How does a rabbit feed a fish?

Rabbits live in a cage above a fish pond.  The rabbit droppings feed the fish.  The Fish droppings enrich the soil.  The grass growing from the soil feeds the rabbits. 

In the mean time, the rabbits thrive and have sex like mad.  They produce many more rabbits. 

The fish also reproduce and spawn many more fish. 

The farmer eats some of the rabbits and sells many.  Also eats some of the fish and sells many more. 

Everybody is happy, sort of.