How do fire ants protect itself?

It attacks or chases off, or even kills and eats, any animals that seem to be threatening the plant or the aphids feeding on that plant. If you find such a plant you can experiment with it. Take a hair or some similar little thing and poke at the ants and their aphids and you will see how the ants attack you or your hair. Or tap the twig gently to alarm them without touching the aphids or ants. You might get results just by blowing gently on them. Or if you find a ladybird or other insect to put on the same branch, see how the ants try to attack them. Most aphid-eating insects either clamp down and leave the ants trying to bite their smooth, hard shells, or they give off a nasty taste that stops most kinds of ants from attacking them.