Figure 17.8 The effect of ants on acacia trees. (a) When ants were removed from acacia trees, the trees experienced a large increase in the number of herbivorous insects feeding on them. (b) As a result, the removal of ants caused much lower survival of the acacia trees.

Data from D. H. Janzen, Coevolution of mutualism between ants and acacias in Central America, Evolution 20 (1966): 249–275.