The ground trembles and clouds of dust swirl as large herds thunder across the African savanna. The annual migration of 1.3 million wildebeests through Tanzania’s Serengeti and Kenya’s Maasai Mara attracts hundreds of thousands of tourists, the phenomenon putting Serengeti on UNESCO’s list of World Heritage Sites.
As the animals cross the Mara River, a number are picked off by lions, hyenas and crocodiles, but the march continues, the animals egged on by new grass shoots in Kenya’s Masai Mara. The migration of this emblematic species is important for the ecological functioning of ecosystems.
Unfortunately, the epic annual migrations of this scale are now only found in a few places on the African continent. Roads, fences, farms and urban sprawl threaten to fracture the historic migratory routes of the wildebeest herds and prevent them from roaming far and wide in search of fresh grass and water.
And now, a new study led by researchers from the University of Copenhagen shows that the genetic health of wildebeests has suffered as a consequence of the human activities.
“No one ever knew that this affected the genetics of wildebeest,” says Rasmus Heller, an associate professor at the Department of Biology and one of the new study’s lead authors. “Our results clearly show that 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.
According to the scientific study published in the journal, Nature Communications, the genetic decline of non-migratory populations is reflected in several of the parameters by which genetic health is measured in nature conservation.
“Wildebeest that can no longer migrate have lower genetic diversity, are more genetically isolated and are more inbred. We expect this to lead to lower survival, reduced fertility and other negative effects on fitness,” says Xiaodong Liu, one of the study’s first authors.
While the savannah grazers are not currently threatened, wildebeest herds that can no longer migrate will likely be worse off in the face of climate change.
“The long-term consequence is that populations with low genetic diversity are less equipped to cope with the effects of environmental changes. Their evolutionary potential is reduced. So, if climatic changes continue to occur, there isn’t as much genetic variation for them to work with to adapt. This could ultimately threaten their survival,” says Heller.
Researchers analysed the whole genomes of 121 wildebeest from their entire range, which spans from South Africa to Kenya. This is the first time that scientific researchers have studied the genetic effect of migration in wildebeest.
“Because we studied the genomes of many wildebeest from virtually their entire range, we have been able to make a general genetic comparison of migratory versus non-migratory populations. And because we witness a consistent difference across multiple locations, the conclusion is clear. Indeed, we can say that the overall negative effect is evident in those wildebeest that have been prevented from migrating, regardless of where they live on the continent,” says Liu.
About 150 years ago, many wildebeest populations made great migrations, but in the last 40 years, only two large intact wildebeest migrations remained in Africa: the famed Great Migration of the Serengeti-Mara and one in the Kalahari Desert of southern Africa.
In the Serengeti-Mara migration, approximately 1.3 million wildebeests, accompanied by about 200,000 zebras and 400,000 gazelles, cover up to 3,000 kilometers annually in a clockwise cycle that follows seasonal rainfall patterns. However, only the wildebeests and zebras from the Serengeti cross the Mara River into Kenya’s Masai Mara National Reserve.
Due to limited source material on African wildlife populations prior to the mid-1800’s, the total number of historical wildebeest migrations is uncertain. However, it is known that migrations comparable to that of the Serengeti-Masai Mara population have been lost.
For example, two wildebeest subspecies including the Western white-bearded wildebeest which makes up the Serengeti-Mara population, and the Eastern white-bearded wildebeest, had a migration centered around Kajiado County during the time of the first big game hunters.
While the Western white-bearded wildebeest was protected in the Serengeti-Mara from the early 1950s, increased human presence and activities dating back to the early 1900’s put mounting pressure on the Eastern white-bearded subspecies.
Today, only 6,000-8,000 Eastern white-bearded wildebeest remain and are divided into small isolated populations.
In both East Africa and southern Africa, fences erected to protect cattle from coming into contact with migratory wild animals has contributed to reduced numbers of the animals. For example, Botswana’s Kalahari population declined from roughly 260,000 in the 1970’s to fewer than 15,000 in the late 1980’s.
“Today, the only remaining large population is that of the Serengeti-Mara. But the Serengeti-Mara migration which is also threatened by plans for roads and rail corridors through the area,” says Mikkel Sinding from the Department of Biology, another of the study’s first authors.
Joseph Ogutu, a senior statistician in the Biostatistics Unit at the University of Hohenheim says wildebeests are dependent on migrations to support their large numbers. He says the animals can survive in resident, non-migratory populations, but their numbers shrink when they cannot migrate. In Kenya, the Loita-Mara migration has almost collapsed as a result of numerous fences.
“The migrations of wildebeests make them a keystone species in ecosystems, as their grazing keeps vegetation healthy, transports and distributes nutrients, while they serve as prey for predators and carrion for scavengers. We might add the enormous amount of tourism revenue that benefits governments and local communities.”
The researchers hope the new results will inspire investigations into the genetic effects of reduced migration among other species.
“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. We hope people will be more cautious about continuing to disrupt these routes. This concern is not just with regards to wildebeests, but also other migratory species in Africa and elsewhere,” says Heller. “If we want the species to not just exist for the next 50 years but to thrive and actually survive in the longer term, we need to halt the genetic decay caused by curtailing their natural migration routes.”
