Life Science

The Complicated Relationship Between Ants and Whistling Thorn Acacia Trees

In most relationships, a third wheel is a major problem. If you happen to be a symbiotic ant colony in a relationship with a whistling thorn acacia tree (Vachellia drepanolobium), however, an ecological triangle might hold the key to your long-term success.

That is the conclusion reached by researchers Todd Palmer, Maureen Stanton, Truman Young, Jacob Goheen, Robert Pringle, and Richard Karban following a 2008 study in Kenya. The researchers were working in an enclosed research site within the Kenya Long Term Enclosure Experiment. They noticed that the whistling thorn trees in an area devoid of large herbivores were beginning to die.

Now, that was really surprising. If anything, the lack of large herbivores should mean that the plants (and the ant communities they supported) should be under less stress and should thrive. So why were the trees doing so poorly? The experiments launched by that initial question eventually suggested that what was previously considered to be a relatively simple symbiotic interaction between ants and whistling thorn trees was actually far more complex.

Vachellia drepanolobium whistling thorn acacia tree details of leaves and domatia.
Vachellia drepanolobium photographed near Nanyuki, Kenya.
Wikimedia Commons (http://goo.gl/pK8oMZ)

Whistling Thorn Acacia Trees Attract and Support Ants

The whistling thorn acacia tree (Vachellia drepanolobium – formerly Acacia drepanolobium) is a dominant plant species in the heavy-clay soils of upland East Africa. These trees combine structural defenses (thorns) with ant mutualisms in a strategy to deter grazing by large herbivores like elephants and giraffes.

These trees go all out to attract and support its resident ants. Aside from the usual slender stipular thorns (at the base of their leaves), the trees produce swollen hollow thorns known as domatia. Symbiotic ants subsequently use these oversized thorny structures as shelter and nesting space.

The benefits of residence don’t stop with shelter. Not only do they get rooms at this hotel, they get a continental breakfast. Glands near the base of the tree’s leaves secrete a carbohydrate-rich nectar. These glands are called nectaries and symbiotic ant colonies can use them as a reliable and substantial form of sustenance.

Ant Species Symbiotically Associated with Whistling Thorn Trees

In Kenya, under normal environmental conditions, the researchers identified four different species of ants in symbiotic relationships with whistling thorn trees. Though all the symbionts enjoyed a productive mutualism, they went about it in different ways.

Crematogaster mimosae were the most prevalent symbiotic ant species on whistling thorn trees in the study area. They occupied approximately 52% of the trees surveyed. This species of ant is an aggressive defender of its host whistling thorn trees. It relies heavily on the host’s domatia for colony housing and the nectaries for nutrition, so it must protect its vital holdings.

Crematogaster gerstaeckeri sjostedti ants (formerly C. sjostedti) occupied approximately 16% of the whistling thorn trees in the study area. This ant species is a less aggressive defender of its host compared to Crematogaster mimosae. Additionally, rather than using the domatia as nest space, Crematogaster gerstaeckeri sjostedti exploits spaces created by one of its host tree’s pests. The larvae of tree-boring long-horned beetles (Family: Cerambycidae) excavate large cavities in host trees. Those cavities are subsequently exploited by Crematogaster gerstaeckeri sjostedti as shelter.

Crematogaster mimosae ant frontal head view scientific photograph. Tetraponera penzigi ant frontal head view scientific photograph. These ants are mutual symbionts of whistling thorn acacia trees.
Crematogaster mimosae head view. Source: AntWeb (http://goo.gl/x9Djhe)
Crematogaster gerstaeckeri sjostedti ant frontal head view scientific photograph.Tetraponera penzigi ant frontal head view scientific photograph. These ants are mutual symbionts of whistling thorn acacia trees.
Crematogaster gerstaeckeri sjostedti head view. Source: AntWeb (http://goo.gl/aOoX03)

Crematogaster nigriceps ants occupied approximately 15% of the surveyed trees. This species will be handedly outcompeted by the other two species we’ve discussed so far. It needs to reduce the probability of coming in contact with rival colonies if it’s going to be successful. To accomplish that, Crematogaster nigriceps prunes the axillary buds and apical meristems of its whistling thorn host. This subsequently curbs canopy spread and reduces the likelihood of their host tree coming in contact with other vegetation which might harbor rivals.  

Lastly, Tetraponera penzigi ants occupied approximately 17% of the studied trees. This symbiotic ant species employs what the researchers referred to as a “scorched-earth strategy.” They targeted the part of the plan most important to their Crematogaster species rivals – the nectaries.  Rather than derive any nutrition from the host tree’s nectaries, Tetraponera penzigi actively destroys them! Instead, these ants subsist on small food items on the surface of their hosts.

Tetraponera penzigi ant frontal head view scientific photograph. These ants are mutual symbionts of whistling thorn acacia trees.
Crematogaster nigriceps head view. Source: AntWeb (http://goo.gl/wLcXB3)
Tetraponera penzigi ant frontal head view scientific photograph. These ants are mutual symbionts of whistling thorn acacia trees.
Tetraponera penzigi head view. Source: AntWeb (http://goo.gl/ly3iiZ)

Destabilizing the Symbiosis Between Ants and Acacias

While the former symbiotic situations apply to natural conditions, the unnatural conditions of the study area created a different dynamic. Rather than developing their usual amount of nectaries and domatia, the whistling thorn trees in the enclosure substantially reduced their energy investment in supporting resident ants.

The exception to that trend was the trees hosting the “pruning” ant species, Crematogaster nigriceps. Those whistling thorn trees did not reduce their production of domatia nor nectaries. That suggests that structural damage is the stimulus behind the trees’ production of domatia and nectaries in the first place. Under natural conditions, that destructive pressure is exerted by large grazing herbivores. In the enclosure, however, it only came from Crematogaster nigriceps.

A thorny domatia among leaves on a whistling-thorn tree.
A great view of thorny domatia on a whistling-thorn tree. Source: http://goo.gl/mMHpI7

Ecological Fallout

The consequences of disrupting the natural dynamic were dramatic.

The local number of Crematogaster mimosae ant colonies – the dominant symbiotic ant species – dropped by 30%. With fewer domatia and nectaries, this highly dependent species just couldn’t thrive. A new dominant symbiotic ant species subsequently rose to fill the gap: Crematogaster gerstaeckeri sjostedti. That turned out to be very bad news for the whistling thorn trees because Crematogaster gerstaeckeri sjostedti actively facilitates the invasion of those tree-boring larvae we mentioned earlier. If you recall, those larvae excavate the cavities used by Crematogaster gerstaeckeri sjostedti as nesting space.

The remaining Crematogaster mimosae colonies also became net-destructive. Without nutritious nectaries, they began tending destructive sap-sucking homopteran scale insects. They then harvested the rich honeydew produced by those pests as a substitue for the usual whistling thorn nectar.

With their symbiotic ants effectively facilitating their destruction, the whistling thorn trees began to lose their vigor and/or die. It became very clear that the symbiotic relationship between the ants and the trees was not a simple arrangement. It was a delicate ecological “triangle” dependent on the region’s large herbivores. Without that ecological third wheel, the mutual symbiosis collapsed.

This study really drives home how complex – yet subtle – the ecological relationships can be between species. It also puts particular emphasis on the importance of fields that study those interactions. Lastly, it lends some support to the idea that ecosystems are best preserved in their natural states, rather than as walled gardens built on the incomplete perspectives of man.

This post has been updated from an earlier version published in 2014.

References and Further Reading

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