There have been three ships mentioned in Trek canon that were crewed (almost) completely by members of one species, Vulcans. This, I suppose, leaves a reasonable change there are ships out there with a same-species crew. Now, is it me, or does this go against the very idea behind the UFP, where interspecies co-operation is the key?
Well, good points ofcourse, about the amount of humans seen. But still, that is explained very simple by the amount of budget for make-up. There were atleast hints to more alien crewmembers on the ships and stations. So, the idea itself is still a bit odd IMO.
If you're talking about ships of planets' own armadas, then that self explanatory. If you're talking about Starfleet ships, then there was only one example of this. It was in an episode where Odo plays an umpire. Let's be real.
Simple biological needs can explain it for the most part. What's comfortable temperature, humidity, and gravity for a Vulcan may well be deadly to an Andorian, and vice versa. Humans need their sleep patterns set on a roughly 24 hour clock, with reasonably consistent timing, while a Caitan or Tellarite might have two sleep periods a day because they are biologically set up to be active at dawn and dusk. You can have some overlap, but the more species you throw into the mix the worse things get, especially for the long-term. To an extent you can customize quarters and passengers can just deal with things for a few days or weeks, but even with custom quarters for everyone crew scheduling issues will be a major problem. A human might literally go crazy if stuck on a Denobulan ship for too long if he's not allowed to keep his biological clock set properly. Space is not an environment forgiving to sleep deprivation. The Ent-D, with a mostly-human crew with a few aliens, is probably pretty typical of a Starfleet ship, and I imagine closely-compatible species like humans and Bajorans may see a much greater mixture. I would also expect Starfleet to mandate that everyone serve at a posting where they're not in the majority at least once in their early career.
What could possibly give you the impression that this is one of the "key" ideals of the Federation? There are logical reasons that individuals from all the member species of the Federation couldn't serve aboard the same ship. One that is brought forward frequently is they are unlikely to all breath the exact same gases, at the same pressure, or live in the same temperature and gravity range. Trek fiction sometimes depicts Mister Spock's living quarters with the gravity system turned up, and the heat too. For all we know, Humans couldn't live in the atmosphere on the vast majority of Starfleet's ships. 95% of all Starfleet vessels could have fully integrated crews, with a 140 species all serving together. And 5% of Starfleet vessels are reserved for the 8 or so species who breath a "Earth average" atmosphere. Humans are among the odd ones out. It's also possible that various Federation species could not serve together for psychological reasons. Or sociological ones. Medusians (not sure if they're a Federation species) can't serve with Humans, and Vulcans can only be in their presence wearing a special visor. Others might have no problem working with them. Some of the Federation's specie wouldn't be able to serve aboard the Enterprise because they physically wouldn't fit in the corridors and turbolifts. Imagine an sapient species with the adults being the size of a elephant.
It's also possible ships are primarily staffed by the species indigenous to the system where that ship was constructed. With about 150 planets, there must be more major shipyards than San Francisco and Utopia Planitia.
Is a US Navy ship of only Californians "un-American"? Provided that the bulk of ships in the fleet were multi-world/species (human, Vulcan, Andorian, Betazoid, Tellarite, Trill, etc.) then a few sole-species ships is no real issue IMO.
In Captain Solok's case, it's understandable, as his evident racial prejudice against Humans would almost certainly preclude him serving on the same ship as them. He believes Vulcans are a superior race (which is in itself an anti-Federation and illogical view), so naturally he would want only Vulcans on his crew.
There was also the Intrepid in "The Immuninuty Syndrome" and the Hera in "Interface", though the latter wasn't entirely Vulcan. Would a captain even be allowed to do that? And what if Bolian, Andorian, Trill, Bajoran, or Betazoid crewmembers were assigned to the ship. What would his attitude be towards them?
That was going to be my answer as well. Breathing the same gases, at the same temperature, in the same environment would be incredibly tricky. Alter the oxygen and nitrogen levels a few points either way and humans can't breathe. Come to think of it, it's quite amazing to get more than 2 species on the same ship given the environmental requirements, even if a self-contained breathing unit was used.
Yeah, so this is why all members signed a charter, or entered into an interstellar federation in the first place....
Mutual defense is one of the Federation's key ideals. Also, one of the Federation's beginning purposes could have be as a intergovernmental trade bloc. Access to each others markets and lower tariffs.
I'm assuming that a captain has some degree of control over who is assigned to a ship. At least Solok does. He could probably pull some strings at Starfleet Command to ensure that the racial purity of his crew was maintained.
Yeah, biological (in)compatiibility is the best and most logical explanation. I imagine Starfleet tries it best to integrate compatible species and overcome the incompatibilities with other species through technical and medical means (that's actually a large theme of the USS Titan novel series) but in the end you can only do so much before simple practicality kicks in. Though when it comes to there being some Vulcan only ships, I could also see that as a historical perk Vulcans got thanks to being a founding species. After all, no organization as large and complex as Starfleet (and the Federation) can be completely consistent in regards to it's ideals without any special cases and compromises. Or maybe it's just Starfleet policy to have a couple of single-species ships for each of it's species in addition to the multi-species ships, just for the diversity of crew organizing approaches. Or it's a remnant from the earlier, presumably less integrated (for political, not biological reasons) days of previous centuries. Most likely it's ALL of the above reasons.