Ancient Migration Regained as Fences Come Down
Posted by Richard Conniff on August 10, 2013
For decades, a network of veterinary fences disrupted traditional wildlife migration routes in Botswana, in southern Africa. The idea was to keep wild buffalo from transmitting disease to domestic cattle. But the horrific result was that dead zebras, giraffes, wildebeests, and other species sometimes piled up by the thousands along the fence lines, unable to find their way to grass or water.
Then in 2004, some key fences came down, and what seemed to be a kind of miracle happened. Zebras whose parents and grandparents were too young to have experienced the old migratory route nonetheless picked up as if the fences had never happened.
They made their way across 180 miles of sparse, flat countryside from the Okavango Delta south to the Makgadikgadi Salt Pan, just in time for the November rains to fill the potholes and drive up a fresh crop of grass for grazing. Then, when the salt pan dried out again, they turned around and walked 180 miles back to the Okavango Delta. It was a remarkable instance of the irrepressible power of nature.
Now, in an article in the Journal of Geophysical Research–Biogeosciences, a team of co-authors, combining data from ground level and from 220 miles up in geostationary orbit, has set out to discover just how the zebras did it.
A young researcher named Hattie Bartlam-Brooks, working with the Botswana Herbivore Research project, first picked up on the recovering migration in 2008, from GPS collars placed on some Okavango zebras.
The recovery of the Makgadikgadi migration—the second longest zebra migration in the world—seemed like a big deal, and Bartlam-Brooks began to puzzle over just how the zebras had rediscovered the salt pans. Was it some internal biological clock? Or some primordial compass directing the zebras to the right places at the right seasons?
She turned for help to Pieter Beck at the Woods Hole Research Center in Massachusetts and Gil Bohrer at Ohio State University, specialists in remote sensing and modeling of vegetation. They combined her GPS movement data with hour-by-hour satellite imagery showing how rainfall and vegetation changed over the course of the migration.
The bottom line: The zebras set their course by traveling where the rainfall and the green grass lead them.
The researchers also quickly realized that by adding daily rainfall and weekly vegetation data from satellite images to their migration models, they could predict with astonishing accuracy when the zebras would move and how fast they would move.
That doesn’t mean the zebras are robots, responding mindlessly to cues in the environment. On the contrary, it means they are smart enough to explore their habitat, see where conditions are most promising, and do the best they can to ensure their own survival.
For Beck, understanding the mechanisms of migration is especially important in a world where human intrusion has already disrupted many old migratory routes, and where those that remain face dramatically shifting conditions.
“We need to know what the fate of those migrations is under climate change,” says Beck.
That means understanding what the animals are looking for at different times, when they are likely to move through an area, and what drives them from season to season. “Being able to predict that into the future is very useful information to managing those landscapes so that migratory animals and humans can coexist.”
The new study also provides reason for hope. It demonstrates “how important the consistency and strength of the rainfall cues were for migration success,” says Bill Fagan, who specializes in combining field data with theoretical models for his research at the University of Maryland (and who did not participate in the zebra study).
The study suggests that, even when species have had their migration patterns disrupted, they can relearn them from “exploratory walks” driven by environmental cues. “With so many ungulate migrations declining worldwide,” says Fagan, “it is nice to have an optimistic result about migration for a change.”