Breakdown of an ant-plant mutualism follows the loss of large herbivores

Large herbivores were excluded from experimental plots of African savanna for a decade, with unfenced plots as controls. Acacia investment in ant rewards — swollen thorns and nectar — was measured, along with which ant species occupied the trees and how the trees fared.

Studied in

Acacia drepanolobium, Crematogaster

Sample size

long-term large-herbivore exclusion plots against unfenced controls

With large herbivores excluded, trees reduced their production of swollen thorns and nectar. Occupancy shifted from a defending ant species towards a less protective one that tolerates lower rewards and does not defend the tree, and that species is associated with wood-boring beetles. Trees in exclusion plots grew more slowly and had higher mortality than those exposed to herbivores.

• One savanna system, one acacia and its particular ant associates.
• Exclusion removes several herbivore species at once and changes more than browsing pressure alone.

• Fencing out the browsers, and watching a partnership fail

  Acacias feed and house ants that defend them from browsers. What happens to the arrangement if the browsers are removed?

• Mutualism

  Fence the large herbivores out of a savanna and, within a decade, acacias stop paying their guard ants. A non-defending ant species takes over, and the trees grow more slowly and die more often than the browsed ones.