Lemmings

Lemmings account for around 90% of the food source of snowy owls. Their population is normally characterized by large booms over regular periods of time (usually every 4 years) to help combat this large level of predation. In recent years, however, these populations have not been booming. There are many factors as to why this is happening, but the largest seems to be the change in habitat for these creatures.

http://polardiscovery.whoi.edu/expedition1/images/postcard_lemming.jpg

Lemmings rely on large sub-terrestrial tunnel networks for safety from predators and food storage. These tunnels are usually made out of snow, and in recent years, arctic warming has compromised the structural integrity of these tunnels. When tunnels collapse due to melting some lemmings will be either buried under large layers of snow, or their food will and they will starve. This recent trend has not allowed for population booms of lemmings, and snowy owl predation ranges have had to shift to adjust to a relative lack of food.