About 1%. For the details (and a one-question poll at the end), read on.
I went back to my nearly-comprehensive list of every N. American TT asst. professor in ecology or an allied field hired during the 2016-17 job season (or in a very few cases in 2015-16), and looked up how many still held the position into which they were hired in 2016-17. These folks have been asst profs for ~2 years now, long enough that you might expect at least a few of them to have switched positions.
Turns out only a few have switched positions: just 2 out of 178 definitely switched, and there’s a third about whom I’m unsure (she may have switched, remained in the same position, or left academia). The rest definitely still hold the positions into which they were hired in 2016-17.
This is further proof, if further proof were needed*, that the TT ecology faculty job market is not dominated by TT asst. profs playing musical chairs. Contrary to speculation I’ve seen on ecoevojobs.net and heard from other sources.
As I hope is obvious, these data tell you nothing about what fraction have tried to switch positions (well, except that it’s at least 1%), or about your own personal odds of success if you are a TT asst. prof who wants to move to another institution. UPDATE: And in case it wasn’t clear, these data also don’t tell you what fraction will have moved to another position within (say) five years of being hired (save that it will be at least 1%). Like Brian in the comments, I suspect that more people move 3-5 years after being hired than move within the first 2 years. I’ll check back in a few years and find out.
I’m guessing these results won’t surprise too many people, but let’s find out. Take the poll below!
*which it wasn’t