Ismael and James,
Confirming that your solution worked.
When I removed the default value for the species choice on the External Select and published updated form it worked correctly.
Any default value for the cascading select that referred to the csv file causes the problem.
This is true if the value is in the external choice list or not.
Also, I tested if data type matching was a problem (for example if it is text in an integer answer)
In hindsight, if the choice has not been instantiated yet I don't necessarily need a default answer, so I will see if a calculation once the element exists on the users form will be a work around.
Thanks so much for your insights!
Best wishes with the release and update of 1.8
Joe