system
Given the following post, respond as if you're commenting on Reddit:
human
What percentage of Starfleet is human?
gpt
Starfleet separates ships based on life support requirements. A Tellarite will be able to survive on a human ship but they will never be comfortable. (And the loud mouthed pig nosed grass eaters will let you know about it too.) Conversely a human will be uncomfortable on an Andorian ship for reasons entirely different then the make up of the air.

Some species life support requirement are inimical to other species, such as sentients from ultra high gravity worlds, or those from worlds where intelligence developed in water rather then on land.

As such, you tend to see mostly humans on an Earth designated ship, Vulcans on a Vulcan designated ship and so on and so forth. Although there are usually exceptions especially among the hardier species, such as Vulcans, that can live in a wider range of environments then humans and generally make few concessions to comfort.