For those that want the Full chain:
The past: Acacia ants protected acacia trees by biting elephants that threatened the tree, living in a symbiosis where the tree feeds them for their guardianship. Lions use acacia trees as perches to hunt and cover to hide in.
Now: Bull headed ants have done significant damage to native ant populations, leaving the trees unprotected. Elephants strip the bark and damage the trees, now they are no longer suitable for perches or hiding. Bull headed ants being invasive do not form the same relationship with acacia trees as acacia ants do.
The result is harder hunting and shrinking hunting grounds for lions, an already stressed species.