In addition to these three mammals, Norway lemmings are also found in the park. The number of lemmings varies extremely, with massive spikes in the population during some years, immediately followed by a very rapid decline. This phenomenon is not completely understood; it appears that favourable weather, and therefore a surplus of food, results in sudden population growths, but the reason for the decline is less obvious, although it is certain that contagious diseases play some role. These cycles are also reflected in the populations of animals who prey on the lemmings.