In article ,
says...
Any chance that frogs and/or tadpoles eat duckweed?!

Something must eat it because it will not spread on my pond stays at one
end and that's the end the frogs use to lay their spawn, but its most
likely cover rather than food supply.
--
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea