Why do the numbers of foxes and rabbits chase each other up and down over the years?

Imagine drawing a graph of the foxes in a wood over many years, then drawing the rabbits on top. In some places scientists really do see two lines like this, and oddly, they seem to take turns, one rising just as the other falls. They are locked together like partners in a dance. Why do foxes and rabbits chase each other up and down year after year?