western limit is bounded by Lake Victoria and Mount Kenya. The range does not include montane or temperate grasslands. These wildebeest are rarely found at altitudes over . With the exception of a small population of Cookson's wildebeest that occurs in the Luangwa Valley (Zambia), the wildebeest is absent in the wetter parts of the southern savanna country, and particularly is not present in miombo woodlands. Three African populations of blue wildebeest take part in long-distance migrations, timed to coincide with the annual pattern of rainfall and grass growth on the short-grass plains, where they can find the nutrient-rich forage necessary for lactation and calf growth. The timing of the migration in both directions can vary considerably from year to year. At the end of the rainy season, they migrate to dry-season areas in response to a lack of drinking water. When the rainy season begins again a few months later, the animals trek back to their wet-season range. These movements and access to nutrient-rich forage for reproduction allow migratory wildebeest populations to grow to much larger numbers than resident populations. Many long-distance migratory populations of wildebeest existed 100 years ago, but currently, all but three migrations (Serengeti, Tarangire, and Kafue) have been disrupted, cut off, and lost. Threats and conservation Major human-related factors affecting populations include large-scale deforestation, the drying up of water sources, the expansion of settlements and poaching. Diseases of domestic cattle such as sleeping sickness can be transmitted to the animals and take their toll. The erection of fences that interrupt traditional migratory routes between wet and dry-season ranges have resulted in mass death events when the animals become cut off from water sources and the areas of better grazing they are seeking during droughts. A study of the factors influencing wildebeest populations in the Maasai Mara ecosystem revealed that the populations had undergone a drastic decline of around 80% from about 119,000 individuals in 1977 to around 22,000 twenty years later. The major cause of this was thought to be the expansion of agriculture, which led to the loss of wet-season grazing and the traditional calving and breeding ranges. Similarly, drastic declines have recently occurred in the Tarangire wildebeest migration. The total number of blue wildebeest is estimated to be around 1,550,000. The population trend overall is stable and the numbers in the Serengeti National Park (Tanzania) have increased to about 1,300,000. The population density