Q:

A word for a skill that if lost may never be able to be reacquired

I'm not sure if there's a word for this. 
It would describe an athlete at their prime who may never get back to the same level if injured, an academic who if they don't constantly keep studying will lose their edge and may never get it back, or a technology that if not continually worked on could disappear never to appear again. 
I want to use it in a sentence to describe that if you do not continually dream big, you may lose the ability and it may never return. I hope the sentence would be "Dreaming big is a ______ skill/attribute". 
Does a word exist?

A:

There's a term in pistolcraft that refers to skill with a pistol as being a naturally depreciating asset.  If you stop practicing, you become less accurate.  
'Perishable', 'ephemeral', 'elusive', 'erosive', 'transitory' and, 'evasive' also come to mind.  So does, 'wasting' and, 'debilitating'.  One of these adjectives should work!  

