mutualisms and resources
- Nutrient fixing bacteria and archaebacteria in root nodules.
- Nitrogen is essential for growth in plants and animals, but most animals and plants cannot
fix atmospheric nitrogen into the useful forms of nitrate or nitrite.
- Legumes are peas, beans, clover, carob, peanuts, lupins etc.
- Rhizobia are nitrogen fixing bacteria – they are free-living within the soil. As a legume grows,
the rhizobia detect the flavonoids secreted by the roots.
- The rhizobia then attach to and eventually enter the root hairs of the legume plant.
- This results in cell proliferation by the plant – resulting in the formation of a root nodule.
- Inside the root nodule, the bacteria change morphologically into a bacteroid – in the soil it
was free living. In this nodule, it becomes bigger and longer and is non-motile.
- The bacteroids are taken up by plant cells in symbiosomes. This is a specialised compartment
in the plant cell.
- The symbiosome surrounds the bacteroid – separated by a symbiosomes space.
- The bacteroid fixes nitrogen into a chemically usable form of ammonium.
- The plant, in return, provides carbohydrates to the bacteroid.
Mycorrhizal fungi and plant roots
- Roots of higher plants are actually a mutualistic association between fungi and root tissue.
The fungi obtain carbohydrates from the plant. The plant obtains minerals extracted from
the soil by the fungi.
Mutualism and protection
- Examples include the ants and acacias.
- In the bull-horn acacia, the acacia produces horn-like thorns filled with a soft pithy material.
- Ants can hollow this out easily and obtain a nest.
- The plant benefits from the ants keeping herbivores away, and sometimes competing plants
too.
- In return, the acacia produces beltian bodies – protein rich food nodules at the ends of
leaves.
Obligate mutualism
- In some cases the mutualistic relationship is so tight that both parties die without the other.
- Example: lichens (a combination of algae and fungi).
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