slugs and snakes

A garden is a mini-ecosystem, and I want mine to include as many different kinds of life as possible. I let "weeds" like dandelions, clover, and various wildflowers run rampant, wild and free between my cultivated vegetables. I love the earthworms, ants, beetles, centipedes, ladybugs, creepy crawly bugs of all shapes and sizes, garter snakes, occasional frogs and other little creatures I have found living in the lush mix of grass, clover, and weeds. A tilled garden with neat rows cannot provide a home for so many little creatures, most of which are beneficial.
One creature which seems to find my garden a comfortable place to live is the slug. My garden is home to several species of them, all of which I detest. I have laid out cardboard where I want to plant vegetables, to kill the weeds growing there, and thereunder lie thousands of slugs which I have been almost daily collecting and destroying. Sometimes I cut them in half with a trowel, sometimes I poke them with a stick, sometimes they are too tough to kill this way so I collect them in a container where they die slowly. I don't feel guilty. If I let them live, they would slowly take over the world. I don't think they have brains, so I doubt they care that they are dying slowly.
Knowing that I can't possibly control the slug population on my own, I have gone to great lengths to make my garden an attractive home for garter snakes, which are supposed to be a predator to slugs. I dug a hole in a random place, put some sticks, dry leaves, and large rocks in the bottom of it, then covered the hole with a pallet to keep out precipitation and to keep my kids from falling in it. This is supposed to make a nice home for snakes to hibernate in or go to when it's too cold or wet to be out. I also piled all the stones I could find in random places throughout the garden, to make a nice spot for the snakes to rest to keep warm when they need to. I've also found that they like to hide under the same cardboard that the slugs collect on. I've found fewer slugs on the cardboard that I've found snakes under.