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A good example of mutually beneficial symbiosis, or mutualism, can be found in the relationship between bull's horn acacias (Acacia cornigera) and pseudomyrmex ants (also known as acacia ants), found in Central America and Mexico. The acacia, which bears thick pairs of horn-like thorns at the base of each leaf, provides a home and nutrients for the colonies of small ants. The ants not only make their nests inside the thorns, but also get much of their sustenance from the oval structures on the tip of each leaf. These structures are known as Beltian bodies, after naturalist Thomas Belt, who first reported on this relationship. They are extremely rich in nutrients, almost like yeast.

Until Belt's discovery of the nutrient-rich Beltian bodies, it was largely assumed that the ants were simply taking advantage of the safe harbour provided by the thorny acacia. However, later studies have shown conclusively that the ants play a major role in the acacia's growth, by protecting the plant from the predations of large mammals and phytophagous insects. The worker ants also clear away other, competing plants from the acacia's vicinity - plants which could otherwise overshade the acacia, which needs direct sunlight to survive.

D. H. Janzen, who gathered most of the early evidence regarding this symbiosis, believed that a bull's horn acacia shoot must be occupied by the pseudomyrmex ants for a substantial portion of its life in order to survive and reproduce, just as the acacia ants depend on its shelter and the Beltian bodies to compete with other insect species. This has since been proven by numerous studies. It also seems that no less than twelve varieties of pseudomyrmex ants have independently developed similar relationships with other types of plants in different parts of the world.


Sources:
  • "Insect Behavior", Robert W. Matthews and Janice R. Matthews
  • http://carbon.cudenver.edu/~gcronin/acacia.htm
  • http://entomology.ucdavis.edu/faculty/ward/antplant.html

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