Acacia drepanolobium
Encyclopedia
Acacia drepanolobium, commonly known as Whistling Thorn (family
Fabaceae
), is a swollen-thorn acacia
native to East Africa
. The whistling thorn grows up to 6 meters tall. It produces a pair of straight thorns at each node, 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 the observation that when wind blows over bulbous thorns in which ants have made entry/exit holes, they create a whistling noise.
Whistling thorn is the dominant tree in some areas of upland East Africa, sometimes forming a nearly monoculture woodland, especially on "black cotton" soils of impeded drainage with high clay content. It is browsed upon by giraffes and other large herbivores. It is apparently fire-adapted, coppicing readily after "top kill" by fire.
Whistling thorn is used as fencing, tool handles, and other implements. The wood of the Whistling Thorn, although usually small in diameter, is hard and resistant to termites. The branches are also be used for kindling, and its gum is sometimes collected and used as glue. The ability to coppice after cutting make it a possibly sustainable source for fuel wood and charcoal. Conversely, Whistling Thorn also has been considered a weed of rangelands, and a bush encroachment species.
s that have formed a mutualistic relationship with some species of ants. In exchange for shelter in the bulbous thorns (domatia
) and nectar secretions, these ants appear to defend the tree against herbivore
s, such as elephants and giraffes, as well as herbivorous insects.
At one site in Kenya, four ant species compete for exclusive possession of individual whistling thorn trees: Crematogaster mimosae, C. sjostedti, C. nigriceps, and Tetraponera
penzigi. Ants vary in their level of mutualism with whistling thorn trees. The most common ant symbiote (~ 50% of trees), C. mimosae, has the strongest mutualistic 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. Crematogaster nigriceps ants trim the buds of trees to reduce lateral growth in trees, thereby reducing chances of contact with a neighboring tree. Tetraponera penzigi, the only species which does not utilize the nectar produced by the trees, instead 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.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Fabaceae
Fabaceae
The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...
), is a swollen-thorn acacia
Acacia
Acacia is a genus of shrubs and trees belonging to the subfamily Mimosoideae of the family Fabaceae, first described in Africa by the Swedish botanist Carl Linnaeus in 1773. Many non-Australian species tend to be thorny, whereas the majority of Australian acacias are not...
native to East Africa
East Africa
East Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
. The whistling thorn grows up to 6 meters tall. It produces a pair of straight thorns at each node, 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 the observation that when wind blows over bulbous thorns in which ants have made entry/exit holes, they create a whistling noise.
Whistling thorn is the dominant tree in some areas of upland East Africa, sometimes forming a nearly monoculture woodland, especially on "black cotton" soils of impeded drainage with high clay content. It is browsed upon by giraffes and other large herbivores. It is apparently fire-adapted, coppicing readily after "top kill" by fire.
Whistling thorn is used as fencing, tool handles, and other implements. The wood of the Whistling Thorn, although usually small in diameter, is hard and resistant to termites. The branches are also be used for kindling, and its gum is sometimes collected and used as glue. The ability to coppice after cutting make it a possibly sustainable source for fuel wood and charcoal. Conversely, Whistling Thorn also has been considered a weed of rangelands, and a bush encroachment species.
Symbiosis with ants
Like other acacias, Whistling Thorns have leaves that contain tannins, which are thought to serve a deterrents to herbivory. Like all non-Australian acacias, they are defended by spines. In addition, Whistling thorn acacias are myrmecophyteMyrmecophyte
Myrmecophyte is a plant that lives in a mutualistic association with a colony of ants. There are over 100 different genera of myrmecophytes. These plants possess structural adaptations that provide ants with food and/or shelter. These specialized structures include domatia, food bodies, and...
s that have formed a mutualistic relationship with some species of ants. In exchange for shelter in the bulbous thorns (domatia
Domatia
Domatia are tiny chambers produced by plants that house arthropods.Domatia differ from galls in that they are produced by the plant rather than being induced by their inhabitants...
) and nectar secretions, these ants appear to defend the tree against herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
s, such as elephants and giraffes, as well as herbivorous insects.
At one site in Kenya, four ant species compete for exclusive possession of individual whistling thorn trees: Crematogaster mimosae, C. sjostedti, C. nigriceps, and Tetraponera
Tetraponera
The genus of ants Tetraponera is in the family Formicidae and are characterised by their arboreal nature and slender bodies. There are 86 described species of Tetraponera all of them living in hollow structures of plants and trees, such as thorns or branches, these hosts are known as myrmecophytes...
penzigi. Ants vary in their level of mutualism with whistling thorn trees. The most common ant symbiote (~ 50% of trees), C. mimosae, has the strongest mutualistic 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. Crematogaster nigriceps ants trim the buds of trees to reduce lateral growth in trees, thereby reducing chances of contact with a neighboring tree. Tetraponera penzigi, the only species which does not utilize the nectar produced by the trees, instead 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.