The phrase 'straw dogs' is drawn from the Tao Te Ching, and refers to ceremonial objects of ancient China, carefully prepared and employed in ritual with great reverence, and then thrown away like rubbish. In this manner are Heaven and Earth said to treat the ten thousand things or myriad creatures.

'Heaven and Earth are ruthless, and treat the ten thousand things like straw dogs.'

The lesson, I suppose, is that one shouldn't rely on the favour of heavenly beings for one's survival.