Plastics choke birds, fish and other animals that try to eat them. Or they fill up the animals’ stomachs so that they can’t eat proper food, and starve to death.
They clog up drains and sewers and cause flooding.
Plastics collect in rivers, and get in the way of fish. Some river beds now contain a thick layer of plastic
Plastics blow into trees and onto beaches. So the place looks a mess. Tourists become put off.