Humans are a big part of nature. Bird migration is another good example. When it gets cold where birds are, and the snow or frozen ground cuts off the normal food supply, birds fly to wherever they can find food. It's a matter of survival - affected by a changing climate. Maybe that means flying all the way to South America or to north Texas (depending largely on their diet) from a place that has no food available, like Canada or Minnesota.
Diet is important not just for a plant's yield, but for timing. Our native plants go to seed or make berries precisely when our native or migrating birds are looking for these things (exotic plants may look nice to you and me, but they produce food when our native birds aren't "in the market". Also, our native trees drop their leaves on schedule each fall, providing millions of protected hiding places for tiny bugs, which birds eat.
Also, all those fallen leaves, as they decompose they loosen the soil and add nutrients; making for more fruit/seed production in the following season.
See how every piece of nature works together with every other piece!