Duckweed, as the name suggests is food to ducks and fish as well as several species of birds. Duckweed also supplies shelter and protection for aquatic animals like the frogs, fish, insects, crustaceans, snakes and others animals.
2 Additional Answers
Pigs, deer, elephants, bobcats, ducks and a few other birds will eat duckweed. There are also fish that will eat duckweed, including carp. You can find more information here: http://schmidling.com/duckwd.htm