Who is in charge of law enforcement at a national park? Is that a park ranger’s job or is there park police?

Is there a special law enforcement branch that covers a national park? If so, what would the title of the person in charge of that park be? I.e., the job title of whoever’s in charge of a park like Yosemite? And what exactly would they do? What does their job entail? Thank you!

Depends on the law being broken and the park you are in. Most older parks the Park Rangers are the law enforcement, in big city national parks, the Park Police. In newer National Parks, there is a mix of agreements with city, county, state, and Park Rangers, and it depends on the enabling legislation of the park and the park agreements with the local government. For example, if you were to speed in Death Valley, you would most likely be stopped by CHP; if you were shooting bottles, either Inyo County Sheriff or the Park LE; but if you try stealing artifacts, only the Park Rangers will nab you. In Yellowstone, all of the above would Park Rangers.

The person in charge of law enforcement in Yosemite would be the Chief Ranger. The Chief Ranger is a senior cop, someone who has been on the beat for 10-15 years, so no longer goes out on a lot of patrols or calls, he (or she) is primarily a manager, administrator, and liaison. He (or she) would still go out on major callouts, during major events, and when LE is “spread thin.” Note, I am tailoring this answer to your example of Yosemite — in a smaller park, the Chief Ranger may “wear several hats” — they may also be the Chief of Interpretation, or Maintenance, or Safety Officer…or all of those and more in a really tiny park.