It's a staggering illustration of the moral of a story told over three and a half centuries ago by the most famous French fable writer. "One often needs someone smaller than oneself," wrote Jean de La Fontaine in The Lion and the Rat (Fables, 1668).

This modern fable also involves lions, as reported in the January 25 issue of Science. Not only hunting lions and invasive ants but also grazing elephants and fleeing zebras – not forgetting the vegetable protagonist, the whistling-thorn tree. In fact, everything turns on the branches and around the trunk of this tree, where all the creatures gravitate – provided that it survives, as we shall see.

Above all, this story is a dramatic example of the ripple effect, how a seemingly tiny event has cascading repercussions which end up producing a spectacular effect. In this case, a tiny creature forces the king of the beasts to change his diet from zebras, his favorite prey, to buffaloes, a much tougher prey. Ultimately, the whole ecosystem was disrupted.

It all began in 2003, when a fearsome creature invaded the savannah of central Kenya: the big-headed ant (Pheidole megacephala), one of the hundred most invasive species, according to the International Union for Conservation of Nature (IUCN). The origin of this invader remains a mystery. One thing that's certain is that its spread is linked to the movement of people and goods, and so to human activities.

The story, shown by teams from the Universities of Wyoming (United States) and Nairobi (Kenya), is a mind-boggling series of twists and turns that followed this disruptive event. Douglas Kamaru and his colleagues meticulously reconstructed every link in this chain reaction, combining field observations, experiments and animal monitoring throughout this 18-year natural study.

The first effect was that the big-headed ant drove out its native cousin, the "acacia ant" (Crematogaster spp.). This is the ant that the lion (Panthera leo) needed, as it normally protects the whistling-thorn tree (Vachellia drepanolobium), which in turn provides cover for the lion. The tree also offers shelter to the ant, which in return protects it from voracious herbivores. When an elephant comes into contact with an acacia colonized by a swarm, it is bitten by the insect and retreats.

This fine example of symbiosis was disrupted by the arrival of the big-headed ant. Highly prolific – a queen can lay 1,500 eggs a day – the invader decimated the acacia ant. Losing its six-legged defenders, the tree became vulnerable to overbrowsing by elephants, which browsed and broke trees five to seven times more often in invaded areas than in uninvaded ones, as the researchers showed. The result was a much more open landscape. After three years, the visibility index was 2.67 times higher in invaded areas, now accessible to large herbivores, than in uninvaded ones.

You have 40% of this article left to read. The rest is for subscribers only.