Which type of scientist would observe the spread of a species of red fire ants

Answer is: myrmecologist.

Myrmecology is a branch of entomology focusing on the scientific study of ants.

Ants are model for the study of questions on the evolution of social systems because of their complex and varied forms of eusociality.

Red ants (fire ant) are the common name for several species of ants in the genus Solenopsis.