Around the world, large gatherings of animals, driven by the need to flee, feed or reproduce, play a key role in the survival of species.
Land mammals
Wildebeest and zebra cross rivers on their migratory journey. Image: Gudkov Andrey
One of the largest animal migrations by weight takes place across the Serengeti grasslands each year when more than 2 million wildebeest move in search of green grass during the wet season. This great migration is one of the biggest wildlife events in Africa and is vital if wildebeest are to feed and help their young survive. This is not an easy task due to the herds of lions and hyenas wandering around. Their pursuit of food is joined by many other herbivores such as antelopes and zebras. However, as they run through grasslands and cross rivers, more and more predators like crocodiles are attracted.
This may be one of the heaviest gatherings of animals on the planet, but not the largest. In the mammal class, that title belongs to the possum bat Tadarida brasiliensis, which lives in caves in groups of up to 15 million animals. These migratory animals spend the winter in caves, but when February arrives, they head to Texas and form large herds to raise their young.
In the air
The red-billed dragon bird moves like a long cloud. Image: Saffa Capture
Red-billed dragon birds fly in flocks of several million birds, moving through the bush like a noisy cloud looking for food. This bird species has been studied very carefully because it can damage agriculture and threaten food security in some areas. They exist in huge flocks that take hours to fly overhead. This is the most abundant bird species on Earth with an estimated 1.5 billion birds.
Under the ocean
Krill are a food source for some of the largest animals on the planet. Image: Andrea Izzotti
From October to November, red crabs (Gecarcoidea natalis) leave the cave in the forest. 50 million crabs will crawl across Christmas Island, where special highways and bridges have been built for the crabs to help them avoid traffic. There are about 100 crabs/m2 on spawning night when female crabs gather on the beach, ready to release their eggs.
Farther offshore, krill are most numerous, tiny crustaceans that provide food for some of the planet’s largest animals such as humpback whales and minke whales. Krill can form massive colonies in the Southern Ocean, stretching up to 20 km and containing more than a million tons at a density of over 10,000 per cubic meter, the largest density of any animal in the world, according to the Antarctic Fund.