How an ant invasion led to lions eating fewer zebra in a Kenyan ecosystem

The invasion of non-native species can sometimes lead to large and unexpected ecosystem shifts, as Douglas Kamaru and colleagues demonstrate in a unique, careful study that traces the links between big-headed ants, acacia trees, elephants, lions, zebras, and buffalo at a Kenyan conservancy. The invasive big-headed ant species disrupted a mutualism between native ants and the region’s thorny acacia trees, in which the native ants protected the trees from grazers in exchange for a place to live. Through a combination of observations, experimental plots, and animal tracking at Ol Pejeta Conservancy, Kamaru et al. followed the ecosystem chain reaction prompted by this disruption. When the invasive ants pushed out the native ants, the trees were left vulnerable to overgrazing by elephants, who browsed and broke trees at five to seven times the rate in areas with invasive ants compared to areas without the invaders. The result was a much more open landscape, where lions were left without hiding places to observe and stalk their preferred prey of zebra. Zebra kills were 2.87 times higher in uninvaded areas, compared to areas with the big-headed ants. The lions responded by shifting their prey to include more African buffalo. From 2003 to 2020, the proportion of zebra kills made by lions dropped from 67% to 42%, while the proportion of buffalo kills rose from 0% to 42%. The study offers an important glimpse at how disruption of a mutualism can have reverberations throughout an ecosystem, Kaitlyn Gaynor notes in a related Perspective. “Ultimately, the conservation of healthy ecosystems requires not only the prevention of species extinction but also the identification and preservation of the most important interactions between species,” Gaynor writes.