Disruption of an ant-plant mutualism shapes interactions between lions and their primary prey.

Mutualisms often define ecosystems, but they are susceptible to human activities. Combining experiments, animal tracking, and mortality investigations, we show that the invasive big-headed ant ( Pheidole megacephala ) makes lions ( Panthera leo ) less effective at killing their primary prey, plains zebra ( Equus quagga ). Big-headed ants disrupted the mutualism between native ants ( Crematogaster spp.) and the dominant whistling-thorn tree ( Vachellia drepanolobium ), rendering trees vulnerable to elephant ( Loxodonta africana ) browsing and resulting in landscapes with higher visibility. Although zebra kills were significantly less likely to occur in higher-visibility, invaded areas, lion numbers did not decline since the onset of the invasion, likely because of prey-switching to African buffalo ( Syncerus caffer ). We show that by controlling biophysical structure across landscapes, a tiny invader reconfigured predator-prey dynamics among iconic species.