The period of rapid warming that ushered in the Holocene had many important implications for human populations around the world. Major climatic changes brought about shifts in the range and even existance of many Pleistocene plants and animals that humans had come to depend on for food. Changes in global weather patterns affected the distribution of major biotic provinces, reducing the huge expanses of tundra and plains, and replacing them with vast forested areas. Vegetation zones that had once been fairly homogenous, now became fragmented and compressed into patchwork systems of local ecozones. Migration routes that had once been open were closed by rising sea levels, the formation of new river systems, or the expansion of deserts. In short, these global phenomena resulted in human populations becoming increasingly dependent on highly localized resources, and those populations themselves becoming increasingly isolated from population groups elsewhere.