The global activities of humans have led to the movement of species from their original habitats to distant regions across the world. The displaced species can significantly affect the existing ecology of their new location, such as the harmful effect of invasive ants on African lions.
Threat From Invasive Species
On the plains of the East African savannah, it is common to see whistling-thorn trees (Vachellia drepanolobium), a type of acacia whose bark emits an eerie sound. They are home to a species of acacia ant that drinks nectar from the trees. To return the favor, the insects swarm and attack herbivores like elephants, rhinoceros, or giraffes, which attempt to eat their home.
The mutually beneficial relationship has allowed the trees and the ants to prosper. However, this symbiosis has been threatened by the arrival of the big-headed ants (Pheidole megacephala), which originated in southern Africa.
Experts believe these ants arrived in Kenya 15 years ago but were not noticed since they are not aggressive towards big animals, including humans. They are currently transforming landscapes in very subtle ways but with devastating impacts.
Big-headed ants are not only the world's worst invasive species, but they are also vicarious predators. They catch and kill various animals, from insects to small birds. Upon encountering species like the acacia ant, they will kill all adults they come across and feed on their eggs and young.
These insects do not have a symbiotic relationship with the whistling-thorn tree. Instead, they leave each tree without the insects that defend them from herbivores. As a result, more whistling-thorn trees are eaten by elephants, leading to reduced tree cover in the savannah.
A Change of Diet
Researchers from the University of Florida tried to investigate how this invasion has affected the relationship between other predators and prey living in Kenya. Led by Douglas N. Kamaru, the researchers divided the study into two sections. Their findings are reported in the paper "Disruption of an ant-plant mutualism shapes interactions between lions and their primary prey."
In the first part of the study, the research team divided the landscape of Laikipia County, Kenya, into plots of equal sizes. Half of them were within an area invaded by big-headed ants, while the other half were in areas with no invasive insects. The plots were further divided, with some fenced off to prevent large herbivores from entering and others left open. This enabled the experts to observe the impacts of ants and herbivores on tree cover.
It was found that the visibility was almost three times greater in the unfenced areas invaded by ants since elephants knocked over and consumed the whistling-thorn trees. Meanwhile, the visibility was lower in invaded areas where elephants were excluded. This indicates that the invasive ants alone were not responsible for the difference in tree cover.
For the second part of the study, the team used a group of six lionesses to look at the impact of invasion on other animals. They found that the visibility increased in areas where ants were present, and zebra kills were about three times lower.
Scientists were puzzled as to why the number of lions remained stable when catching fewer zebras. It turned out that these animals learned to switch to alternative food sources and preferred buffalo. This indicates that the lions have had to change their feeding behavior and work together in larger groups to take down their prey.
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