Why does Great Migration happen?
The earth vibrates with the thundering of millions of hooves as heaving herds of wildebeest, zebra and gazelles fill the plains to chase the rains in the epic annual struggle for survival known as the Great Migration. It is the greatest show on earth, a natural spectacle like no other and a life-changing experience.
But why does Great Migration happen?
The Great Migration is the largest herd movement of animals on the planet. In fact, with up to 1,000 animals per km2, the great columns of wildebeest can be seen from space.
The numbers are astonishing: over 1,2 million wildebeest and 300.000 zebra along with topi and other gazelle move in a constant cycle through the Mara-Serengeti ecosystem in search of nutritious grass and water. Guided by survival instinct, each wildebeest will cover 500 to 1,000 km on its individual journey along age-old migration routes. Hungry predators – lion, leopard, cheetah, hyena, wild dog and crocs – make sure only the strongest survive.
Wildebeest during Great Migration – Masai Mara 2012
Photo by Emanuele Trabucco
WHY ALL OF THIS HAPPENS?
Let’s try to understand it together…
“Fantastic experience in Mara in June 2019. Sara and Emanuel have been great guides and hosts. Thanks Paka!” Joe W. (London)
Analysing the event from a generic point of view, this is an easy thing to understand. In fact there are two very important environmental gradients across the ecosystem and the wildebeest, together with the other herbivores, are trying to maximise their access to essential resources. So, we start with the two important gradients: rainfall and nutrients.
Starting with rainfall, we have to consider that the northern and western Mara-Serengeti ecosystem is interested by a greater amount of rains compared to the southern and eastern area. And more important is the seasonal difference in rainfall scheme. Obviously most rain falls during the wet season and the wet season rainfall shows a similar pattern to the general scheme.
But the key is dry season rainfall. In fact the far north and the far west have an average of 400 mm of rain even during the dry season, and that is very reliable rainfall, while the rest of the ecosystem is compeltely dry, or there is only a shower every few years. Also, there is only one permanent river in the ecosystem, which is the Mara river in the north. So dry season rainfall means there’s green grass to eat, and the Mara river means there’s water to drink during the dry season in the far north (Masai Mara): a good reason for migrant animals to be there during that season. By the way, the animals move around quite a lot at this time, following local patterns of rainfall and often crossing and recrossing the Mara river throughout their time up there.
As the rains spread more widely in November the ungulates quickly head to the south, moving away from the woodlands to the short grass plains of the Serengeti NP – Ngorongoro border. Why? Well, now the other important gradient comes into play: soil nutrients. And you will understand this better by looking at the geology of the Mara-Serengeti ecosystem. North-east area is 540 – 1500 Million years old, south-east area in Tanzania is fairly recent (not more than 65 Million years – most only 3 Million years old), north-west area (Masai Mara) is over 2500 Million years old, among the most ancient areas on the planet. West area in Serengeti in Tanzania is also relative recent alluvial soil, derived from ancient Lake Victoria shorelines.
Generally speaking, infact, there are three geological areas in Mara-Serengeti ecosystem: the southern areas in Tanzania show very recent soils formed on top of the ash deposits from the crater highlands. The north area (Masai Mara in Kenya) is characterised by rocks formed over 2500 million years ago, while the western area in Serengeti NP in Tanzania has more recent deposits from the rivers and different shores of lake Victroia. Unsurprisingly, the nutrients from the ancient rocks in the north have been washed away since long time, leaving the north in a particular extremely nutrient poor condition, while the short grass plains of the south are still very rich, particularly in phosphorus and calcium, both essential nutrients for pregnant and lactating wildebeest. The recent soils of the west part of Serengeti are rich too, but mainly in nitrogen, which is important but not necessary for pregnant animals. So here, as soon as it starts raining, there is a strong instinct for animals to move away from those wet, but nutrient poor northern woodlands, down to the dry but nutrient rich grasslands of the south. Obviously they can only get here when it’s wet, so to choose this place and time for breeding is surely a great idea. What’s more, predation down here is much lower too, as the hard pan and low rainfall prevents trees since they can’t get their roots through, and for lions it is very difficult to hunt away from rivers and woodlands.
So, now you have understood, I think, clearly the broad-scale movements of the migration. During the dry season, you’ve got to be near the Mara river, in the far north. Once the rains come you want to move as fast as possible down to the nutrient rich grasslands of the south, where it’s wise to give birth. But then once the rains stop, the bad news is that even though the grass stays green for a while, the standing water at Masek and Ndutu is so rich in nutrients that becomes actually toxic. So even though the food is still there and still good you’ve got to start moving away. But instead of heading straight up to the north, you move west, where there’s still rich grazing and water in the Grumeti and Mbalageti rivers, the so called Western Corridor. The migration stays there as long as the grass remains before gradually filtering north again as the good grazing is eaten. Dates may change because rainfall is more and more influenced by climated changes.
Mara North Conservancy – 2019
Photo by Sara Gastaldi
Masai Mara – baloon view – 2019
Photo by Sara Gastaldi
So now you have the broad pattern – a triangular migration in a clockwise direction, covering between 500 and 1000 kms, and one of the most spectacular wildlife sights on earth. But, as always, the broad scale picture isn’t all there is to it. Individual animals take some remarkably different routes around the ecosystem, as some data from gps collared indivudals shows. Why do they do that? No one knows. More recent work in the Masai Mara has made even more exciting discoveries: animals having local migrations, into and out of the Mara, showing some extraordinary movements, even joining the main Serengeti migration in some years, but not others.
So, now, do you wish to see the Great Migration at Masai Mara personally? Come with us then on safari!