Historically, migrations or long-distance seasonal movements were common in a wide range of marine, freshwater and terrestrial taxa (Berger, 2004). Terrestrial mammal migrations, defined by Berger (2004) as ‘seasonal movements between discrete areas that are not used at other times of the year’, used to occur in most grassland and boreal forest ecosystems in Africa, Asia, Europe and North America (Fryxell et al., 1988; Dingle, 1996). These may have evolved to enhance access to high-quality food or to reduce predation risk (Fryxell & Sinclair, 1988). Because ungulate survival has been linked to the quality rather than absolute abundance of available resources (Fryxell, 1987), any strategy that allows ungulates to select higher quality resources may improve survival chances. Thus in the Serengeti–Mara ecosystem, wildebeest Connochaetes taurinus, zebra Equus burchelli and Thompson’s gazelle Eudorcas thomsonii move in response to seasonal variations in mineral and protein content of preferred forage rather than to variations in forage abundance (Sinclair & Arcese, 1995), although overall survival is regulated by the abundance of dry season forage (Mduma et al., 1999).
In Africa migrations numbering millions of animals were common until the 19th century (Roche, 2008). However, these have declined dramatically in both number and size over the last century and many of those that still occur are believed to be under threat (Berger, 2004), primarily from uncontrolled hunting and habitat fragmentation because of human encroachment from arable farming, pastoralism and urbanization (Harris et al., 2009). The large ranges required by migratory populations are particularly affected by habitat fragmentation because few migration routes are completely within protected areas. Because migratory populations are only maintained by their ability to undertake seasonal movements to areas of higher resource quality/lower predation risk, they rapidly decline once migration routes are blocked and seasonal ranges are no longer accessible (Williamson et al., 1988; Perkins, 1996; Bolger et al., 2008).
Historically many ungulate migrations occurred in Botswana. The largest existed around the Kalahari Desert, with animals spending the dry season either without access to water or around permanent water before moving to seasonal desert grasslands once rain had filled temporary waterholes and initiated the growth of annual grasses (Williamson et al., 1988). Millions of wildebeest, hartebeest Alcelaphus buselaphus and springbok Antidorcas marsupialis moved from the central Kalahari Desert to the schwelle grassland in south-west Botswana (Williamson et al., 1988) and hundreds of thousands of wildebeest and zebra moved from the Boteti River to the Kalahari and Makgadikgadi grasslands (Kgathi & Kalikawe, 1993). Further north, buffalo Syncerus caffer, elephant Loxodonta africana, wildebeest and zebra moved from the permanent waters of the Okavango and Linyanti river systems to the seasonal grasslands of the Savuti Marsh (Vandewalle, 2000).
Between the 1950s and 1980s, Botswana erected veterinary cordon fences across much of the country to prevent disease transmission between wildlife and cattle, especially those destined for European export (Keene-Young, 1999). These 1.3 m high cable fences were put up rapidly with little information on potential impacts on wildlife movements or ranges (Perkins, 1996). Although the deployment of fences gave increased protection for wildlife in specified areas, many migratory movements were disrupted. Reversing such ecosystem fragmentation is increasingly being seen as an important strategy to ensure long-term species survival (Diamond, 1975). However, so far there is little evidence to suggest that traditional large-scale movements by medium-sized herbivores can be restored.Read more: Download PDF Document