Now that conservationists are around, things are returning to what they once were.
If synonymous with:
Now that conservationists are around, things are returning to what they were before.
Are the sentences above the same as for their meaning?
Yes. The sentences are the same in meaning.
The meaning of "once was/were" is "as before", "like before," "as in the past":
in the past, but not now:
This house once belonged to my grandfather.
Computers are much cheaper nowadays than they once were.
Once-thriving villages stand deserted and in ruins.
From sense B1, once https://dictionary.cambridge.org/dictionary/english/once