At first glance, big-headed ants don't seem particularly dangerous. Workers are just 2.5 millimeters in size and, unlike many other species, they can't even spray formic acid. Pheidole megacephala's soldiers are a little larger and have a broad head, which gave the whole species its name – but they don't really look threatening either. This is misleading.
According to the International Union for Conservation of Nature (IUCN), big-headed ants are among the one hundred worst invasive species in the world. According to a study published in the current issue of the scientific journal Science, the small insects may even force lions to eat fewer zebras.
According to the current study, it all started when fat-headed ants drove out another ant species, the acacia ants (Crematogaster), from part of the Ol Pejeta Conservancy, a protected area in northern Kenya. “They outnumber Crematogaster and destroy them by killing adult ants and eating eggs, larvae and pupae,” write the researchers led by biologist Douglas Kamaru, from the University of Wyoming, in the United States, in Science.
Acacia ants even defend “their” tree against elephants
In this way, the attackers destroyed the community that acacia ants normally form with acacias and from which both insects and plants benefit. Biologists talk about mutualism. Acacia trees offer protection to ants in the form of hollow spines where the insects live and food in the form of sweet nectar that emerges from the leaf stalks. In return, the ants defend “their” acacia against all animals that want to nibble on the tree. Masses of ants that move at the slightest vibration can even make elephants flee.
But after the big-headed ants drove out the acacia's bodyguards, the plants were left defenseless against the appetites of the large herbivores: According to the study, the elephants ate the trees five to seven times more or even broke the trees down completely. The study authors discovered this by comparing the parts of the protected area where the big-headed ants had taken over with those where there were still acacia ants.
According to the researchers, elephants decimated the number of acacia trees in the fat-headed ants' territory so dramatically that a more open landscape with significantly fewer trees was created. This robbed the lions of hiding places from which they launched surprise attacks on their favorite prey – zebras. According to the study results, the number of zebras killed by lions fell by about a third.
It's not entirely clear whether and how lions compensate for this, writes Kaitlyn Gaynor of the University of British Columbia in Vancouver, Canada, in a commentary in the journal Science. According to the study authors, there is evidence that big cats have changed their prey pattern and now hunt more buffaloes. Possibly because they are slower than zebras and big cats catch them without surprising them.
The Science study is one of the few in which it was possible to understand step by step how a seemingly small change in the spectrum of species in an ecosystem can have large effects. In most cases, it is “nearly impossible to attribute an observed system-wide change to a specific cause in the chain,” Gaynor writes. Often there simply isn't enough known about the different creatures in an ecosystem to be able to piece together the puzzle as completely as Douglas Kamaru's team was able to do.