In most relationships, a “third wheel” could be a major problem but if you happen to be an ant or a Whistling-thorn tree, the stability of your symbiotic relationship may depend on the pressure of a third party. 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 inhabiting them) should be under less stress and should thrive. So why were the trees doing so poorly? The experiments spawned by this question would eventually suggest that what was previously considered to be relatively simple symbiotic interaction between ants and trees was actually far more complex.

The Whistling-thorn tree (Vachellia drepanolobium – formerly Acacia drepanolobium) is a dominant plant species in the heavy-clay soils of upland East Africa and combines structural defenses (thorns) with ant mutualisms in a strategy to deter grazing by large herbivores like elephants and giraffes. The Whistling-thorn produces either slender stipular thorns (at the base of their leaves) or swollen hollow thorns known as domatia which serve to house resident ant colonies. To sweeten the deal, the tree also secretes a carbohydrate-rich nectar from glands near the base of the leaves which provide it’s ant defense forces with nutritional support.

The researchers in Kenya noticed that under normal environmental conditions, four different species of ant take advantage of this arrangement, but in different ways. Crematogaster mimosae is the most prevalent, occupying ~52% of the trees. It is an aggressive defender of its host trees and relies heavily on the host tree’s domatia as spaces for colony housing and on the necataries for nutrition. Crematogaster gerstaeckeri sjostedti (formerly C. sjostedti) occupies ~16% of the trees and is an less aggressive defender of its hosts. Furthermore, rather than using the domatia as nest space, C. g. sjostedti instead resides in the cavities excavated by the tree-boring larvae of long-horned beetles (Family: Cerambycidae).

Crematogaster nigricpes occupies ~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 which curb canopy spread and reduce the likelihood of their host trees coming in contact with other vegetation which may be occupied by hostile colonies. Lastly, Tetraponera penzigi occupies ~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 nutrition from the host tree’s nectaries, T. penzigi actively destroys them and instead appears to subsist on small food items on the surface of its hosts.

Now while that may be the case under natural conditions, the researchers noticed something far different within the space devoid of large herbivore grazers. Rather than developing the usual amount of nectaries and domatia, the Whistling-thorns in the enclosure appeared to be reducing their investment in supporting their resident ants. This obviously had a negative impact on the C. mimosae and C. sjostedti colonies which relied on those structures for their colonies’ well-being. Furthermore, they noticed that the trees housing the “pruning species”, C. nigriceps were not experiencing a reduction in domatia or nectaries suggesting that destructive pressures imposed by large herbivores (or by “gardening” ants ) is what stimulates the production of domatia and nectaries in the first place. Therefore, without the usual need to defend as vigorously against herbivorous fauna, the trees not longer produced material for the ants and the previously amicable relationship between the symbiotic species began to deteriorate.

The local number of C. mimosae colonies dropped by 30% (since they relied most heavily on the nectar and domatia) and a new dominant ant species arose in the form of C. g. sjostedti. This was 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 essentially under widespread attack by their previously mutual symbionts and they began to lose their vigor and/or die as a consequence.

Thus, it became very clear that the ant-tree symbiosis in this case was certainly not an isolated ecological development. It was intimately linked to the behavior of the regions large herbivores who seem to have facilitated this relationship in the first place. Without that ecological third-wheel, the relationship with the most mutual benefit for all the involved species began to collapse. It makes you wonder what other symbiotic interactions are structured this way and provides a powerful justification for the preservation of ecosystems in their entirety.

Further Reading