just as wheels inside a clock, or electronic components inside a computer are there all in place to do SOMETHING, so are all the elements of life. simple examples to build on are like- mosquitoes. we hate 'em, but some birds feed on them. without the bird, we wouldn't have food for some other carnivorous animal. the same bird also may eat crop-damaging insects and WE eat the crops! or feed them to cows, pigs, chckens, and we eat them! birds and lower animals also do things other than feed each other. many plants are helped by birds, bugs, animals, or bigger wildlife like deer. but we usually focus on the food chain as it is a direct way of understanding the interacting of animals, bugs, birds, (also animals...) etc.