This game mechanic represents overfishing.

So, I actually have two models.

Stacks

Stacks of tokens, each of which have a random number of null tokens on the bottom, and full tokens on the top.

So, you draw from the stack knowing that at some unknown point suddenly that thing will be gone, but people can't predict when.

Bag

In this case you have a bag of tokens that starts with some number of null tokens and some bigger number of live tokens. You shake the whole thing up.

Now you draw from the bag, removing live tokens, and throwing back null tokens. Every turn you can put some number of tokens back into the bag, but if you spend more than went in, it's unsustainable.

This one has the advantage of more uncertainty. If you draw 3, and you happen to get 2 nulls, you might be like "Woah, we're running low", but also maybe you were just unlucky. You never know when you reach in if you'll get a bounty, or not.

At some point people will just realize that bag's not a good gamble any longer.