When we teach environmental science to our students there's a basic principle that energy flows through the biosphere while nutrients cycle. This is true in terms of global ecology but the facts on the ground are different.
The cold hard fact struck me last year when I was working in an organic dairy farm in upstate New York. Simply put, a hayfield gathers energy and nutrients all season. When we harvest the hay we remove those nutrients. On a macro scale we can acknowledge that the goats cycle the nutrients etc., but the fact is that the hayfield is the poorer after harvest.
I saw this fact at work again when I visited Sri Lanka this year. I had thought that rice paddy ecosystems were "balanced" because of microorganisms in the water that provide nutrients. To an extent this is true, but rice consumes enormous quantities of nutrients. The fields are a one-way flow of nutrients (out). The irrigation ponds (wewas), rich in aquatic life, are the source of nutrients that nourish the rice.
So as I obsessively remove snails from my postage-stamp sized garden in Boston I wonder, what if anything do these creatures do to cycle nutrients at the micro level? By removing them from the garden (I realize there are millions more I never find), how do I impact the nutrient content of my garden? If I were able to remove all the snails, what would the impact be?