Each of them is using different resources and doing different things. You'll have a variety of plants, like trees/bushes/grass. Then animals will eat those, like snails eating leaves, squirrels eating seeds, moles eating roots, and beetles eating wood. Larger animals will eat those animals, and so on.It's like all the businesses in a community. There's the obvious stores, but then all the businesses like the electric company, janitors, and accountants who help those stores do what they do. That's why diversity is so important- if there was only one kind of plant in a forest or one food store in a town, imagine if it went extinct/out of business- without something to take on some of it's role, the entire forest/town may face ruin. Diversity makes an environment/business community more flexible and resilient to losses.