In Star Trek, the Prime Directive is Starfleet General Order 1. Does this mean it does not apply to other species? Can other Federation species openly visit primitive planets?
More importantly, what prevents all the other non-Federation species from colonizing or claiming the thousands (if not millions) of primitive planets in the quadrant (like the Dominion has done with the Gamma quadrant)?
The Federation is not a human government, it is a multi-stellar/multi-racial government that includes, for example, the planet Vulcan.
Any member of Starfleet would be required to obey this order. This order would not apply to a private citizen (such as Spock’s mom or any non-Starfleet officers and non-Federation officials) but I would assume that there is an equivalent Federation law since it would be silly if the Prime Directive applied only to the Starfleet.
Presumably some cultures have their own equivalent of the Prime Directive, and some don't.
Certainly most of the non-Federation cultures we've seen don't, but perhaps that's selection bias; if the Vnerzglians, for example, follow their own Prime Directive, it's more difficult to write an interesting episode about any interaction with them.
The Organians certainly appeared to have their own version of a non-interference directive; they were very hesitant to interfere even with the actions of Kirk, Spock, and the Klingon occupiers on their own planet.
sigh You're making me talk about Angel One. I find this distressing because it was a terrible first season TNG episode.
https://memory-alpha.fandom.com/wiki/Angel_One_(episode)
Ramsey tells the rest of the away team that at first they thought Angel I was great, but then they saw how the men had no respect and were discriminated against. When they spoke out, they were forced to become fugitives. He refuses to leave, and Data adds that they can't force him, as he and his crew are not members of Starfleet, nor do they have to obey the Prime Directive.
So canonically Federation civilians, technically even human civilians are not answerable to the Prime Directive because it's a Starfleet regulation. It only applies to Starfleet. It would not be surprising if the Federation has laws saying things like there are certain planets that civilians aren't allowed to go to because the locals have never made first contact and laws against exporting more advanced technologies to a planet but that remains unconfirmed. We can just surmise that to be the case because in TNG we saw increasing numbers of pre-warp drive civilizations that had not had any open contact with aliens in living memory. As TNG went on the Prime Directive became portrayed more and more as a foundational moral principle of the Federation which is kind of inconsistent with Angel One but consistence was not that strong a suit when it came to Star Trek.
Other governments are well known to not live by the same code as the Federation held itself to. The Klingons, Cardassians, Dominion and Romulans frequently gobble up more primitive planets within their claimed space, bringing them into the technological future more rapidly than they would have on their own development.
Just for a couple of examples: The Jem'hadar were fairly primitive farmers who were taken and changed into soldiers by the Changelings. The Remans were likewise a primitive race who were brought into virtual servitude by the Romulans to serve as laborors and shock troops in battle. The brief-lived Sona subjugated several races into their empire.
Nothing really prevents other governments from doing this. Occasionally border worlds are protected more or less by being in Neutral Zones between the various governments, but for the most part the only thing that prevents a world from being pulled into the future as servants to a subjugating race is that the conquerors must expend the effort required to bring that culture up to a level high enough to be of use. That may be a small effort or a large one, depending on how far that world has already progressed on its own. Even for primitive slave labor, you would still have to teach them enough to be useful.
Add to that the amount of effort required to keep control of that world if it is one taken by subjugation. Are you willing to expend the troops and supplies necessary to keep the locals in line? Or is that not worth your time for the return you're getting? So there's a lot of factors that can "protect" primitive worlds from being taken over by advanced governments.
Only Starfleet is expected to follow the Prime Directive. The Prime Directive only applies to Starfleet officers and Federation officials. The Federation cannot expect its “citizens” to follow the Prime Directive. https://scifi.stackexchange.com/a/72298/89278
The Bajorans are perhaps an example of a species that was greatly affected by failure of Cardassians (and, depending on how you view it, the Federation) to follow anything like the Prime Directive. According to Memory Alpha, the Bajorans had developed "sublight space travel" when they were "annexed" by the Cardassian Empire; as such they were a pre-Warp civilization and fell under the Prime Directive.
The Bajorans were enslaved and their planet's natural resources were plundered for forty years. They had a Resistance, but neither Memory Alpha nor DS9 (which starts right after the occupation ends) detail exactly how a technologically inferior race was able to expel an established occupation, or whether they had help from the Federation or others. The Federation then set up a friendly presence on the former Cardassian space station, originally to provide administrative and scientific assistance in the Bajoran recovery.
While the Bajorans do have some sort of military (the Bajoran Militia), it seems to be relatively weak and perhaps technologically inferior to others. In the first episode of DS9 Season Seven (Memory Alpha doesn't really go into detail on this) Kira, with the militia's backing, confronts a semi-hostile Romulan warbird, and everyone involved - Kira, the Romulan officer, and the Federation officer present - considers the Bajoran vessel greatly outmatched. So it seems likely that they are still technically a pre-Warp civilization, despite their significant role in 24th-century events.
