Transhumance is a type of pastoralism or nomadism, a seasonal movement of livestock between fixed summer and winter pastures. In montane regions (vertical transhumance), it implies movement between higher pastures in summer and lower valleys in winter. Herders have a permanent home, typically in valleys. Generally only the herds travel, with a certain number of people necessary to tend them, while the main population stays at the base. In contrast, horizontal transhumance is more susceptible to being disrupted by climatic, economic, or political change.