In most relationships, a third wheel is a major problem, but if you happen to an ant or a whistling thorn tree, the stability of your symbiosis may depend on the pressure from an outsider.
This is what researchers Todd Palmer, Maureen Stanton, Truman Young, Jacob Goheen, Robert Pringle, and Richard Karban discovered circa 2008 while working in an enclosed research site in Kenya (part of the Kenya Long Term Enclosure Experiment). The researchers noticed that the Whistling-thorns 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 trees was actually far more complex.
Whistling-thorns Attract and Support Ants
The Whistling-thorn 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.
The Whistling-thorn goes all out to attract and support its resident ant. These trees produce slender stipular thorns (at the base of their leaves) and swollen hollow thorns known as domatia which serve to house resident ant colonies. To sweeten the deal, the tree also secretes carbohydrate-rich nectar from glands near the base of the leaves to further attract and sustain its ant defense forces.
The Symbiotic Ants
The researchers in Kenya found that, under normal environmental conditions, four different species of ants take advantage of this arrangement, but in different ways.
Crematogaster mimosae were the most prevalent, occupying ~52% of the trees. It is an aggressive defender of host trees and relies heavily on the host tree’s domatia as spaces for colony housing and on the nectaries for nutrition.
Crematogaster gerstaeckeri sjostedti (formerly C. sjostedti) occupied ~16% of the trees and is a less aggressive defender of its host. Additionally, rather than using the domatia as nest space, C. g. sjostedti instead resides in cavities excavated by the tree-boring larvae of long-horned beetles (Family: Cerambycidae).
Crematogaster nigricpes occupied ~15% of the trees and is competitively inferior to the former two ant species. As such, it has adopted a strategy of pruning axillary buds and apical meristems to curb canopy spread and reduce the likelihood of their host trees coming in contact with other vegetation. Consequently, that means that there is less of change of their tree coming in contact with a tree occupied by hostile colonies.
Lastly, Tetraponera penzigi occupied ~17% of the trees and employs what the researchers referred to as a “scorched-earth strategy” to deter the more competitive nectar-dependent Crematogaster species from moving in. Rather than derive any nutrition from the host tree’s nectaries, T. penzigi actively destroys them and instead subsists on small food items on the surface of its hosts.
Destabilizing the Balance
Now while the above observations may be the case under natural conditions, the researchers noticed something far different within the habitats devoid of large herbivore grazers. Rather than developing the usual amount of nectaries and domatia, the Whistling-thorns in the enclosure reduced their investment in supporting their resident ants.
This obviously had a negative impact on the C. mimosae and C. sjostedti colonies which rely on those structures for their colonies’ well-being.
The researchers also noticed that the trees housing the “pruning species”, C. nigriceps did not reduce their domatia or nectaries. That suggests that the destructive pressures imposed by large herbivores (or by “gardening” ants ) are what stimulates the production of domatia and nectaries in the first place.
Without the usual need to defend as vigorously against the structural damage created by large herbivores, the trees no longer had a need to expend energy and materials on attracting and supporting ants. What was once an amicable relationship between the symbiotic species rapidly deteriorated. They needed their third wheel.
Things Start to Get Ugly
The local number of C. mimosae colonies dropped by 30%. That’s no surprise because they relied most heavily on the nectar and domatia. A new dominant ant species subsequently rose to fill the gap: C. g. sjostedti.
That turned out to be very bad news for the trees because C. g. sjostedti actively facilitates the invasion of Cerambycidae beetles whose destructive tree-boring behavior creates space for C. g. sjostedti nests. Furthermore, without their previously abundant source of nutritious nectar, many of the remaining C. mimosae colonies adopted a new strategy of tending destructive sap-sucking homopteran scale insects in order to sustain their colonies.
The whistling thorns were now being indirectly attacked by their previously mutual symbionts and they began to lose their vigor and/or die.
It became very clear that the ant-tree symbiosis, in this case, was not an isolated ecological development. It was a delicate arrangement dependent on the region’s large herbivores. Without that ecological third-wheel, a relationship that had a decently mutual benefit for all the involved species collapsed.
This discovery really drives home how intertwined environments are and puts particular emphasis on the importance of fields like ecology which study those interactions. It also provides a powerful justification for the preservation of ecosystems in their natural states, rather than as walled gardens dictated by the incomplete perspectives of man.
References and Further Reading
- Hocking, B. (1970). Insect associations with the swollen thorn acacias. Transactions of the Royal Entomological Society of London, 122(pt. 7).
- Palmer, Todd M., et al. “Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna.” Science 319.5860 (2008): 192-195.
- Palmer, T. M., Young, T. P., Stanton, M. L., & Wenk, E. (2000). Short-term dynamics of an acacia ant community in Laikipia, Kenya. Oecologia, 123(3), 425-435.
- Wardle, D. A., Bardgett, R. D., Klironomos, J. N., Setälä, H., Van Der Putten, W. H., & Wall, D. H. (2004). Ecological linkages between aboveground and belowground biota. Science, 304(5677), 1629-1633.
- Young, T. P., Stubblefield, C. H., & Isbell, L. A. (1996). Ants on swollen-thorn acacias: species coexistence in a simple system. Oecologia, 109(1), 98-107.