卫星帮助解释斑马的迁徙

Every year, zebras in Botswana begin a 580-kilometer migration looking for food. Even though it’s an annual event, researchers only had a general idea when it would start and they didn’t know which route the animals would take or why. That is, until now.

Sometime in September, zebras begin their trek from Botswana’s Okavango Delta to the Makgadikgadi Salt Pans.Pieter Beck, a research associate at the Woods Hole Research Center, was among those studying the migration.

“Like most large herbivores that naturally migrate, zebra migrations have been under threat. Across the globe, most of the big migrations already are under threat. So, conserving the ones that exist is of course important; and finding ways where people and wildlife can live together is critical if you want to achieve that,” he said.

One big mystery was how do zebras choose the route to the salt pans? It was not because the knowledge had been passed down from one generation to the next. Researchers knew that between 1968 and 2004 veterinary fences were built across potential migration routes. The fences were put up to help prevent wild buffalo from transmitting disease to cattle.

Within three years of the fences being taken down, the zebras began to migrate. Now, keep in mind that the fences were up for 36 years and that zebras usually live no more than 12 years in the wild.

“The fences were up longer than the lifespan of the zebra. So the zebra that we tracked making this hundreds of miles of migration – they couldn’t have learned it from their parents. So they actually rediscovered it using probably exploratory walks. It’s a very fascinating story in that sense and sort of a good news story in conservation. And these are unfortunately rare these days,” said Beck.

So, researchers turned to the U.S. space agency NASA and took a close look at some of its satellite images and data.

“The NASA instruments that we worked with orbit the Earth and they give us a global picture of rainfall and of vegetation productivity. So those are the two metrics we extracted. The rainfall data you can get for the tropics in particular. And we can see every six hours from satellites an estimate of how much it rained. For the vegetation data we can see globally every day, if there are no clouds, how productive the vegetation is. Is it green or is it wilted or is it dry?”

What Beck and his colleagues found was that when enough rain has fallen there’s a “surge in the greening of grasses.”

Beck said, “We do see that rainfall is what gets them going. That was very clear. But then, once they’re on the move, they appear to rely both on rainfall and the productivity of vegetation because the leg of migration we looked at is at the end of the dry season. So when we look at the vegetation that’s surrounding them and how green it is, really, we can tease out how fast the animals will migrate.”

And they’ll follow the greening vegetation. But Beck said the zebras are in tune with the environment and don’t simply move blindly in one direction.

“If the rains then abate or you get an interruption, then they’ll actually reverse their migration, which is something that had only very rarely been documented. They’ll reverse their migration and wait for the next spell of rains. Same thing when they return from the very productive grasslands where they are in the wet season. If you get a very late rain spell, they’ll actually reverse again and extend the period that they can graze in the grasslands,” he said.

So researchers wanted to know whether being in tune with their environment would help the zebras – and possibly other migrating animals -- adapt to climate change.

Beck said, “In our research we assessed the chance of those weather systems becoming decoupled and really the cues that animals rely on now not becoming reliable anymore in the future – and whether the animals’ behavior is adaptable enough to cope with that.”

That kind of knowledge could help in the conservation of wild animals. He also says knowing which migration routes animals will take may help game park rangers and others better protect them from poachers. Beck and his colleagues presented their findings in the Journal of Geophysical Research – Biogeosciences.