I have dug out my copy of "Cry of the Kalahari" by Mark & Delia Owens (ISBN 0-00-637030) and thought I'd quote some of the relevant sections. It is 1979 and the rains have failed.
This was unlike the Serengeti migration, where herds of wildebeest often mass together in great numbers. Because they live in a marginal, semidesert habitat, the Kalahari population is more mobile and less concentrated to begin with; now it was moving in herds of from 40 to 400 antelope, scattered along a vast front measuring more than 100 miles from east to west.
Not all the herds headed in the same direction. One portion, probably more than 90,000 strong, had taken the route to the north [to Lake Xau, Lake Ngami, the Okavango]; tens of thousands of others began walking toward the Limpopo river 300 miles to the east.... Once underway, they spent little time feeding, for without moisture, they could not digest what they ate. Their aim was to get to water, and perhaps to better forage, as quickly as possible....
The herds covered about 25 or 30 miles each night. From the air the dusty migratory trails looked like gnarled fingers reaching for the lakes and rivers.... The desert was taking its toll of the very young and the old; they were left behind for the scavengers. The physical condition of each animal was pitted against the great distances that had to be traveled, with little to eat and nothing to drink, but evolution had prepared them for the trek, and the strong should survive.
Suddenly, the wildebeest stopped short.... Stretched across their path were strands of high-tensile steel wire - the Kuki foot-and-mouth-disease control fence....
The wildebeest were cut off from the emergency water and riverine habitat that for eons they had counted upon in times of drought. Nothing they had ever learned, none of their instincts could help them deal with this obstacle.
As they plodded along the fence, they encountered many other herds, part of the same migration headed for the lakeshores and riverbanks. Each day they were joined by giraffe, gemsbok and hartebeest, all needing water but trapped by the wire and the posts.
....Animals began to drop from hunger, thirst and fatigue. A giraffe who could have easily stepped over the wire became tangled in it. He struggled to get free, but the coils of high-tensile steel sliced deep into his flesh until he pitched forward, breaking his foreleg at the knee. His hind legs still ensnared, he pawed at the ground for days, building small mounds of sand around him as he tried to rise again. He never did.
This is a heart-rending
story of how well-meaning intervention (fences to stop foot-and-mouth) had catastrophic unintended consequences. To me, this underscores the importance of realising that we are all part of the same natural system, and that the balance is finely tuned. We meddle at our peril.