Butterfly effects: Acacias, Elephants, and the Ants that guide their relationship

Acacia drepanolobium and Crematogaster nigriceps.
Image courtesy of Pharaoh han

It is sometimes difficult for people to imagine how the activities of minute creatures like ants can have major effects at the meter scale world, but a paper I read recently demonstrates such a process clearly.

In Kenya there is a tree called the whistling thorn acacia (Vachellia drepanolobium – formerly Acacia drepanolobium), an iconic shrub of the East African savanna that grows to 6 meters tall and is covered with intimidating thorns, some of which have bulbous bases and are called domatia.

The bulbous bases are hollow, and house symbiotic ants that protect the tree from browsing by herbivores. The tree also provides the resident ants nutrition in the form of sugary secretions from glands at the base of their leaves.

The tree houses one of four different species of ants: Crematogaster sjostedti,  C. mimosae, C. nigriceps, and Tetraponera penzigi, all of which swarm out to attack browsing animals with varying degrees of effectiveness, though the overall efficacy of the ants as guardians is quite high. As shown in the youtube video above, acacias with no ants tend to do very badly when large herbivores are present.

This relationship between the acacias, herbivores like elephants, and the ants that guard the acacias has recently been disrupted by the appearance of Pheidole megacephala (BHA) in the Laikipia region of Kenya within the last 10-15 years. In a research paper in the journal Ecology, Corinna Riginos and her colleagues moved complete trees along with their resident ants from uninfested locations to areas infested with BHA.

Almost immediately, BHA discovered the new trees and started moving up the trunk, only to be met in force by the aggressive Crematogaster species, who streamed down to meet and repel the invaders. In all such cases, however, the resident ants were fairly quickly pushed up the trunk and soon vanquished, their nests looted and any remaining adult defenders killed if they could not escape. The only exception was in trees with the Tetraponera ants, who instead of meeting the invaders head on, instead retreated into their domatia where the BHA could not get to them.

Acacia seedpods. Image courtesy of Chr. Kooyman

The researchers found that even 30 days after the invasion, with BHA patrolling throughout the entire tree, the Tetraponera ants managed to eke out an existence. Foragers who came out lay still and did not fight BHA ants who came to them, and the BHA ants did not seem to consider them to be hostile. In fact, the density of Tetraponera colonies significantly increased in invaded areas, due to the removal of their Crematogaster competitors.

Unfortunately for the acacia trees, the Tetraponera is the least effective defender against encroaching herbivores, and BHA itself does not attack large mammals who decide to browse on the plant. This resulted in significant damage to the host acacias, with the researchers finding up to a seven fold increase in the number of trees catastrophically damaged by elephants in invaded areas versus those trees in uninvaded locations. This damage to the trees due to the disruption of the mutualism between plants and ants has the potential to cause significant changes in the dynamics of the savanna ecosystems where this acacia is dominant.

The relevant paper is:

Riginos, C., M.A, Karande, D.I. Rubenstein & T.M. Palmer. 2015. Disruption of a protective ant–plant mutualism by an invasive ant increases elephant damage to savanna trees. Ecology 96:654-661.3

The abstract of the paper:

Invasive species can indirectly affect ecosystem processes via the disruption of mutualisms. The mutualism between the whistling thorn acacia (Acacia drepanolobium) and four species of symbiotic ants is an ecologically important one; ants strongly defend trees against elephants, which can otherwise have dramatic impacts on tree cover. In Laikipia, Kenya, the invasive big headed ant (Pheidole megacephala) has established itself at numerous locations within the last 10-15 years. In invaded areas on five properties, we found that three species of symbiotic Crematogaster ants were virtually extirpated, whereas Tetraponera penzigi co-occurred with P. megacephala. Tetraponera penzigi appears to persist because of its non-aggressive behavior; in a whole-tree translocation experiment, Crematogaster defended host trees against P. megacephala but were extirpated from trees within hours. In contrast, T. penzigi retreated into domatia and withstood invading ants for >30 days. In the field, the loss of defensive Crematogaster ants in invaded areas led to a five- to seven-fold increase in the number of trees catastrophically damaged by elephants compared to un-invaded areas. In savannas, tree cover drives many ecosystem processes and provides essential forage for many large mammal species; thus, the invasion of big-headed ants may strongly alter the dynamics and diversity of East Africa’s whistling thorn savannas by disrupting this system’s keystone acacia-ant mutualism.