Mutualism is a type of interaction between two different species in which the impact is positive for both individuals. This is thought to be a beneficial interaction. In a mutualistic partnership, sometimes the species reside within close proximity to each other, but this is not required and is not always the case.
For example, the relationship between humans and the bacteria which reside within their stomach, known as gut flora, is a mutualistic partnership. In this mutualism, humans are the host organism and benefit because these bacteria assist in facilitating digestion for foods which humans cannot digest on their own. Bacteria also benefit because they acquire nutrients in the host environment.
There are two common types of mutualistic interactions:
- Facultative Interactions: These are interactions which species do not require for survival. An example would be between pollinating birds and the flowers which they pollinate.
- Obligate Interactions: In this type of interaction, there is reciprocal dependency, which means that the species are dependent upon each other for survival. For example, there is a dependency between mycorrhizal fungi and plant roots. The fungi are essential to increasing the surface area available for absorbing water and nutrients for the plants (and potentially providing protection against pathogens). Conversely, the plants produce carbohydrates for the fungi through photosynthesis.
However, it is critical to mention that in the literature, there is research discussing the instability which exists between these partnerships due to species competing to gain the greater benefit in the relationship. This exploitation can be through cheating or parasitism. For instance, studies on mycorrhizal fungi and plants have observed that in fertilized, high nutrient soils, plants require less support from the fungi and thus, release fewer carbohydrates to the fungi. However, this causes the fungi to become parasitic, negatively impacting the structures of the plant. Thus, mutualistic relationships can be rather complicated.
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