Invasive ants influence the hunting success of lions

Acacias previously provided lions with privacy when hunting – but due to the spread of an invasive species of ants, the small trees are increasingly disappearing. © Victoria Null

Through cascade effects, even the “King of Animals” can ultimately be under the influence of tiny creatures: A study shows that the disruption of an insect-plant symbiosis by an introduced species of ant ultimately led to lions in Kenya preying on fewer zebras. According to the researchers, this is a particularly impressive example of the complex and far-reaching consequences that human-caused disruptions can have in well-established living environments.

In addition to destruction, pollution, overuse and climate change, many of the world's ecosystems are threatened by another danger: “Aliens” are spreading. Through trade, traffic or targeted naturalization, humans have transported numerous animal and plant species that cause problems in their new homeland: certain characteristics cause such invasive species to continue to spread - often at the expense of native species. Due to the complex interconnections in many ecosystems, this can have far-reaching consequences that can hardly be imagined at first. The current study now focuses on the savannahs of East Africa.

As the international research team led by Douglas Kamaru from the University of Wyoming in Laramie reports, an ant-plant symbiosis has ensured that acacia trees of the species Vachellia drepanolobium have been able to survive in the landscape since ancient times. The trees offer a native species of ants nesting opportunities in the cavities of their thickened thorns and food through their nectar supply. In return, the acacia ants vehemently defend their home tree against herbivores such as giraffes and elephants: they attack their lips and trunks with their pincers and by spraying formic acid. As previous research has shown, the tiny “bodyguards” protect the trees extremely effectively from excessive grazing. Above all, they protect them from the particularly destructive activity of elephants.

Tree Guardians fall victim to aliens

The acacia ants therefore play an important role in the development of the typical savannah vegetation in East Africa, to which the creatures in the ecosystem have adapted. But for about 15 years, more and more acacias have been losing their six-legged bodyguards: They are falling victim to an introduced species of ant that probably originally comes from an island in the Indian Ocean. The big-headed ant (Pheidole megacephala) kills and displaces the native acacia ants, but does not protect the trees from the herbivores. The scientists have now carried out extensive research in a nature reserve in Kenya to investigate the direct and indirect consequences this has for the ecosystem.

Basically, the observations showed that in areas where the invasive ants have displaced the native species, the elephants use the acacia trees five to seven times more intensively as a food source. This overuse then leads to significant losses of these trees, resulting in an increasingly open landscape in the areas affected by the invasive ants. Against this background, the scientists then came to suspect that this change triggered by the invaders could have an impact on the lions. It is known that the big cats use the acacia trees as a privacy screen to launch surprise attacks on their preferred prey: zebras.

Cascade effect reaches Leo

In order to demonstrate possible effects on this predator-to-burrow ratio, the researchers collected data over the years on the activity of lions and zebras in the areas of the nature reserve characterized by the invasive ants and in the areas that were still spared. The information, which came from camera traps, satellite observations and lions equipped with transmitters, was then incorporated into statistical modeling.

As the team reports, the evaluation results showed that the number of zebras captured by lions was around three times higher in non-infested areas than in areas that the big-headed ant had already conquered. “We show that the spread of this invasive species has triggered an ecological chain reaction that ultimately reduces the success of lions in hunting their main prey,” the team concludes. Co-author Todd Palmer from the University of Florida in Gainesville adds: “These ants initially appeared about 15 years ago with little attention because they are not aggressive towards large creatures, including humans. But now we see that they are changing the landscape – with far-reaching consequences,” said the scientist.

But what exactly does the change mean for the lions of East Africa? The data shows that the populations of big cats have remained stable so far. As the researchers explain, this is because lions are now increasingly hunting buffaloes instead of zebras. The colossi are significantly more difficult to capture than zebras and the hunt requires more intensive cooperation between the pack. But so far the big cats seem to be able to adapt. “Leos have some ability to find solutions to their problems,” says Palmer. However, it remains unclear to what extent the animals' behavioral adaptations can keep the population stable as the landscape becomes increasingly dominated by ant invasion. We do not yet know what could result from these profound changes. We are very interested in pursuing this story,” concludes Palmer.

Source: University of Florida, University of Wyoming, specialist article: Science, doi: 10.1126/science.adg1464

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