Hello Maggi, I suppose they rely on seed production and those seed finding another suitable habitat. As you know, orchids produce many thousands of seed and the chances are that some will find the right conditions for germination. I found Dactylorhiza purpurella on the local patch some years ago. As far as I have discovered this was a first record for the species at this site. There were 89 plants when it was first found. Last year we counted about 1300. This shows how the plants can increase in 6 years given the right conditions.