Organisms may play only one role in an ecosystem true or false

This is false. Organisms may play several important roles in an ecosystem. Consider for example and ocean shore environment occupied by a particular species of crab. The crabs are scavengers and eat any organic matter they encounter. They therefore play an important role in the cycling of nutrients in the ecosystem. The crabs are however also an important source of food for a range of other species occupying the same habitat, including octopi, certain fishes and sea otters. Therefore, the crabs are an important part of the food web in the ecosystem. Many species similarly occupy multiple important roles in an ecosystem.