I dunno about paramites, but here's my theory on scrabs.
When the ickle scrabs are born in the herd, they have a rather happy life. As they grow up, the Alpha male gets more and more aggresive towards them, forcing them to leave when they near maturity(both males and females). They go to the desert to escape, whenever two males encounter each other they fight to the death.
If a male and female, or two females meet, they won't fight, but become a herd (a herd of two isn't very big, I know). Anyway, if lots of females join the group, then they will move to the grasslands, where there is more food. If the group can be supported in the desert, then they will stay there until the majority of the group matures, and then go to the grasslands.
If the group includes a male, then they will stay in the desert until he matures fully, unless it becomes a struggle to find enough food, in which case they will move on anyway.
The femal group will eventually meet and established herd, which they will join, or a lone male, who will complete the group.
If a scrab dosen't meet another scrab to form a group with, then it will wait until it is mature, and then journey to the grasslands, if it is male it will fight for a herd, and if it is female, it will join a herd.
This theory allows for the sustaining of current herds, and the creation of new ones.