Answer:
Ecosystems have a oxygen factory thanks to plants, so one example the photosynthesis process, we can breath because of it, we exhale carbon dioxide, plants take it and release oxygen.
On the other hand, we are heterotrophs beings which mean that we depend on other beings to get food, like plants, they produce glucose and nutrients that we need to live and have health.
Nowadays, pollution is almost everywhere and one way to have good health is planting trees, so their existence give us health purifying air, making it cleaner to us.
