Disruption of a protective ant-plant mutualism by an invasive ant increases elephant damage to savanna trees

Background/Question/Methods

Mutualisms are ubiquitous and ecologically important species interactions that play key roles in community and ecosystem processes. Invasive species, however, can disrupt these mutualisms, with potential ecosystem-wide consequences. The mutualism between the whistling thorn acacia (Acacia drepanolobium) and four species of symbiotic ants in the savannas of East Africa is both well-known and ecologically important; ants defend trees against elephants and other browsers and in exchange receive carbohydrate rewards from extrafloral necatries. The protective effects of ants against elephants is particularly important in stabilizing landscape-scale tree cover, as elephants can otherwise dramatically reduce tree cover. In Laikipia, Kenya, the invasive big-headed ant (Pheidole megacephala) has recently established; we set out to examine (1) whether P. megacephala is able to invade the native ant-defended acacia overstory; (2) whether there are differences among the native ants in their ability to defend themselves against P. megacephala; and (3) the effects of the invasion on A. drepanolobium trees. We used a combination of field surveys of trees in invaded and uninvaded areas, staged arena-style battles between native and invasive ants, and transplants of whole trees into an invaded zone to test the effects of P. megacephala on the native ant-acacia mutualism.

Results/Conclusions

In invaded areas, we found that three species of symbiotic Crematogaster ants were virtually extirpated, whereas Tetraponera penzigi was able to persist on the distal branches of most A. drepanolobium trees. Tetraponera penzigi appears to persist because of its less aggressive behavior. In staged battles, T. penzigi were half as likely to fight with or be killed by P. megecephala compared to the Crematogaster ants. In a whole-tree transplant experiment, Crematogaster ants defended host trees aggressively against P. megacephala but were extirpated from trees within hours. In contrast, T. penzigii retreated into domatia and withstood invading ants for >30 days. In the field, the loss of defensive Crematogaster ants in invaded sites led to a five-fold increase in the number of trees catastrophically damaged by elephants compared to un-invaded sites. This suggests that the big-headed ant invasion may ultimately lead to large-scale changes in tree cover. In savannas such as this one, tree cover drives many ecosystem processes. Thus, the invasion of big-headed ants is expected to strongly alter the dynamics and diversity of East Africa’s whistling thorn savannas (which include large sections of the Serengeti ecosystem and several other important conservation areas) by disrupting this system’s keystone acacia-ant mutualism.