Two years of dry weather are followed by two wet years, causing the grass population to rise. How will the increase in grass affect the animal populations?

Answer is;

The animal population in the environment will increase.

This is because the grass is the main producer in a habitat. Its plentifulness directly influences that of the consumers up the food web. Increase in grass population means reduced competition for primary consumers and their population will therefore increase. This will also reduce competition for secondary consumers and tertiary consumers up the chain.