Where have all the tadpoles gone?

Some horrible disease of frogs has removed breeding frogs from my garden for a couple of years.  Now imported tadpoles from Surrey have mysteriously disappeared from my ponds and the newts are fewer in number.  The answer is possibly below.

Click on any picture to expand it.

What looked like a blob of excrement moved off!  It went too quickly to get a closer picture.
It is a grass snake, which eats tadpoles, frogs and newts, insects and perhaps slugs.  It is in turn eaten by foxes and badgers, both of which frequent my garden.  The balance of nature should check the growth of the grass snake population, but I fear that it is too late for the tadpoles and frogs, if any.