In the category of monumental natural events, the annual migration of wildebeests across eastern Africa ranks high. The image of vast waves of gray and black animals pouring across oceans of grass, to the sonorous tones of David Attenborough’s narration, is unforgettable.
But this breathtaking annual ritual involving some 1.3 million wildebeests, the world’s largest migratory movement of grazing animals, isn’t a sure thing. Human blockades in the form of fences and roads have already taken a toll. Now there is new evidence that such migratory disruptions can undermine the basic genetic wellbeing of this species.
“Wildebeest populations which no longer migrate, but have historically done so, are simply less genetically healthy than those that continue to migrate. And this weakens their chances of long-term survival,” said Rasmus Heller, a wildlife genetics expert at the University of Copenhagen involved in the new research.
Vast wildebeest herds once swept across large swathes of eastern and southern Africa’s savannas. But that annual migration has disappeared in many places. Today the only remaining epic migration route passes from the Masai Mara plains in Kenya south to Tanzania’s Serengeti. Another group of wildebeests are thought to have once mounted they’re epic migration on a similar scale in another part of Kenya. But that subspecies, known as the eastern white-bearded wildebeest, now numbers around 10,000 animals that don’t travel far. Likewise, a population of 260,000 in Botswana in the 1970s shrank to 15,000 by the late 1980s as a consequence of fencing erected to protect cattle from diseases carried by wild animals. Around the world, the effects of fences are so profound that some scientists have called for a new field of “fence ecology.”
The blue wildebeest, the most common species and the one pictured in all those nature documentaries, is not considered an endangered species. But there are fears that the last major migratory route is in danger, said Mikkel Sinding, another University of Copenhagen scientist involved in the genetic work. “The Serengeti-Mara migration is also threatened by plans for roads and rail corridors.”
Besides being cut off from crucial feeding and watering grounds, the scientists wanted to know if this loss of migration posed a less visible threat to the animals through their DNA. They gathered genetic samples from wildebeests spanning all the different subspecies and locations, from Kenya to South Africa. They then mapped the entire genomes from 121 animals, creating a picture of the location of each of the four basic nucleotides that form every living organism’s distinctive genetic fingerprint.
By comparing those fingerprints, the scientists could see how the different herds were related. They also looked for signs of genetic problems such as inbreeding, which can lead to a prevalence of genetic problems, and lower genetic diversity, which can make a species less adaptable to changing circumstances.
The genetic consequences of isolated populations created by fencing have been illustrated elsewhere, such as South African elephants that lost their tusks over several generations.
The analysis revealed that the DNA of wildebeests that continue to migrate looked different – and healthier – than their non-migrating brethren. They showed more genetic connections between animals living far apart, greater diversity and less inbreeding, the scientists reported last week in Nature Communications.
For the animals that don’t migrate, their genetic shortcomings will likely “lead to lower survival, reduced fertility and other negative effects on fitness,” says Xiaodong Liu, a postdoctoral researcher at the University of Copenhagen who worked on the study.
The findings are more evidence of the toll taken by the wildlife-blocking infrastructure we are spreading across the landscape. The lessons could also extend beyond Africa’s wildebeests to other migrating grazers, such as reindeer. “The study shows us that wild animal species, for whom migration is an essential part of their biology, struggle to survive in an increasingly human-dominated world, unless special attention is paid to preserving their old and natural migratory routes,” said Heller.
Just because a vivid documentary captures a natural wonder teeming with wildlife on a scale that seems almost immune to human meddling, that doesn’t mean it’s safe without concerted efforts to keep it that way.
Liu, et. al. “Introgression and disruption of migration routes have shaped the genetic integrity of wildebeest populations.” Nature Communications. April 12, 2024.
Photo credit: Dickson Kaelo/Research Features
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