Several species of ducks, geese, beavers and turtles, such as eastern painted turtles and common snapping turtles, eat duckweed. There are also several species of fish that eat duckweed, primarily grass-eating carp such as koi and grass carp, although grass carp will likely eat all other aquatic vegetation before moving on to duckweed.
Although Canada geese and mallard ducks eat duckweed, it is primarily domesticated waterfowl that tend to eat it, not wild species. Some snails, including stagnant pond snails, and gastropods also eat duckweed. Duckweed also provides shelter for a much greater number of species, including snakes, frogs, muskrats and a number of different fish.
There are three different species of duckweed: Wolffia, Lemna and Spirodela. Of the three, Wolffia can be eaten by humans and is quite nutritious. Because most duckweed grows in dirty water with high calcium levels, which the plant then absorbs, most Lemna species have very high levels of calcium oxalate, which can be dangerous for humans. Provided it is grown in clean water, Lemna could also be eaten by people. Lemna is often dried and used as feed for cattle, as it contains more than 45 percent protein and only a small amount of fat.