When wildebeest, such as those famous for crossing the Mara River in Tanzania during their annual migration, run into a crocodile or some other danger it is often the first time they've seen that particular thing. This is because most wildebeest don't live very long so many are on their very first migration. One wonders what would happen if you killed all of the wildebeest migrating in a particular year and set new ones out on the landscape to take their place. Would the migration continue?
Probably not, initially. Something like this did in fact happen on the Botswana-South Africa-Namibian border when large fences put up between unfriendly nations, more or less in a single year, stopped an annual migration in its tracks killing most of the animals at the fence line. Now, there is no major migration active in that area and a small number of wildebeests wander back and forth across the arid landscape looking for some grass and getting their water from boreholes in the dry riverbeds that crisscross the area.
And that is only one of many stories of migrations thwarted by human activities.
Almost everything in or about Africa is somehow misunderstood by people outside of Africa, and the topic of the great migrations of wildebeest and other mammals is one of them. First, most people think only of the Serengeti migration, but there are several other migratory circuits. Second, most people think of thundering herds of animals crossing the landscape. But really, the only time they thunder across the landscape is when they are being chased by photographers in big trucks or helicopters. What actually happens is this: You set up a tent and see some wildebeest and zebras hanging around in small family groups. You go to bed. The next morning and for the next day or so, there are a lot more of these family groups. Then one day there are fewer. Then almost none. That was the migration by your tent. If you head in the right direction for a few hours the next day, you'll find them again. In other words, they walk, slowly, and often don't move at all for a given period of time.
The other thing that is often not understood not only of Africa but of much of the world is that vast areas of wilderness are circumscribed by human-altered landscapes so that migrations of land mammals have by and large stopped on much of this planet. The animals that were migrating did not have to migrate (often) had their numbers been kept down, but with the famous Malthusian over-production always going on, they needed to move often in a seasonal cycle, either moving from one green pasture to another, or more often, moving between a dispersed pattern in one habitat to a concentrated pattern in a different habitat, either in a back and forth pattern or a cycle. It can get rather complicated.
With migratory routes closed down, the animals are still there but in much smaller numbers. So one week I spent in Bostwana looking at elephant damage to vegetation, I saw dozens of antelopes. But every antelope was actually an aborted fetus... there were no adults to be found, though they were out there somewhere. The females had all "dropped the fetus" as a strategy to deal with lack of water and food, which in turn occurred because of the fences, which in turn were put up either as international borders between unfriendly states (and not yet taken down though the states are now friendly) or between cattle ranching areas and wild lands. A year before that, in the Kalahari, I had to stop the vehicle we were driving in and wait as a 'migration' of springbok ... maybe three hundred of them ... crossed the road. There were many springbok that year because the rather random rainfall patterns of that region happen to produce two wet seasons a year for two years so there was continuous, copious grass. The springbok produced calves as fast as they could. Down the road from the springbok herd we found a cheetah female with five ... yes, five! ... cubs. The first chance that I got to talk to a ranger about this, he said "Ya, five cubs on that one, and the cat in the next territory over (pointing to the west as he spoke) ... she's got five of her own as well!" So in that unprecedented year there were herds of hundreds, instead of dozens, of springbok because almost every female in every herd of 20 or so had three or four calves over the previous two years (and of course the first and second round was now having their own calves).
I was impressed but should not have been. Before fences went up, and farmsteads in nearby Namibia, and the closure of access to the Gariep (nee Orange) river to the south, and so on, the herds of springbok, in good years, would number in several thousand or even tends of thousands. The migrations did not require you to stop your truck for ten minutes. The migrations required you to build a barrier around your camp and hunker down for a week.
No Way Home: The Decline of the World's Great Animal Migrationsby David Wilcove chronicles, documents, and discusses the changes over the last century or so in migratory patterns, looking at all sorts of animals, all sorts of causes, and many regions. From the publisher:
We may be witnessing a dying phenomenon among many species. Migration has always been arduous, but today's travelers face unprecedented dangers. Skyscrapers and cell towers lure birds and bats to untimely deaths, fences and farms block herds of antelope, salmon are caught en route between ocean and river, breeding and wintering grounds are paved over or plowed, and global warming disrupts the synchronized schedules of predators and prey. The result is a dramatic decline in the number of migrants.
Wilcove is a professor of biology and public affairs at Princeton and provides an authoritative and accessible study of this important modern phenomenon. He also wrote The Condor's Shadow: The Loss and Recovery of Wildlife in America. This is a powerful and important book. It is available on the Kindle, by the way, and I think it could be read that way. There are a few illustration and they are important, but they are black and white drawings and should be usable in that format.
You say your most recent fieldwork was in South Africa. I was hoping to find a little more than this post. I have just joined the NBN network and came to see the topranked blog. I see a busy news feed so I understand your popularity.
Diana of Elephant's Eye
mmm I did read your other SA posts ...
Have you read this?