The dragons (i.e. mankind) are multiplying fast, so that there are more of them than the earth can sustain. Their dwelling places spread all over the planet and their demand for food and drink outgrows the resources. Their rubbish contaminates the environment. By blowing hot air (i.e. cars) they cause the temperatures to rise and the polar caps to melt. Soon other animals become extinct. In the end the dragons see the error of their ways and change their habits.