Is the "of" necessary?

So, the choice is between "At first people thought that they were different species" and "At first people thought they were (members) of different species.

Either is acceptable although I think the latter is a little clearer.

The 'of' is redundant.

Must you parrot others? Do the research yourself if you are in doubt.

"Males are mostly bright green, females are predominately bright red. The male and female eclectus were once thought to be different species."

https://en.wikipedia.org/wiki/Eclectus