Why ups and downs?
The decrease of vole after its peak, could be caused by the vole eaters, who after the vole's peak grow so common that they thin out the number of vole. Moreover, during a peak year the vole may exhaust its food resources (seeds, buds, bark and roots) so there is nothing left to eat. Scientists have no final answer.
Spiral of fluctuation
When vole-numbers are reduced, vole-eaters also thin out fast since their food becomes scarcer. A vole-hunter eats vole even when the vole is rare, which slows down its propagation. And when vole-hunters decline, the vole is able to multiply again. And so the spiral of fluctuation continues among voles and hunters.
