Forgot to answer your doubt about rodent ability to chew through liner. As the guys said above, mice and rats will chew up and through the liner no problem ability-wise, it may be like a bubble gum to them. They can chew up far, far harder materials, if hard pressed - on par with their relatives squirrels and beavers. Now their motivation to do so may be your saving grace.
In abandoned houses, seasonally inhabited cottages, etc. hungry overwintering rodents eat up wallpaper with the glue, cloth, plastic, wire harnesses, plastic wire insulation, wooden furniture, etc. Hunger is a great motivator to try to feed even on inedible or hardly edible / digestible items.
Why they'd chew on rubber in the "wild", that is your yard, IDK. It seems to me to be a hit and miss. The risk of the "hit" Ken is indicating is surely worth avoiding. You will only taste the frustration of it when you have to repair or replace your punctured liner.
Now a hole well below ground level and protected from being washed out and turned into an underground cavity by the underlayment and/or by the soil being hard might not even be noticed. I am sure many liner pond owners have such holes and don't even know it. Still, this is a rather careless approach. Instead of guessing what if, address this most serious potential problem from the design stage.... is our advice.
But shoving knowledge and advice down one's throat, no matter how well motivated, is repugnant. You are a free man in a free country. We will appreciate whatever you do as long as you share it and let us learn with you and through you. Maybe not all, maybe not equally, but most of us will appreciate it.
