subnivean

Loss of habitat is to blame, specifically, the lack of the pocket of air between snow and ground called the “subnivean space.” There lemmings winter and give birth to prodigious litters of young.

环境的缩小是一个原因，更主要的是它们在冰雪和地面之间被称为“雪下空间”里得不到保护，有旅鼠会在冬天下数量惊人却无法活的小旅鼠。