Nobody knows for sure, but there are up to 15 million different types of life on our planet. All of them are interconnected and humans are just one small part of those connections. If you damage some of the life, you destroy some of the connections and affect lots of other life as a result. So if you people kill a forest by cutting it down, you also condemn to death all the animals and birds that lived there even though you may not have killed them directly. You destroy some more biodiversity