Life developed in the water in the forms both of grazers and predators-upon-grazers. When life developed on land, it took the form of both plant-eating and planteating-eating creatures. In the air there are birds that eat insects, and there are hawks and kites that eat birds-that-eat-insects. We might say, indeed, that wherever life evolves it takes this form of beast and predator.
It's true also of liminal places: the beach, half-land, half-sea; the treetops, half-land-half air, both territories with their specific inhabitants and specific predators. But, wait: what about the surface of the water? That interface between water and air? All those ducks and swans and geese ... why is there no hawk-swan, or falcon-duck? Why, of all the animals adapted for life on the surface of the water, are none of them predators? What makes that environment so special?