I haven't seen the census but I spoke to the census taker. The big increase in population is as you said: economic refugees.The population of Caye Caulker is now overwhelmingly what we call aliens. Our economy has little to do with serving needs of expats. For each one of us there are a 300 tourists and a hundred aliens serving their needs.
Not sure about unsustainable feedback.There are mitigating factors.The expats brought a respect for the environment, esp marine, that simply did not exist before.They brought the idea of eating vegetables daily. They cleaned the streets in person and the idea caught on. In some cases they grew the economy in less destructive ways than before.