Other important habitats are mudflats, shell sands, bird cliffs and coastal heaths. Ecosystems can be defined at a variety of scales, from a single pond, a fjord, an ocean or the entire globe. Functional diversity can also include differences between populations' or species' response to various stress factors. Seaweeds provide substrate, shelter and food for a rich associated flora and fauna, which in turn provide food for a large variety of animals including many fish species.