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.