Acacia drepanolobium
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Acacia drepanolobium, commonly known as Whistling Thorn (family Fabaceae), is a swollen-thorn acacia native to East Africa.[1] The whistling thorn has an average height of 2 meters and is covered with long thorns, some of which have large bulbous bases — these swollen thorns are naturally hollow and occupied by any one of several symbiotic ant species. The common name of the plant is derived from this: when wind blows over bulbous thorns in which ants have made entry/exit holes, they create a whistling noise.[2]
Whistling thorn is the dominant tree in some areas of upland East Africa, sometimes forming a nearly monoculture woodland.[3][4] It is browsed upon by giraffes and other large herbivores. The wood of the Whistling Thorn, although small in dimension, is hard and resistant to termites.[1][5] It is used as fencing, tool handles, and other implements. The branches are also be used for kindling, and its gum is sometimes collected and used as glue. It's ability to coppice after cutting make it a possibly sustainable source for fuel wood and charcoal.[6] Conversely, Whistling Thorn also has been considered a weed of rangelands, and a bush encroachment species.[7][8]
[edit] Symbiosis with ants
Like all acacias, Whistling Thorns have leaves that contain tannins, which are thought to serve a deterrents to herbivory, and like all non-Australian acacias, they are defended by spines.[9] In addition, Whistling thorn acacias have formed a mutualistic relationship with some species of ants. These ants apparently defend the tree against herbivores in exchange for shelter in the bulbous thorns (domatia) and nectar secretions.
In Kenya, there are four predominant species of ant which compete for exclusive possession of whistling thorn trees: Crematogaster mimosae, C. sjostedti, C. nigriceps, and Tetraponera penzigi.[4] Ants vary in their level of symbiosis with whistling thorn trees. The most common ant symbiote (~ 50% of trees), C. mimosae, has the strongest symbiotic relationship, aggressively defending trees from herbivores while relying heavily on swollen-thorns for shelter and feeding off of nectar produced by glands near the base of leaves.
Because the ants compete for exclusive usage of a given tree, some species employ tactics to reduce the chance of a hostile ant invasion. C. nigriceps ants trim the buds of trees to reduce lateral growth in trees, thereby reducing chances of contact with a neighboring tree. T. penzigi, the only species which does not utilize the nectar produced by the trees, destroys the nectar glands in order to make a tree less appealing to other species.
The symbiotic benefits the trees provide to the ants is maintained by the effects of browsing by large herbivores; in the absence of being browsed, trees reduce the number of nectar glands and swollen buds they provide to ants. In response, the usually dominant C. mimosae increase their farming of parasitic sap-sucking insects as a replacement food source. In addition, the number of C. mimosae-occupied trees declines while twice as many become occupied by C. sjostedti, a much less aggressive defender of trees. Because C. sjostedti is dependent on the holes created by boring beetle larvae, this species facilitates parasitism of trees by the beetles. As a result, the mutualistic relationship between whistling thorn trees and resident ants breaks down in the absence of large herbivores, and trees become paradoxically less healthy as a result.
[edit] References
- ^ a b Acacia drepanolobium (Whistling Thorn). ZipcodeZoo.com. Retrieved on 2008-01-18.
- ^ Whistling Thorn. Retrieved on 2008-01-18.
- ^ Young, T.P.; Cynthia H. Stubblefield, Lynne A. Isbell (December 1996). "Ants on swollen-thorn acacias: species coexistence in a simple system". Oecologia 109 (1): 98-107.
- ^ a b Palmer, Todd; Maureen L. Stanton, Truman P. Young, Jacob R. Goheen, Robert M. Pringle, Richard Karban (January 2008). ""Breakdown of an Ant-Plant Mutualism Follows the Loss of Large Herbivores from an African Savanna"". Science 319 (5860): 192-195.
- ^ The Whistling Thorn. Retrieved on 2008-01-18.
- ^ Okello, B.D., O’Connor, T.G. & Young, T.P. (2001). "Growth, biomass estimates, and charcoal production of Acacia drepanolobium in Laikipia, Kenya.". For. Ecol. & Manag. 142: 143-153.
- ^ Pratt, D.J. & Gwynne, M.D. (1977). Rangeland Management and Ecology in East Africa. Hodder & Stoughton, London.
- ^ Dall, G., Maass, B.L. & Isselstein, J. (2006). "Encroachment of woody plants and its impact on pastoral livestock production in the Borana lowlands, southern Oromia, Ethiopia.". Afr. J. Ecol. 44: 237-246.
- ^ Ward, D. & T.P. Young (2002). "Effects of large mammalian herbivores and ant symbionts on condensed tannins of Acacia drepanolobium in Kenya". Journal of Chemical Ecology 28: 913-929.
