These species share a mutual symbiotic relationship meaning both benefit from one another.
Each bullhorn acacia tree is home to an entire colony of stinging ants that carve out the thorns of the tree providing shelter, the tree also provides the ants with two natural food sources. One of which being a nectar that seeps from the trees nectaries. The other food source is a form of food called beltian bodies found on the leaves of the tree. The ants yield sustainable shelter and food just as long as the tree stays alive and healthy, thats where the ants come in.
The stinger ants are extremely territorial and will attack any other living organisms that touches the tree such as other bugs, deer, humans. The ants even climb onto branches of a neighboring tree that touches their tree and kill the entire branch. So the stinging ants protect the bullhorn acacia tree from threats, allowing it to grow and prosper much more than it would without the ants.
"Symbiotic Relationships: Mutualism, Commensalism & Parasitism" n.p., n.d. Web. 7 sep. 2014