Relationship Advice: Acacia Trees and Ants

http://www.marietta.edu/~biol/biomes/tropdry.htm 

Introduction:

If one is to seek relationship advice from nature, the acacia tree and acacia ants are the ones to consult! Not only do they live in very close proximity – one literally lives within the other – but both partners also depend heavily on each other for fitness. Many species of acacia tress that are deficient in chemical defenses have developed a mutualistic relationship with stinging ants in which protection is exchanged for nutrients and a home [1]. Acacia trees and their symbiotic partner can be found all over the world in temperate, desert, and tropical regions, especially since some species of acacia trees are highly invasive [2]. They reach sexual maturity typically three years after germination, and the adult trees can be used for industrial or decorative purposes [3]. During development, the acacia trees form symbiotic relationships with ants to promote healthy growth for both the ant and the tree. Not only are the trees vigorously protected, but they also provide ants and their larvae a ready home and available nutrients.     

http://www.youtube.com/watch?v=Li_tdhxMs90 

Description of the Relationship:

Some species of acacia trees, like the Acacia macrantha of Central American, produce bitter alkaloids to ward off predators [1]. However, for those acacia species that are deficient in chemical defenses, acacia ants act as aggressive protectors. For example, Acacia cornigera (commonly known as the bullhorn acacia) found in Mexico and Central America house Pseudomyrmex ferruginea in swollen, hollow thorns [1]. The ants, with their powerful stingers, protect the trees against destructive insects, feeding herbivores, and competing vines and vegetation that pose a threat to overcrowding and sunlight availability. In return, the tree provides shelter and nutrition [4]. Ants live within the swollen, hollow thorns of the tree, and the tree produces nutrients: protein-lipid Beltian bodies from its leaflet tips and carbohydrate-rich nectar from glands on its leaf stalk [1].

http://earthlysolutions.wordpress.com/2012/02/08/biological-communities-and-species-interactions/
A hollow, swollen thorn: home for the ant larvae.

http://www.alexanderwild.com/Ants/Taxonomic-List-of-Ant-Genera/Pseudomyrmex/8709998_bQRJk8/575648985_FG4cj#!i=575648985&k=FG4cj
A Beltian Body. 

The acacia trees that produce alkaloids grow slower than the ant-protected acacia trees because of the allocation of resources and energy. Additionally, they must grow in drier habitats where the competing vegetation grows slower since they do not have the ant gardeners [1]. Therefore, evolutionarily, this mutualistic association developed as a means to increase the fitness of both partners. Ant-plant mutualism is not rare with at least 100 other species of plants and ants exhibiting this relationship [5]. Not only can the acacia trees grow faster without any threat from insects, herbivores, and other plants, but the ants can also develop with a plentiful source of nutrition and shelter.

The mutualistic relationship is established when a newly mated queen is attracted to a tree by its odor and starts nesting inside the large, hollow acacia thorns. She lays 15-20 eggs to produce the first generation of workers. As the colony grows, more thorns become inhabited, and when the colony reaches around 400 individuals, the ants start to protect the plant [5]. The mutualistic relationship develops at this time, when both the tree and the ants benefit from the other. The association between these two partners perfectly describes mutualism because, as Claude Combes describes in our text, this bond develops “with reciprocal benefits” (11) [6].

Cost/Benefit Analysis:

In this mutualistic association, the benefits obviously outweigh the costs. The benefits to the trees include protection from feeding insects and herbivores and from competing vegetation. Costs include energy and resources used to make the nutritious Beltian bodies for the ants. They have no known function other than to provide food for the ants [1]. Acacia drepanolobium (commonly known as the whistling thorn acacia) in South Africa is called home by Crematogaster mimosa, but the tree does not provide Beltian bodies, so the ants have to forage for food elsewhere. Therefore, the ants allow insects to feed on the tree and do not provide aggression protection [1]. However, because the benefits of mutualism provide an increase to fitness, some acacia trees have developed a means to maintain the mutualistic relationship. Most plant species contain sucrose in their nectar and sap, but acacia trees provide their ants with nectar containing digested sucrose (glucose and fructose). Therefore, through evolution, some acacia ants have lost the gene encoding for the enzyme required to digest sucrose, invertase [4]. This means that these ants are bound to the tree since the only food source they can utilize is the acacia tree nectar.

Similar to the costs for the tree, the cost for the ant includes energy usage in providing protection and defense for the tree. The benefits, however, include shelter and a readily available source of nutrition. The ants require less energy in finding a suitable home for their larvae to develop, and they are not required to forage extensively for food. Therefore, the energy used in protecting the tree is compensated for by the energy saved in other tasks, so the benefits outweigh the costs. The same logic applies for the acacia tree. Less energy is devoted to producing chemical defenses, so more energy can be applied to producing food for the ants. Therefore, this mutualistic relationship works amazingly well!

References:

[1] Wolffia. "Central American Swollen-Thorn Acacias."Wayne's Word. Web. 22 Mar 2012. <http://waynesword.palomar.edu/acacia.htm>.

[2] Brennan, John. "Relationship Between Acacia and Ants." eHow. eHow, Web. 22 Mar 2012. <http://www.ehow.com/facts_6977979_relationship-between-acacia-ants.html>.

[3] Richards, Bailey. "Acacia Development." eHow. eHow, 11 Apr 2011. Web. 22 Mar 2012. <http://www.ehow.com/facts_7986996_acacia-development.html>.

[4] Cheshire. "Cheshire: Insects, Evolution, and Random Splatters from the Windshield of the Blogosphere." 26 Feb. 2009. Web. 22 Mar. 2012. <http://tachinid.wordpress.com/2009/02/26/perhaps-i-should-start-asking-acacia-trees-for-relationship-advice/>.

[5] Piper, Ross. Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Westport, CT: Greenwood Press, 2007. 1-3. Print.

[6] Combes, Claude. Parasitism: The Ecology and Evolution of Intimate Interactions; Translated by Isaure De Buron and Vincent A. Connors; with a New Foreword by Daniel Simberloff. Chicago: University of Chicago, 2001. Print.