Answer: The short answer is that we, like all animals, provide carbon dioxide as we exhale; plants use carbon dioxide during photosynthesis. Also, in nature as plants and animals die their bodies decompose and provide nutrients to future generations of plants. Unfortunately, since humans are generally embalmed and buried in caskets we don't provide this benefit.
I suppose you could say that as we tend our gardens, we provide nutrients in the form of fertilizer, supplemental water during droughts, and in other ways make life easier for the plants we decide are desirable. Plants we deem weeds, however, are pulled out and discarded! (Well, hopefully, at least composted so the nutrients can be returned to the soil.)
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