Arrival of invasive ant remodels environment in the savanna – 01/27/2024 – Reinaldo José Lopes

What difference can a single ant species make in an ecosystem? Apparently, such a gigantic difference that the lives of trees, lions, zebras and buffaloes can change a bit simply because of that little ant.

For this, it is enough that it is an invasive species. In other words, it is a newcomer that evolved in a completely different place and is not inserted into the web of interrelations, millions of years old, that prevails in its new environment.

That, in short, is the fascinating and frightening story told in a study that has just been published in the specialized journal Science. In it, an international team of researchers studied the seemingly inexorable advance of Pheidole megacephala, or –seriously, the name is the same– big-headed ant. (No one, after all, takes the Greek nickname “megacephalus” in vain.)

At an average rate of about 50 meters per year, the loggerhead ant, probably originating from an island in the Indian Ocean, has been spreading across the savannah of Kenya in East Africa, in places such as the Laikipia region. Its first direct victim is other ants, of the genus Crematogasterbut things are far from stopping there.

This is because the Crematogaster, or acacia ants, have formed a lasting alliance with these trees. While acacias provide native insects with nectar and shelter in special structures called swollen spines, African ants ardently defend the plants from incursions by herbivores.

Not even elephants are capable of destroying acacias when their resident ants are at hand – the effect of herbivory by pachyderms falls to just a tenth of what would be expected thanks to them. Thus, under normal conditions, acacias are the dominant plant species in the region’s savannah.

Invasive big-headed ants, however, are capable of exterminating native ants, killing adult individuals and devouring eggs, larvae and pupae. Result: acacia trees become vulnerable to elephants and almost disappear from the landscape.

But other dominoes in this game also fall. A series of experiments and observations, coordinated by Jacob Goheen, from the University of Wyoming (USA), revealed that the war between ants modified the interaction between the main large vertebrates in the savannah. With the terrain much more open, without acacia trees, the tactic normally used by lions to hunt zebras was hampered: taking advantage of the cover provided by the trees to hide.

The result of this is that zebras became considerably more skittish, going from 67% to 42% of the total prey killed by lions in Laikipia. To compensate for the shortage of zebra meat, the big cats began attacking buffaloes. Previously, cattle were never hunted by local lions, but now they account for 42% of the prey devoured by them.

At least for now, the consequences for large vertebrates have not been apocalyptic — lions apparently are not yet starving, nor has there been a population explosion in zebras or an appreciable decline in buffalo.

In part, this may have to do with the fact that the introduction of big-headed ants into the region’s environment is quite recent, having only started in the 2000s. Still, it is still impressive what happens when a single piece of key to an ecosystem is exchanged. Environmental change is often inevitable – but the human effort to grasp the complexity of its consequences is essential if we are to achieve some kind of responsible coming of age as a species.

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