Why are the roles in different ecosystems the same but the species that fill them often different

Ecosystem is a biological community, where living beings interact with each other and with their non-living environment. It includes two factors living or biotic factor like plants, animals, microbes and humans and abiotic factors such as sunlight, wind, water, soil and others. The role of different ecosystem is the same as they provide an environment where a living species can survive and reproduce. But the roles of species may vary from ecosystem to ecosystem. Like, mouse act as a predator,as it's function in one ecosystem is to feed upon insects, and grasses. But in another ecosystem it may be fed by other organisms like snakes, hawk, therefore, it will be prey for them.

the index of refraction of the second medium is higher. i just took the test trust me !