I'm afraid it's called nature- and we should leave well alone. More tadpoles are eaten every year by gold fish and gold carps, etc, than by dragonfly larvae. Our pond is full of those larvae- and we have been watching the absolutely amazing spectacle of them climbing on a reed- latch on and begin the spectacular transformation. Pushing the head hard back until it pierces the skin, and then continuing to pull slowly back for about 1hr until the whole body of the dragon fly has emerged- then having to turn quickly to latch on the reed to wait until dry, and then fly off - leaving the shell of the larvae attached to the reed- looking like it is still there but it is just the
skin! It is just a miracle- as the dragonfly that emerges is quite a bit bigger than the larvae.
They then flutter at great speed all over the garden, with their huge blue and yellow eyes- eating mosquitoes and other bugs.
The pond is also full of black alpine newts, with bright orange bellies- that also eat tadpoles- but as said, this is nature. Some tadpoles will always survive- but I really believe interfering is just plain wrong.