This article in Science Daily says yes. "They found that the presence of worms increased nitrogen content of plants and reduced the number of leaves damaged due to slugs by 60%."
I'm not sure I believe it. I have lots of earthworms, and lots of slugs,, and lots of slug damage. Chickens will eat slugs, but chickens will also destroy a garden in a few hours. I know... I've seen the result. So I use an organic slug bait, made from iron phosphate - it does work.
Interesting to read about garden ecosystem interactions, even if it's not clear cut. Plus, I like the worms, as did Charles Darwin. Darwin determined that earthworms were responsible for sinking Stonehenge deeper into the ground, among other ideas.
Whether they reduce slug damage or not, my garden loves worms and so I do also.
That's an interesting read about Darwin and worms.
Still, the suggestion to increase plant diversity in the garden might be useful. On the other hand a highly diverse plant mix might be far more difficult to tend.
... in high diverse ecosystems slugs eat less in total because they have to switch their diets more often since plants of the same species are less available. Therefore gardeners are to help protect earthworms by increasing plant diversity in the garden in order to keep slug damage low.
That would go along with the concept of companion planting, too. Slugs in my garden seem to specialize in a handful of plants, and eat those with great relish. Not pickle relish, the kind that just means enjoyment.
I use diatomaceous earth (DE) sprinkled around certain plants (i.e., strawberries) that slugs like. Seems to do the trick. Of course, there's the old fashioned beer baths!