Quote Originally Posted by chris-uk View Post
What would be the minimum number to form a sustainable population?
One gravid female with a bit of luck.

What sort of range do individual garters cover within a breeding population?
Typically, they move a few hundred meters to a few kilometers during a season.

I suspect that the number required within a relatively small area would be quite high.
The density would matter, yes. No idea how many per hectare would be sufficient.

I expect there are odd escapees that survive for years in the wild, but not a population.
I'd have to agree. It also takes more than survival to actually establish a population and climate is the best predictor.