Alien Species

The following is a list of the creatures that compose the forces of Earth's intruders. The list is sorted by rank, with the high-ranking species coming first.
Above all are the sovereigns. Very few and rare, killing one is a step closer to bringing about the end of the organized alien threat, according to Ryan. 
Next are the birth Mothers, a species created especially to give birth to the rest aliens species.
==List of Species==
*Sovereign
*Birth Mother
*Tracker
*Gunner
*Zombie