Query: The failure of the 2007 polar bear survival model is a simple fact that explodes the myth that polar bears are on their way to extinction.

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Holocene extinction <=SEP=> The Holocene extinction , otherwise referred to as the Sixth extinction or Anthropocene extinction , is the ongoing extinction event of species during the present Holocene epoch , mainly due to human activity . The large number of extinctions spans numerous families of plants and animals , including mammals , birds , amphibians , reptiles and arthropods . With widespread degradation of highly biodiverse habitats such as coral reefs and rainforest , as well as other areas , the vast majority of these extinctions is thought to be undocumented . According to the species-area theory , and based on upper-bound estimating , the present rate of extinction may be up to 140,000 species per year , making it the greatest loss of biodiversity since the Cretaceous -- Paleogene extinction event .   The Holocene extinction includes the disappearance of large land animals known as megafauna , starting at the end of the last Ice Age . Megafauna outside of the African continent , which did not evolve alongside humans , proved highly sensitive to the introduction of new predation , and many died out shortly after early humans began spreading and hunting across the Earth ( additionally , many African species have also gone extinct in the Holocene ) . These extinctions , occurring near the Pleistocene -- Holocene boundary , are sometimes referred to as the Quaternary extinction event .   The arrival of humans on different continents coincides with megafaunal extinction . The most popular theory is that human overhunting of species added to existing stress conditions . Although there is debate regarding how much human predation affected their decline , certain population declines have been directly correlated with human activity , such as the extinction events of New Zealand and Hawaii . Aside from humans , climate change may have been a driving factor in the megafaunal extinctions , especially at the end of the Quaternary .   The ecology of humanity has been noted as being that of an unprecedented `` global superpredator '' that regularly preys on the adults of other apex predators and has worldwide effects on food webs . Extinctions of species have occurred on every land mass and ocean , with many famous examples within Africa , Asia , Europe , Australia , North and South America , and on smaller islands . Overall , the Holocene extinction can be characterized by the human impact on the environment . The Holocene extinction continues into the 21st century , with meat consumption , overfishing , ocean acidification and the amphibian crisis being a few broader examples of an almost universal , cosmopolitan decline in biodiversity . A ballooning human population along with profligate consumption are considered to be the primary drivers of this rapid decline .

Quaternary extinction event <=SEP=> The Quaternary period saw the extinctions of numerous predominantly larger , especially megafaunal , species , many of which occurred during the transition from the Pleistocene to the Holocene epoch . However , this extinction wave did not stop at the end of the Pleistocene , but continued , especially on isolated islands , in human-caused extinctions , although there is debate as to whether these should be considered separate events or part of the same event . Among the main causes hypothesized by paleontologists are natural climate change and overkill by humans , who appeared during the Middle Pleistocene and migrated to many regions of the world during the Late Pleistocene and Holocene . A variant of the latter possibility is the second-order predation hypothesis , which focuses more on the indirect damage caused by overcompetition with nonhuman predators . The spread of disease is also discussed as a possible reason .

Cretaceous–Paleogene extinction event <=SEP=> The Cretaceous -- Paleogene ( K -- Pg ) extinction event , also known as the Cretaceous -- Tertiary ( K -- T ) extinction , was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time approximately 65 million years ago . With the exception of some ectothermic species like the leatherback sea turtle and crocodiles , no tetrapods weighing more than 25 kg survived . It marked the end of the Cretaceous period and with it , the entire Mesozoic Era , opening the Cenozoic Era that continues today .   In the geologic record , the K -- Pg event is marked by a thin layer of sediment called the K -- Pg boundary , which can be found throughout the world in marine and terrestrial rocks . The boundary clay shows high levels of the metal iridium , which is rare in the Earth 's crust but abundant in asteroids .   As originally proposed in 1980 by a team of scientists led by Luis Alvarez , it is now generally thought that the K -- Pg extinction was caused by a massive comet or asteroid impact , estimated to be 10 km wide , 66 million years ago and its catastrophic effects on the global environment , including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis . The impact hypothesis , also known as the Alvarez hypothesis , was bolstered by the discovery of the 180 km Chicxulub crater in the Gulf of Mexico in the early 1990s , which provided conclusive evidence that the K -- Pg boundary clay represented debris from an asteroid impact . The fact that the extinctions occurred at the same time as the impact provides strong situational evidence that the K -- Pg extinction was caused by the asteroid . It was possibly accelerated by the creation of the Deccan Traps . However , some scientists maintain the extinction was caused or exacerbated by other factors , such as volcanic eruptions , climate change , or sea level change , separately or together .   A wide range of species perished in the K -- Pg extinction . The best-known victims are the non-avian dinosaurs . However , the extinction also destroyed a plethora of other terrestrial organisms , including certain mammals , pterosaurs , birds , lizards , insects , and plants . In the oceans , the K -- Pg extinction killed off plesiosaurs and the giant marine lizards ( Mosasauridae ) and devastated fish , sharks , mollusks ( especially ammonites , which became extinct ) and many species of plankton . It is estimated that 75 % or more of all species on Earth vanished . Yet the devastation caused by the extinction also provided evolutionary opportunities . In the wake of the extinction , many groups underwent remarkable adaptive radiations -- a sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches resulting from the event . Mammals in particular diversified in the Paleogene , producing new forms such as horses , whales , bats , and primates . Birds , fish and perhaps lizards also radiated .

Extinction event <=SEP=> An extinction event ( also known as a mass extinction or biotic crisis ) is a widespread and rapid decrease in the biodiversity on Earth . Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms . It occurs when the rate of extinction increases with respect to the rate of speciation . Because the majority of diversity and biomass on Earth is microbial , and thus difficult to measure , recorded extinction events affect the easily observed , biologically complex component of the biosphere rather than the total diversity and abundance of life .   Extinction occurs at an uneven rate . Based on the fossil record , the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years . Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals .   The Great Oxygenation Event was probably the first major extinction event . Since the Cambrian explosion five further major mass extinctions have significantly exceeded the background extinction rate . The most recent and debatably best-known , the Cretaceous -- Paleogene extinction event , which occurred approximately million years ago ( Ma ) , was a large-scale mass extinction of animal and plant species in a geologically short period of time . In addition to the five major mass extinctions , there are numerous minor ones as well , and the ongoing mass extinction caused by human activity is sometimes called the sixth extinction . Mass extinctions seem to be a mainly Phanerozoic phenomenon , with extinction rates low before large complex organisms arose .   Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty . These differences stem from the threshold chosen for describing an extinction event as `` major '' , and the data chosen to measure past diversity .