Snails are a minor part of human life, while breathing air with enough oxygen is a major part. However sad a genocide of multicellular species may feel, strangling ourselves by destroying forests is much worse.
Are much more influent that most people think. Some species are among the animals in the planet with a higher bodycount.
Oxygen is produced by plankton. Trees did not appear on this planet before oxygen, so you are breathing air not because of trees. Human species also are multicellular species...