Each June on Tanzania’s Serengeti plains, animals resume a journey they’ve been making for millions of years. It’s the drying of the grasses that triggers them and the scent of rain in the north that beckons them. Then, as if a herdsman had cracked a whip, wildebeest, zebra, gazelle and antelope sweep over the plains, and for a few weeks the Serengeti thrums with hoofs pounding against hard earth. These are sounds our hominid ancestors would have heard. And it is a scene they may have watched from a hillside overlooking the plains.

This is the largest mammal migration on the planet, and although ecologist Tony Sinclair and I had witnessed it on other occasions, we were still dumbstruck by the huge number of animals massed together. We were nothing more to them than a rock, an obstacle in their path. Our Land Rover parted their flood, and then the parted streams merged back together and the wildebeest continued en masse, doing what they have always done: eating their way across the plains, as much a force of nature as a wildfire or sudden storm.

It is this migration that many people picture when they think of wild Africa, and it was this migration that led the British colonial government to create the Serengeti National Park in 1951. By saving the Serengeti, one of the park’s advocates wrote, they would preserve a small remnant of Africa’s “original splendor, so that the black and white men who follow us will be able to see it in its awe-filled past glory.”

“They thought the Serengeti was untouched, ‘primeval’ Africa,” said Sinclair, an ecologist at the University of British Columbia at Vancouver. His 40-plus years of study here have left him weathered and freckled, and earned him the nickname Mr. Serengeti. “But of course it was neither of those things. We—humans, that is—had shaped it to a great degree, although no one realized it at the time.”

Sinclair pointed to the wildebeest around us. “The wildebeest population now is about one and a half million,” he said. “In the 1960s there were barely 200,000.” After dropping to this low point, the wildebeest numbers began to rise again, as did the buffalo population. “The wildebeest and buffalo were increasing by leaps and bounds. And that’s why the park service invited me here. They wanted to know why.”

In pursuing that question, Sinclair discovered two things about the Serengeti that had escaped the park’s founders. The first is that this land is far from being a slice of unchanged nature, a part of the wild frozen in time. “Everyone looks at the Serengeti and wants it to be lasting and unchanging,” Sinclair said. “It may very well last, but it will most certainly change.” The second is that wildebeest—these most unlovely ungulates with their scruffy beards and tails, twisted horns and goggle eyes—play the crucial role in the dynamic life of the Serengeti. “Without the wildebeest, there would not be a Serengeti. They are its lifeblood.”

It’s relatively easy to understand how the wildebeest might affect something like the number of lions in the park. The ungulate is one of the biggest meals a lion can bring down; at the height of the migration, lions need only wait by a gully or river crossing, then slap out a paw to put meat on their table. They lie brazenly on the roads, fat and dazed from the gorging. This leaves the wildebeest nervous and alert. A herd may be loping along rhythmically one moment, seemingly oblivious to anything other than the need to move, and in the next is heaving with panicked animals darting this way and that, trying to escape.

Once, some years ago, I camped with Sinclair, his wife, Anne, and his colleague Simon Mduma of the Tanzania Wildlife Research Institute on the banks of the Serengeti’s Grumeti River. The migration was coming to an end, and several big herds of wildebeest settled near our tents for the night. I listened to them snorting and shuffling about, and then I heard the first lion’s roar. Actually, I felt rather than heard it, the deep rumbling coming up through the ground as if the earth had a bad, dry cough. More lions joined in, and the thin walls of my tent seemed to vibrate with their roars. Involuntarily, I curled into a fetal position. My breathing sank to something less than shallow. Here I was, a skinny, little, clawless, hornless primate...on the ground. “Take the wildebeest,” my racing heart chanted. “Take the big, fat, succulent wildebeest.”

Maybe that’s what the panic of the wildebeest is about: “Take him! Not me!”

Had the increased number of wildebeest led to a greater number of lions? I now asked Sinclair.

“Yes,” he replied. “But not in the way you might think. Certainly, more prey means more to eat. But what has really helped the lions is the increasing forest cover—which is actually because of the wildebeest.”

To understand how the wildebeest affect the trees and thus the lion population, you must know what was here in the past and what is here now. And that is what Sinclair has learned. Over the years, he has studied everything from the population dynamics of the herd animals and their predators to the effect grasshoppers have on the various types of grasslands. He has traveled to almost every corner of the 5,700-square-mile park, counting birds and acacia trees to assess how the various species of trees affect the number and variety of birds and butterflies. He has collected piles of buffalo bones to analyze the fat in their marrow, and he has counted seedlings and shrubs to measure the density of woodland thickets. When he sees a particular species, he jots its name and location in a pocket-size notebook. He also photographs key areas, such as the forests on certain hillsides or bordering major rivers—a record that shows how the tree cover comes and goes. And for the past two decades, he and Mduma have overseen a biodiversity survey of the park, slowly building up a record of its creatures. “None of this is very high tech,” Sinclair said, “but if you keep at it long enough, you begin to see patterns, and then you can pick out changes in those patterns.”

Sinclair has pushed his base line back to the 1850s, using the accounts of explorers, hunters, slave and ivory traders and colonial administrators who ventured here. At that time, and presumably for thousands of years before, the Serengeti ecosystem encompassed some 13,000 square miles. The cattle-herding Maasai people, like the wildebeest, followed the greening of the grasses. To the Maasai, the plains were so vast they said you could walk your entire life and never reach the end. The grasslands were, in their Maa language, siringet: endless plains.

Today about three-quarters of the original habitat is protected in various reserves, the largest being Serengeti National Park with its adjoining game and conservation areas. Much of the rest has been converted into wheat farms, cattle ranches, villages and towns that hem the Serengeti to the north and west, limiting the movement of the animals. The wildebeest face a more direct human threat too. Poachers kill some 20,000 of them each year, both to eat and to sell in bush meat markets. (Sinclair estimates that wildebeest meat feeds about one million people.) But as the human population grows (at a hefty 4 percent annually in the towns along the Serengeti’s western border), the number of poached wildebeest could double or triple. “If that happens, it will be a huge threat to the migration,” said Sinclair. “It’s simply not sustainable.”

For now, though, the migration endures. The animals move from their southern home at the base of Ngorongoro Crater west into the heart of the Serengeti, then north and east across the Mara River into Kenya and its Maasai Mara National Reserve. In November they travel back, completing a clockwise journey through the plains—a distance of some 300 miles.

Did the migration follow this same route before the arrival of the Europeans in the interior of Africa in the mid-1800s?

“We don’t know,” said Sinclair. “I couldn’t get a fix on the size of the herds, or what migration route they followed. What was clear was that the herds collapsed in the early 1900s. There was a horrific die-off. It was a seminal event for the Serengeti, with many long-range consequences. Let me show you what I mean.”

He turned the key in the ignition, and the wildebeest that had been loping past us stopped abruptly to stare. For a few moments we held back the tide of animals. But then, having decided that we were not some deep-throated predator, they bent their heads toward the earth and pushed on. Sinclair drove past the herds, turned up another track, and headed straight north toward a low-slung range of hills. As we bumped through the dust and heat, he told me the story of the disease that changed the Serengeti.

It’s common these days to worry about epidemics from SARS, the Ebola virus or the bird flu coming in from the wild to kill off humankind. But “civilized” diseases can also wreak havoc in the natural world. In the late 1800s, Europeans unintentionally introduced rinderpest, a viral cattle disease, into East Africa. Although the virus was found in European and Asian cows, cattle in Africa had never been exposed to it, and, throughout the continent, they died. Rinderpest also swept through ungulates closely related to cattle, such as the wildebeest and buffalo.

“There were so many dead animals and so much food to scavenge that it was said the vultures forgot to fly,” said Sinclair. He pulled to a stop below a tree-covered hill. “And when the animals—domestic and wild—died, the people died. There was dreadful starvation among the Maasai.”

Over time the disease reached a sort of equilibrium. Every decade or so it would rise up and kill off cattle, wildebeest and buffalo. “The populations eventually stabilized,” Sinclair said, “so that there were 15,000 buffalo and 100,000 to 200,000 wildebeest, and over the years everyone came to think that this was the way the Serengeti had always been.”

The disease brought other changes. Without prey to hunt, the number of lions, hyenas and other predators also dropped. And the number of trees increased.

A forest isn’t the first thing that comes to mind when one thinks of the Serengeti. Yet when the colonial administration set up the first game reserve here in 1929, dense woodlands of thorny acacia trees had sprung up across the plains. The trees were even thicker when the park was officially established two decades later. But then they began to disappear.

“The older trees were all dying,” said Sinclair, “and there weren’t any seedlings. Something was killing the forest. And park officials were very worried. They were losing their ‘natural, pristine’ forest. They wanted to know why, and they wanted to know what to do. It was all very confusing. The wildebeest and buffalo were booming, and the trees were dying.”

As it happens, an effective vaccination against rinderpest had been developed in the early 1960s, and veterinarians inoculated the cattle around the Serengeti. The wild ungulates began to rebound. But the trees continued to dwindle. One day in 1980 Sinclair came to the very spot where we were now sitting to photograph what he thought was one of the last trees in the Serengeti. “Here’s the picture I took of this hillside,” he said, pulling a yellowed photograph from a folder and pointing to the slope immediately across from us. In the photograph, a single acacia spreads its limbs over a barren, rocky slope.

When he returned to the spot five years later, Sinclair found that the tree was surrounded by hundreds of tiny, spindly seedlings. “There was an explosion of trees here, and in fact all over this part of the Serengeti. And ever since, the trees have been increasing and increasing. Just look at this hillside.” Indeed, it was virtually hidden under a cover of dense young trees. “That’s how quickly things can change here,” Sinclair said.

Two things led to the forest’s rebound. First, the wildebeest population grew to its current size, between 1.2 million and 1.5 million animals. And all those wildebeest ate more grass each year, in the process turning the long-grass plains into short-grass plains. As a result, human-set fires that sweep through the Serengeti each dry season don’t burn as hot as they did when the grass stayed long and so do not consume as many acacia seedlings. “The elephants were also ‘culled,’” Sinclair said, bracketing the word with his fingers. “Illegal hunters poached thousands of elephants here in the late 1970s and throughout the 1980s, leaving very few of them to eat the young trees. So, yes, the lions are increasing,” Sinclair said. “But that’s because of these dense forests that have grown up because the wildebeest keep the grasses short.” More forest means more cover for the lions; they catch more prey, have more babies and form more prides. Their numbers go up.

But what if something happened to the wildebeest? “That is the one change it cannot withstand: the total loss of the wildebeest,” Sinclair said. “They keep the entire ecosystem going. We would not have a Serengeti without them. It would be something else.”

The Serengeti of the early 21st century is different from the Serengeti of 1950 and 1850. Aside from being smaller, its rhinos, elephants, roan antelope and wild dogs are far fewer. The Maasai people, who lived in the national park until the 1950s, have been replaced by tourists, researchers and park employees in cars. One can argue about whether these changes made the park less natural. But in a time of great extinctions, here in the Serengeti we humans have done something rare: we have set aside a corner of the earth for the animals.

Early one evening I joined Sinclair’s four Tanzanian research assistants on a nighttime survey. Ernest Eblate sat behind the wheel of the Land Rover, and I took a seat next to him while the other three crowded into the back. Eblate handed me a clipboard, saying, “You’re our secretary tonight.” The men in the back would be spotting and calling out the animals, and I would record their numbers, species and other details.

Eblate pointed the Land Rover north along a track. Once a month, the team drives a set distance on each track and counts the number and types of mammal species they see. Our track led past a steady stream of wildebeest munching their way across a short-grass plain, then through grasslands, riverine acacia forests and rocky hills. Eblate kept a careful eye on the odometer, and as soon as it registered the requisite 40 kilometers (about 25 miles), he swung the car around.

Night had settled and a chill breeze blew through the car. We pulled on jackets and sweaters, and the men pointed their lights into the hidden Serengeti. Eblate crept along, giving his colleagues time to search the tall grass and the thorny trees for the bright glow of an animal’s eyes.

It didn’t take long. In the trees, saucer-eyed bush babies leapt from branch to branch to escape the light, while reedbuck stood momentarily frozen in the grasses before bounding into the darkness. There were smaller antelopes, dik-diks, hovering close to the road, and hyenas slinking among the bushes in the distance. We inched down the track, spotting bat-eared foxes, black-backed jackals that turned tail and ran, and then two unexpected prizes: an aardwolf and a diminutive hedgehog, two of the Serengeti’s rarest species. The hedgehog was the size of a tennis ball and sat tucked in a rut in the road beside some low bushes. Eblate had spotted this wee fellow, and we all praised him for his sharp eyes.

As we inched on into the night, my register grew ever longer. We found a cheetah mother with four half-grown cubs huddled in the grass, then spotted three sturdy klipspringers, small rock-climbing antelope that for some reason had left the safety of their boulders to stand in the road. “Get back to the rocks,” Eblate ordered them. “There are lions out here.”

And indeed a few miles farther on, we drove into the middle of a pride, five females and two young males. They’d been feeding, probably on an unlucky wildebeest, and watched us with sleepy eyes. One male stepped into the track and trotted ahead of us, his huge, meat-swollen belly swinging from side to side. He looked at us over his shoulder, then stumbled into the bushes, dropped to the ground and fell instantly asleep.

We found the wildebeests again, thousands of them. They loped steadily and silently over the grasses and under stands of acacia trees, a dark ribbon of animals winding through the night. The spotters’ lights caught the wildebeests’ eyes, and they glowed back at us like the candles of the faithful in a church processional.

“Wildebeest,” someone said softly from the back seat. I marked it on the register. Wildebeest, too many to count. Wildebeest, on the move again, making their way over the Serengeti.

In the wildebeests’ mowed path, the trees will sprout. In the trees’ shadows, the lions will lurk. And in the canopies, birds and butterflies and insects of all sorts will flourish. This is the path that leads through the knotted tangle of life and change in the Serengeti.