Megafauna are large animals. Commonly used criteria for this classification are minimum weights of 100 kilograms for mammals, ray-finned fish and cartilaginous fish and 40 kilograms for reptiles, amphibians and birds. The term megaherbivore is sometimes used to refer to the largest herbivorous terrestrial megafauna.
The largest still extant megafauna species is the blue whale, whose weight can reach up to nearly 200,000 kilograms. This contrasts with a maximum of about 11,000 kilograms for the African elephant, the largest land animal. Other large megafauna include the slightly smaller Asian elephant, the hippopotamus, the rhinoceros, the giraffe, the polar bear, which is the largest carnivore, and the bison, which is North America's largest land animal. Examples of megafauna birds include ostriches, the largest weighing up to 156 kilograms, cassowaries at up to 85 kilograms, and emus, at up to 60 kilograms. Among the few surviving megafauna amphibians, the largest by far is the Chinese giant salamander, which can weigh up to 140 kilograms.
As impressive as the still living terrestrial megafauna are, they are dwarfed by the sizes of ancient megafauna. For example, the largest known terrestrial animal, the Argentinosaurus, which lived about 94 to 97 million years ago, is estimated to have had a weight of 63,500 kilograms or more.
Megafauna have play several important roles in ecosystems. One is nutrient cycling, which occurs by their breaking down large quantities of tough, fibrous vegetation that is otherwise resistant to decomposition and releasing nutrients from it back into the soil through their manure.
Another is reducing competition from dominant species, particularly woody plants, thereby helping to maintain open habitats such as grasslands and savannas and the species diversity that they can support.
The migrations of megafauna contribute to seed dispersal, particularly for large-seeded fruits that may require long-distance transport for germination, thereby helping to maintain plant genetic diversity and helping them to adapt to changing environmental conditions. In addition, the trampling and digging by megafauna can create diverse microhabitats that benefit various species.
A series of mass extinctions of megafauna began roughly 50,000 years ago, coinciding with the arrival of humans and major climatic changes, and by 11,700 years ago about 80 percent of mammal species weighing over 1,000 kilograms had become extinct. The remaining megafauna continue to face threats mainly from human activity, including poaching, habitat destruction and human-caused climate change. Some species are critically endangered and close to extinction, including the Javan rhinoceros, with a population of less than 75 individuals, the Sumatran rhinoceros, with approximately 80 individuals, the vaquita (a small porpoise found in the Gulf of California), with possibly less than 30 individuals, and the saola (a two-horned mammal of Vietnam with some similarities to both antelopes and oxen), with an unknown population size and possible extinction.
Efforts to conserve megafauna include habitat protection, anti-poaching efforts, rewilding, education, legal protection and international cooperation. One of the biggest successes has been with regard to the giant panda, for which extensive habitat restoration and strong anti-poaching measures have resulted in a substantial increase in its population. Other examples of the recovery of nearly extinct species include the European bison, bald eagle, Przewalski's horse (a wild horse native to Central Asia), bontebok (a South African antelope), California condor, kakapo (a flightless parrot from New Zealand) and musk ox.