There’s an urgency to find quality food and water that forces many large mammals to migrate. A new study finds that human activities increasingly threaten their ability to do so.
Photo of zebra migration by Stuart L. Pimm
By Stuart L. Pimm
for NatGeo News Watch
Midnight and there’s no moon. The elephants moving near my tent have only starlight to guide them to the river nearby. There’s an urgency to their thirst.
In August, on the Okavanga River in the southern African country Botswana, it’s well into the dry season.
During the day, the elephants–some still small enough to fit under their mothers’ bellies–have to trek 20 kilometers [12 miles] away from the river to find food. They’ve eaten everything that’s closer. So back and forth they go each day and night, with ever-longer treks as the dry season progresses, drinking hurriedly before turning around.
In another few months, the rains will come. We know from our satellite collars that a handful of females carry around their necks, that the breeding herds will give up their nightly commute and head north, away from the river, as far as they can, knowing there will now be ephemeral pools from which to drink.
Summertime, at last, and the living will be easier–but still constrained. Fences along the borders with Namibia and Angola restrict how far they can move.
Newly published work, in the journal Endangered Species Research, shows that such frustrations harm many species of large mammal.
Grant Harris, a wildlife biologist with the U.S. Fish and Wildlife Service, and an international group of collaborators have scoured the scientific literature to catalogue migrations of large mammals. “There are a lot of migrations, most are severely threatened, some are extinct, and we just don’t know enough to save many of them,” he told me.
NGS photo of elephant herd on the march in Chad, Africa, by Michael Nichols
For elephants and many other species of large mammal, movement is survival–they must eat and drink every day and food and water are usually in different places.
As the seasons change, so does where the best food is and, in dry-season Africa, water is very sparse and precious.
Elephants move one breeding herd at a time, each matriarch seeking her own solution for her grown daughters and their children. The migration from the Okavanga river in the dry season northwards with the summer rains is a diffuse one.
For other large mammals, seasonally changing food and water makes everyone move together. Such aggregations are among the most spectacular wildlife spectacles on Earth–and it’s these that Harris and his team wanted to document.
“By far the most famous is that of wildebeest, zebra, and other species in the Serengeti-Mara ecosystem of Tanzania and Kenya,” Harris said. “About two million animals are involved and it’s the worlds largest.”
NGS photo of caribou migrating in Arctic Wildlife Refuge by George F. Mobley
Like most other conservation biologists, I’ve been vaguely aware that there are other less famous migrations, with other species, in other places–and ones that human actions threaten as do fences the elephants I’ve described. “There’s a lot more of these migrations than I thought when I started this study,” Harris confirmed.
Harris and his colleagues find that large mammal migrations fall into two broad classes. One, like the elephants, involves animals driven by the seasonally changing distribution of good quality food and access to water in the dry ecosystems in southern and eastern Africa.
Snow Forces Animals to Move
In the second class, snow forces animals to move off grazing lands to snow-free areas. Examples include the caribou (reindeer) migrations across the Arctic tundras of North America and Eurasia, and Mongolian gazelle, chiru and saiga antelopes in central Asia.
The Serengeti-Mara ecosystem in East Africa not only holds the largest migration, but it’s one that isn’t fenced and its mammals are reasonably well-protected. Even so, hunters still kill about 40,000 wildebeest each year, illegally.
The massive migration “attracts a lot of tourists–and their money–and that helps protect it,” Harris said. He explained that there were once similar migrations in and out of what are now Kruger National Park (in South Africa) and Etosha National Park (in Namibia). “The park protected the animals, but in doing so, this stopped the migration and the numbers of animals plummeted.”
NGS photo of wildebeest migrating across the Serengeti Plain, Tanzania, by Joe Scherschel
Most of the other migrations in Africa are in trouble, too, either from hunting, or from fences that shut off migration routes or exclude animals from what is now agricultural land.
“The situation may be better for caribou and reindeer, but especially in Siberia, we have no idea how climate change might change things,” Harris says.
So what did Harris learn from assembling the list of migrations, I asked. He replied: “I was shocked that apart from one or two well-known examples, these migrations have been overlooked by conservation science–and we’re losing them from lack of attention.
“Let’s understand the state of play, what we know and don’t and what we need to know to preserve them. Only with foresight, can we keep these wonderful natural phenomena.”
Professor Stuart L. Pimm is a conservation biologist at Duke University, North Carolina. A former member of the National Geographic Committee for Research and Exploration, Pimm is the author of dozens of books and research papers, including the book “The World According to Pimm: A Scientist Audits the Earth.”
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