Acacia and Pseudomyrmex:

This is one of the best-known examples of an obligate mutualism. These Pseudomyrmex ants live only in this type of Acacia plant, receiving food and a place to live from the plants. In return, the ants aggressively protect the plants from vertebrate and invertebrate herbivores, and cut away any plants that might encroach on the Acacia.