The bull's horn Acacia, Acacia colensii, produces pairs of large thorns where species of Pseudomermex ants live. The trees produce food (nectar and protein rich Beltian bodies) that the ants consume. The ants attack herbivores and clip off nearby vegetation resulting in the formation of halos of bare soil surrounding the stems.
We hypothesized that small trees would benefit more from ant protection, and wondered whether we might find a relationship between tree size and the defensive activity of ants. We distinguished between a red and black species of Pseudomermex. . We found black ants more commonly on larger trees. Our results show that red ants are more active than black ants. Both species of ants increase in activity equally when disturbed by tapping on the tree trunk but active ants respond more to the disturbance. The active red ants may therefore provide smaller trees more protection against herbivores and competitors.
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