Mountains, deserts, and grasslands are ecosystems, each characterized by its own climate, vegetation, and animals. However, these ecosystems are closely related. Mountains often form boundaries between hot and cold regions, and grasslands form between mountain ranges. Therefore, mountains influence the climate of desert areas and the adjacent grasslands. Human activities have disrupted the balance between these ecosystems. For example, trees have been cut down in some areas to make way for agricultural development, housing, or recreation. Most scientists believe that human-caused global warming is the primary threat to these ecosystems.