Timing and possible causes over the last 450,000 years (based on Antarctic temperatures and global ice volume), showing that there were no unique climatic events that would account for any of the megafaunal extinction pulses
Numerous extinctions occurred during the latter half of the
Last Glacial Period when most large mammals went extinct in the
Americas,
Australia-New Guinea, and
Eurasia, including over 80% of all terrestrial animals with a body mass greater than . Small animals and other organisms like plants were generally unaffected by the extinctions, which is unprecented in previous extinctions during the last 30 million years. Various theories have attributed the wave of extinctions to
human hunting,
climate change,
disease,
extraterrestrial impact,
competition from other animals or other causes. However, this extinction near the end of the
Pleistocene was just one of a series of megafaunal extinction pulses that have occurred during the last 50,000 years over much of the Earth's surface, with
Africa and
Asia (where the local megafauna had a chance to evolve alongside modern humans) being comparatively less affected. The latter areas did suffer gradual attrition of megafauna, particularly of the slower-moving species (a class of vulnerable megafauna epitomized by
giant tortoises), over the last several million years. Outside the mainland of
Afro-Eurasia, these megafaunal extinctions followed a highly distinctive landmass-by-landmass pattern that closely parallels the spread of humans into previously uninhabited regions of the world, and which shows no overall correlation with climatic history (which can be visualized with plots over recent geological time periods of climate markers such as
marine oxygen isotopes or
atmospheric carbon dioxide levels).
Australia and nearby islands (e.g.,
Flores) were struck first around 46,000 years ago, followed by
Tasmania about 41,000 years ago (after formation of a land bridge to Australia about 43,000 years ago). The role of humans in the extinction of Australia and New Guinea's megafauna has been disputed, with multiple studies showing a decline in the number of species prior to the arrival of humans on the continent and the absence of any evidence of human predation; the impact of climate change has instead been cited for their decline.
North America 13,000 years ago and
South America about 500 years later,
Cyprus 10,000 years ago, the
Antilles 6,000 years ago,
New Caledonia and nearby islands 3,000 years ago,
Madagascar 2,000 years ago,
New Zealand 700 years ago, the
Mascarenes 400 years ago, and the
Commander Islands 250 years ago. Nearly all of the world's isolated islands could furnish similar examples of extinctions occurring shortly after the arrival of
humans, though most of these islands, such as the
Hawaiian Islands, never had terrestrial megafauna, so their
extinct fauna were smaller, but still displayed
island gigantism. In at least some areas, interstadials were periods of expanding human populations. An analysis of
Sporormiella fungal spores (which derive mainly from the dung of megaherbivores) in swamp sediment cores spanning the last 130,000 years from
Lynch's Crater in
Queensland, Australia, showed that the megafauna of that region virtually disappeared about 41,000 years ago, at a time when
climate changes were minimal; the change was accompanied by an increase in charcoal, and was followed by a transition from rainforest to fire-tolerant
sclerophyll vegetation. The high-resolution chronology of the changes supports the hypothesis that human hunting alone eliminated the megafauna, and that the subsequent change in flora was most likely a consequence of the elimination of browsers and an increase in fire. from analysis of
Sporormiella fungal spores from a lake in eastern North America and from study of deposits of
Shasta ground sloth dung left in over half a dozen caves in the American Southwest. Continuing human hunting and environmental disturbance has led to additional
megafaunal extinctions in the recent past, and has created a
serious danger of further extinctions in the near future (see examples below). Direct killing by humans, primarily for meat or other body parts, is the most significant factor in contemporary megafaunal decline. A number of other
mass extinctions occurred earlier in Earth's geologic history, in which some or all of the megafauna of the time also died out. Famously, in the
Cretaceous–Paleogene extinction event, the non-avian dinosaurs and most other giant reptiles were eliminated. However, the earlier mass extinctions were more global and not so selective for megafauna; i.e., many species of other types, including plants, marine invertebrates and plankton, went extinct as well. Thus, the earlier events must have been caused by more generalized types of disturbances to the
biosphere.
Consequences of depletion of megafauna Depletion of herbivorous megafauna results in increased growth of woody vegetation, and a consequent increase in
wildfire frequency. Megafauna may help to suppress the growth of invasive plants. Large herbivores and carnivores can suppress the abundance of smaller animals, resulting in their population increase when megafauna are removed. In South America's
Amazon Basin, it is estimated that such lateral diffusion was reduced over 98% following the megafaunal extinctions that occurred roughly 12,500 years ago. Given that
phosphorus availability is thought to limit productivity in much of the region, the decrease in its transport from the western part of the basin and from floodplains (both of which derive their supply from the uplift of the
Andes) to other areas is thought to have significantly impacted the region's ecology, and the effects may not yet have reached their limits.
Effect on methane emissions Large populations of megaherbivores have the potential to contribute greatly to the atmospheric concentration of
methane, which is an important
greenhouse gas. Modern
ruminant herbivores produce methane as a byproduct of
foregut fermentation in digestion and release it through belching or flatulence. Today, around 20% of annual
methane emissions come from livestock methane release. In the
Mesozoic, it has been estimated that
sauropods could have emitted 520 million tons of methane to the atmosphere annually, contributing to the warmer climate of the time (up to 10 °C (18 °F) warmer than at present). This large emission follows from the enormous estimated biomass of sauropods, and because methane production of individual herbivores is believed to be almost proportional to their mass. One study examined the methane emissions from the
bison that occupied the
Great Plains of North America before contact with European settlers. The study estimated that the removal of the bison caused a decrease of as much as 2.2 million tons per year. Another study examined the change in the methane concentration in the atmosphere at the end of the
Pleistocene epoch after the extinction of megafauna in the Americas. After early humans migrated to the Americas about 13,000
BP, their hunting and other associated ecological impacts led to the extinction of many megafaunal species there. Calculations suggest that this extinction decreased methane production by about 9.6 million tons per year. This suggests that the absence of megafaunal methane emissions may have contributed to the abrupt climatic cooling at the onset of the
Younger Dryas. The decrease in atmospheric methane that occurred at that time, as recorded in
ice cores, was 2 to 4 times more rapid than any other decrease in the last half million years, suggesting that an unusual mechanism was at work. == Current studies ==