Information

Can the Quarternary Extinction event be linked to the spread of mankind around the world?

Can the Quarternary Extinction event be linked to the spread of mankind around the world?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Evidence is presented in a variety of media by Paul S. Martin as his 'Overkill hypothesis', particularly in Twilight of the Mammoths.

The internet describes this idea as 'controversial.' Other than the works of Paul Martin, what evidence has been presented supporting this hypothesis?


Only in islands can this hypothesis be supported. But islands are NOT continents, and that raises problems with the hypothesis. The most inconspicuous of which is timing.

In Australia, the gap between man setting foot on Australia and the extinction of Australia's megafauna was 17,000 years. Too long.

Man left Africa and set foot in Eurasia more or less than 50,000 years ago, yet the extinction of Eurasia's megafauna happened 10,000 years ago. Too long.

The oldest discovered American, "Eve", was discovered in a cave in Mexico's Yucatan Peninsula and dated to be 13,500 years old, 500 years older than the supposed time that man crossed Beringia. This could imply that man crossed Beringia at a far earlier date, yet the dating of the extinction of America's megafauna coincided with that in Eurasia. Too long.

There's also into consideration the Younger Dryas climate chaos phenomenon, which did coincide with the extinction of the 30 genera of Pleistocene megafauna, a handful of microfauna and plants, not to mention the Clovis culture itself.

If man was the cause of the extinction event, the gap between man setting foot in new lands and the megafaunal extinction would've been a few generations, not a few millennia.


Holocene extinction

The Holocene extinction, otherwise referred to as the sixth mass extinction or Anthropocene extinction, is an ongoing extinction event of species during the present Holocene epoch (with the more recent time sometimes called Anthropocene) as a result of human activity. [3] [4] [5] The included extinctions span numerous families of plants [6] and animals, including mammals, birds, reptiles, amphibians, fish and invertebrates. With widespread degradation of highly biodiverse habitats such as coral reefs and rainforests, as well as other areas, the vast majority of these extinctions are thought to be undocumented, as the species are undiscovered at the time of their extinction, or no one has yet discovered their extinction. The current rate of extinction of species is estimated at 100 to 1,000 times higher than natural background extinction rates. [4] [7] [8] [9] [10] [11]

The Holocene extinction includes the disappearance of large land animals known as megafauna, starting at the end of the last glacial period. Megafauna outside of the African mainland, 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 [12] [13] many African species have also gone extinct in the Holocene, but – with few exceptions – megafauna of the mainland was largely unaffected until a few hundred years ago. [14] These extinctions, occurring near the Pleistocene–Holocene boundary, are sometimes referred to as the Quaternary extinction event.

The most popular theory is that human overhunting of species added to existing stress conditions as the extinction coincides with human emergence. 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 Pleistocene.

Ecologically, humanity has been noted as an unprecedented "global superpredator" [15] that consistently preys on the adults of other apex predators, and has worldwide effects on food webs. There have been extinctions of species on every land mass and in every ocean: there are many famous examples within Africa, Asia, Europe, Australia, North and South America, and on smaller islands. Overall, the Holocene extinction can be linked to the human impact on the environment. The Holocene extinction continues into the 21st century, with meat consumption, overfishing, and ocean acidification and the decline in amphibian populations [16] being a few broader examples of a cosmopolitan decline in biodiversity. Human population growth and increasing per capita consumption are considered to be the primary drivers of this decline. [11] [17] [18] [19]

The 2019 Global Assessment Report on Biodiversity and Ecosystem Services, published by the United Nations' Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, posits that roughly one million species of plants and animals face extinction within decades as the result of human actions. [19] [20] [21] [22] Organized human existence is jeopardized by increasingly rapid destruction of the systems that support life on Earth, according to the report, the result of one of the most comprehensive studies of the health of the planet ever conducted. [23]


Climate

Scientists have evidence of more than 60 periods of glacial expansion interspersed with briefer intervals of warmer temperatures. The entire Quaternary Period, including the present, is referred to as an ice age due to the presence of at least one permanent ice sheet (Antarctica) however, the Pleistocene Epoch was generally much drier and colder than the present time.

Although glacial advancement varied between continents, about 22,000 years ago, glaciers covered approximately 30 percent of Earth&rsquos surface. Ahead of the glaciers, in areas that are now Europe and North America, there existed vast grasslands known as the &ldquomammoth steppes.&rdquo The mammoth steppes had a higher productivity than modern grasslands with greater biomass. The grasses were dense and highly nutritious. Winter snow cover was quite shallow.


Published by the Royal Society. All rights reserved.

References

2014 Patterns of maximum body size evolution in Cenozoic land mammals: eco-evolutionary processes and abiotic forcing . Proc. R. Soc. B 281, 20132049. (doi:10.1098/rspb.2013.2049) Link, ISI, Google Scholar

Lyons SK, Smith FA, Brown JH

. 2004 Of mice, mastodons and men: human-mediated extinctions on four continents . Evol. Ecol. Res. 6, 339–358. ISI, Google Scholar

Sandom C, Faurby S, Sandel B, Svenning JC

. 2014 Global late Quaternary megafauna extinctions linked to humans, not climate change . Proc. R. Soc. B 281, 20133254. (doi:10.1098/rspb.2013.3254) Link, ISI, Google Scholar

Alcover JA, Sans A, Palmer M

. 1998 The extent of extinctions of mammals on islands . J. Biogeogr. 25, 913–918. (doi:10.1046/j.1365-2699.1998.00246.x) Crossref, ISI, Google Scholar

. 1999 Putting North America's End–Pleistocene megafaunal extinction in context: large scale analyses of spatial patterns, extinction rates, and size distributions . In Extinctions in near time: causes, contexts, and consequences (ed.

), pp. 105–143. New York, NY : Kluwer Academic/Plenum . Crossref, Google Scholar

. 2015 Expected time-invariant effects of biological traits on mammal species duration . Proc. Natl Acad. Sci. USA 112, 13 015–13 020. (doi:10.1073/pnas.1510482112) Crossref, ISI, Google Scholar

. 2013 Body size and extinction risk in terrestrial mammals above the species level . Am. Nat . 182, E196–E214. (doi:10.1086/673489) Crossref, PubMed, ISI, Google Scholar

Davidson AD, Hamilton MJ, Boyer AG, Brown JH

. 2009 Multiple ecological pathways to extinction in mammals . Proc. Natl Acad. Sci. USA 106, 10 702–10 705. (doi:10.1073/pnas.0901956106) Crossref, ISI, Google Scholar

Bartlett LJ, Williams DR, Prescott GW, Balmford A, Green RE, Eriksson A, Valdes PJ, Singarayer JS, Manica A

. 2015 Robustness despite uncertainty: regional climate data reveal the dominant role of humans in explaining global extinctions of Late Quaternary megafauna . Ecography 39, 152–161. (doi:10.1111/ecog.01566) Crossref, ISI, Google Scholar

. 1967 Prehistoric overkill . In Pleistocene extinctions: the search for a cause (eds

, pp. 75–120. New Haven, CT : Yale University Press . Google Scholar

. 1984 Coevolutionary disequilibrium and Pleistocene extinctions . In Quaternary extinctions: a prehistoric revolution (eds

), pp. 223–249. Tucson, AZ : University of Arizona Press . Google Scholar

. 1987 Pleistocene extinctions: the pivotal role of megaherbivores . Paleobiology 13, 351–362. Crossref, ISI, Google Scholar

. 1997 Humans, hyperdisease, and first-contact extinctions . In Natural change and human impact in Madagascar (eds

), pp. 169–217. Washington, DC : Smithsonian Institution Press . Google Scholar

. 2010 Timing of Quaternary megafaunal extinction in South America in relation to human arrival and climate change . Quat. Int . 217, 10–29. (doi:10.1016/j.quaint.2009.11.017) Crossref, ISI, Google Scholar

2007 Evidence for an extraterrestrial impact 12 900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling . Proc. Natl Acad. Sci. USA 104, 16 016–16 021. (doi:10.1073/pnas.0706977104) Crossref, ISI, Google Scholar

Smith FA, Lyons SK, Ernest SKM, Jones KE, Kaufman DM, Dayan T, Marquet PA, Brown JH, Haskell JP

. 2003 Body mass of late quaternary mammals . Ecology 84, 3403. (doi:10.1890/02-9003) Crossref, ISI, Google Scholar

. 2015 A species-level phylogeny of all extant and late Quaternary extinct mammals using a novel heuristic-hierarchical Bayesian approach . Mol. Phylogenet. Evol. 84, 14–26. (doi:10.1016/j.ympev.2014.11.001) Crossref, PubMed, ISI, Google Scholar

2009 PanTHERIA: a species-level database of life history, ecology, and geography of extant and recently extinct mammals . Ecology 90, 2648. (doi:10.1890/08-1494.1) Crossref, Google Scholar

. 2003 Life history characteristics of placental nonvolant mammals . Ecology 84, 3402. (doi:10.1890/02-9002) Crossref, ISI, Google Scholar

. 2014 Ancestral character estimation under the threshold model from quantitative genetics . Evolution 68, 743–759. (doi:10.1111/evo.12300) Crossref, PubMed, ISI, Google Scholar

Breiman L, Friedman J, Stone CJ, Olshen RA

. 1984 Classification and regression trees . Boca Raton, FL : Wadsworth/CRC Press . Google Scholar

Mercado N, Olden JD, Maxted JT, Hrabik TR, Vander Zanden MJ

. 2006 Forecasting the spread of invasive rainbow smelt in the Laurentian Great Lakes region of North America . Conserv. Biol . 20, 1740–1749. (doi:10.1111/j.1523-1739.2006.00508.x) Crossref, PubMed, ISI, Google Scholar

. 2010 Phylogenetic logistic regression for binary dependent variables . Syst. Biol . 59, 9–26. (doi:10.1093/sysbio/syp074) Crossref, PubMed, ISI, Google Scholar

. 2005 One equation fits overkill: why allometry underpins both prehistoric and modern body size-biased extinctions . Popul. Ecol . 47, 137–141. (doi:10.1007/s10144-005-0213-4) Crossref, ISI, Google Scholar

. 2002 Determinants of loss of mammal species during the Late Quaternary ‘megafauna’ extinctions: life history and ecology, but not body size . Proc. R. Soc. Lond. B 269, 2221–2227. (doi:10.1098/rspb.2002.2130) Link, ISI, Google Scholar


18 signs we're in the middle of a 6th mass extinction

The phrase "mass extinction" typically conjures images of the asteroid crash that led to the twilight of the dinosaurs.

Upon impact, that 6-mile-wide space rock caused a tsunami in the Atlantic Ocean, along with earthquakes and landslides up and down what is now the Americas. A heat pulse baked the Earth, and the Tyrannosaurus rex and its compatriots died out, along with 75% of the planet's species.

Although it may not be obvious, another devastating mass extinction event is taking place today — the sixth of its kind in Earth's history. The trend is hitting global fauna on multiple fronts, as hotter oceans, deforestation, and climate change drive animal populations to drop in unprecedented numbers.

These alarming extinction trends are driven by one key factor: humans. According to a 2014 study, current extinction rates are 1,000 times higher than they would be if humans weren't around. A summary of a United Nations report released last month put it another way: "Human actions threaten more species with global extinction now than ever before," the authors wrote.

That report, which assessed the state of our planet's biodiversity, found that up to 1 million plant and animals species face extinction, many within decades, due to human activity.

Other recent research has led to similar conclusions: A 2017 study found that animal species around the world are experiencing a "biological annihilation" and that our current "mass extinction episode has proceeded further than most assume."

Here are 18 signs that the planet is in the midst of a sixth mass extinction, and why people are primarily to blame.


3 Late Quaternary Extinctions

Causes of extinction of mammalian megafauna (adult weight >45 kg) during the Late Quaternary, the so-called Late Quaternary Extinction (LQE), have been the subject of prolonged debate (e.g., Koch & Barnosky, 2006 Martin, 1967 Stuart, 2015 ). Prior to

13 ka, the mammal assemblage of the Americas included large-bodied animals such as mammoths, horses, camels, saber-tooth cats, and the short-faced bear. Extinction was total for mammals larger than 1,000 kg, >50% for size classes between 32 and 1,000 kg, and

20% for those between 10 and 32 kg (Koch & Barnosky, 2006 ). Within a short time window, >150 species were lost in the Americas, including all mammals over

600 kg. An analogous catastrophic size-controlled LQE affected 14 of 16 Australian mammalian genera however, extinction ages are at least

30 kyr older than in North America. Although fewer species were affected by the LQE in Africa and Eurasia, a similar size-biased extinction has been observed with end-Pleistocene (

13 ka) ages being prominent. Current explanations for the LQE in North America and Australia involve a combination of two hypotheses: climate change, and “overkill” by human hunting, modulated by the knock-on effect of herbivore extinction on the environment and on the survivability of other groups (e.g., Owen-Smith, 1987 ). Although “overkill” was originally used to explain North American extinctions (Martin, 1967 ), a forerunner of the hypothesis was popular in 19th century Europe, before it was eventually abandoned as archaeological evidence for human migration showed little evidence for the impact of human hunting on the LQE. Grayson and Meltzer ( 2003 ) argued that island settings (e.g., New Zealand or the West Indies), where human hunting and habitat degradation can be unequivocally associated with extinction, should not be the model for continental extinctions. Extraterrestrial impact as a contributing cause for the LQE in North America, and for the onset of the Younger Dryas cold period (Firestone et al., 2007 ), has not been supported by subsequent analyses (Holliday et al., 2014 Pinter et al., 2011 ).

In North America, the brief (

200 year) duration of Clovis-tool finds at

According to Faith and Surovell ( 2009 ), the LQE in North America was abrupt and requires a mechanism capable of wiping out

35 genera across the continent in a “geological instant” in the 13.8- to 11.4-ka interval (Figure 2), with the spread in last appearances being largely explained by an incomplete fossil record and the resulting Signor-Lipps effect (Signor & Lipps, 1982 ). Abrupt versus staggered megafaunal extinction at the LQE is central to determination of cause. In their “continental simulation,” Faith and Surovell ( 2009 ) determined the empirical probability (3.4%) of observing a terminal Pleistocene (10-12 ka) age from 1955 stratigraphic occurrences (from 31 genera) of which 66 taxa (from 16 genera) yield terminal Pleistocene ages, assuming that all occurrences are equally likely to receive a terminal Pleistocene age. The simulation randomly assigned pre-12-ka or post-12-ka ages to all 1955 observations based on this probability (3.4%). The total number of genera that received a terminal Pleistocene age was tallied for each of 10,000 simulations to determine the probability of observing 16 or fewer terminal Pleistocene genera. The authors concluded that the observed pattern is consistent with synchronous (i.e., 10-12 ka) extinction for all 31 genera.

Bradshaw et al. ( 2012 ) proposed a Gaussian-resampled, inverse-weighted McInerny (GRIWM) approach, which weights observations inversely according to their temporal distance from the last observation of a confirmed species occurrence, and samples radiometric ages from the underlying probability distribution. In Figure 2, we show GRIWM estimates of continent-wide European extinctions from the fossil record aided by DNA analyses (Cooper et al., 2015 ), excluding regional disappearances. An extinction age estimate in North America for Arctodus simus (the short-faced bear) at 10.8 ka (Schubert, 2010 ) and the onset of population decline of Bison priscus (the steppe bison) at

37 ka (Shapiro et al., 2004 ) are included in Figure 2. Note that the horse and woolly mammoth (Mammuthus primigenius) persisted in interior Alaska until

10.5 ka (Haile et al., 2009 ), and the woolly mammoth survived on St. Paul Island (Alaska) until

5.6 ka (Graham et al., 2016 ). Zazula et al. ( 2014 ) pointed out that the American mastodon (Mammut americanum) occupied eastern Beringia (Alaska/Yukon) during the last interglacial before its range contracted southward at the onset of glacial conditions at

75 ka. The range of the species appears to have expanded northward again as interglacial conditions returned at the end of the Pleistocene, before extinction of the species at

11.5 ka (10,000 14 C years before present [BP]). Zazula et al. ( 2014 ) posed the question: why was this species stopped in its tracks when favorable conditions beckoned in Beringia?

An important proxy for herbivore population is the abundance in sedimentary sequences of coprophilous (dung) fungal spores, such as Sporormiella. The proxy was first proposed over 30 years ago (Davis, 1987 ), requires careful interpretation and laboratory techniques (e.g., van Asperen et al., 2016 ), but provides a measure of herbivore population independent of the bone-fossil record. Lake sediments in New York and Indiana imply a decline in Sporormiella spores beginning at 14.8 ka (Figure 2) that falls below the 2% threshold by 13.7 ka (Gill et al., 2009 ). This result has been closely replicated at Silver Lake (Ohio) where Sporormiella decline was dated at 13.9 ka (Gill et al., 2012 ). Importantly, the Sporormiella decline at these sites predates Younger Dryas cooling, and concurrent changes in the pollen record, and immediately predates a marked charcoal deposition increase, implying that herbivore decline and resulting landscape changes provide an explanation for subsequent (natural) landscape burning. The onset of the demise of North American herbivores at

14.5 ka (Gill et al., 2009 ) lies within the Bølling-Allerød warm period with the Younger Dryas cold period beginning

2 kyr later (e.g., Deplazes et al., 2013 ).

The South American LQE was even more profound than that in North America (Barnosky & Lindsey, 2010 Koch & Barnosky, 2006 ), with the loss of 50 megafaunal genera (∼83%). Robust dates are scarce for the South American LQE, although it appears that many taxa were lost near the Pleistocene-Holocene boundary (Barnosky & Lindsey, 2010 ). Sporormiella decline in lake sediments from SE Brazil implies herbivore population collapse at

In Northern Eurasia, nine genera (35%) were lost during the LQE. Available age data are consistent with a two-phase extinction in the 45- to 35-ka and 15- to 10-ka intervals (Koch & Barnosky, 2006 ). Up to 50% of worldwide megafaunal extinctions at 15-10 ka apparently occurred in Northern Eurasia (Cooper et al., 2015 ), but the extinction pattern is more complex than in North America with megafaunal range contractions culminating in extinction for some species but not others (Stuart, 2015 ). In Figure 2, we plot continent-wide Eurasian megafaunal extinction events from Cooper et al. ( 2015 ). Several well-studied species disappeared continent-wide at

26-31 ka (Figure 2) hence, their last appearances significantly postdate the Laschamp excursion (at

41 ka). Post-Laschamp extinction for Crocuta crocuta (spotted hyaena) and Crocuta spelaea (cave hyaena) at

26 ka were, however, preceded by severe range contraction from Asia into Europe (Stuart & Lister, 2014 ). Fossils of Ursus spelaeus (cave bear) also indicate E to W range contraction before extinction at

26 ka with abrupt population decline, based on DNA analyses, after 50 ka (Baca et al., 2016 Stiller et al., 2010 Stiller et al., 2014 ).

In Africa, at least 24 species and

10 genera of mammals became extinct in the 13- to 6-ka interval, representing 25% of Pleistocene African megafauna (Faith, 2014 ). Species-level extinction was, again, most intense for larger-bodied megafauna (Koch & Barnosky, 2006 ). The African LQE was considered to have been less severe than elsewhere, accounting for the relatively rich diversity of modern African megafauna. On the other hand, the number of extinct African species dated to the last 100 kyr exceeds the number in Europe and matches the number in Australia, and is only surpassed by the LQE in the Americas (Faith, 2014 ). In east Africa alone, the number of securely dated latest Pleistocene mammal extinctions has risen from two to seven in the last decade, with most being of grazers associated with open habitats (Faith, 2014 ).

Estimation of extinction ages for Australian megafauna (and for some Eurasian genera) is complicated by the majority of last appearances being at or beyond the practical range of radiocarbon dating (i.e., >40 ka). The LQE in Australia was apparently catastrophic for large mammalian megafauna, with the complete loss of all animals heavier than

100 kg. Fourteen of 16 genera of Pleistocene mammalian megafauna disappeared, together with all megafaunal reptiles (6 genera), in the vicinity of, or prior to,

40 ka (Figure 2). Ten Australian genera disappeared in the 44- to 35-ka interval based on a variety of frequentist statistical methods (including GRIWM) to determine extinction ages for 16 megafaunal genera (Figure 2 Saltré et al., 2016 ). The mass extinction of megafauna at this time, including the largest-known (

3,000 kg) marsupial (Diprotodon), has been linked with climate variability and aridity (e.g., Wroe et al., 2013 ), although this linkage has been disputed (e.g., Saltré et al., 2016 ), often in favor of human predation or “overkill” (e.g., Brook & Johnson, 2006 Johnson et al., 2016 Miller et al., 2016 van der Kaars et al., 2017 ). It is noteworthy that the extinction age for the

200-kg flightless bird Genyornis newtoni at

35 ka (Figure 2 Saltré et al., 2016 ) is younger than the

43-ka estimate given by Miller et al. ( 2016 ) based on dated eggshell fragments. Even if final extinction was delayed until

35 ka, the population of Genyornis newtoni crashed close to the time of the Laschamp excursion (

41 ka) based on the egg-shell data (Miller et al., 2016 ), although some egg-shells attributed to Genyornis newtoni may be from other species (Grellet-Tinner et al., 2016 ).

At Lynch's Crater (NE Queensland), an abrupt decline in dung fungi including Sporormiella (Figures 2 and 3) implies abrupt demise of large Australian herbivores at 40-44 ka (Johnson et al., 2015 ). An abrupt increase in charcoal lags Sporormiella decline by

100 years, and evidence for grasses and sclerophyll vegetation lags Sporormiella decline by

300-400 years (Johnson et al., 2015 Rule et al., 2012 ). The charcoal-rich levels can be explained by natural lightning-induced biomass burning as a result of fuel buildup triggered by herbivore extinction (Johnson et al., 2016 Rule et al., 2012 ). Off the southern coast of Western Australia, marine core MD03-2614G records a sharp decline in Sporormiella in the 45- to 43-ka interval, relative to values recorded back to 140 ka (Figures 2 and 3 van der Kaars et al., 2017 ).

A role for humans in the extinction of large animals in Australia remains popular (e.g., Brook & Johnson, 2006 Johnson et al., 2016 Miller et al., 2016 Turney et al., 2008 van der Kaars et al., 2017 ), although the arrival of humans in Australia (Sahul) may have predated the LQE at

25 kyr (Clarkson et al., 2017 ) but the arrival date is not unequivocal (O'Connell et al., 2018 ). There is no evidence for a spike in the human population in Australia at the time of the most prominent extinction event at

40 ka, when the entire Australian human population may not have exceeded a few tens of thousands (Williams, 2013 ). Tasmania had a land bridge to the Australian mainland during the last glacial, becoming an island in the early Holocene. The extinction of megafauna in Tasmania at

40 ka does not correspond to climate or environmental change and has been associated with the late arrival of humans in the region (Turney et al., 2008 ). More recent results place the Tasmanian extinction of Protemnodon anak and other megafauna at

41 ka, predating human arrival on the island at

39 ka and hence precluding human involvement in the extinctions (Cosgrove et al., 2010 Lima-Ribeiro & Diniz-Filho, 2014 ). Extant smaller (more accessible) prey, particularly the common wombat (Vombatus ursinus) and the red-necked wallaby (Macropus rufogriseus), characterize the early archaeological kill-sites on the island (Cosgrove et al., 2010 ).

Apart from Australian extinctions concentrated close to the Laschamp excursion at

40 ka, fossils from the King's Creek Catchment (SE Queensland) indicate additional concentrations of megafaunal last appearances at

122 ka (Price et al., 2011 Wroe et al., 2013 ). The older two dates (107 and 122 ka) correspond to magnetic field intensity minima associated with the Blake excursions (Figure 1b).


The Mass Extinction Periods

Ordovician–Silurian Extinction

Around 439 million years ago, 86% of life on Earth was wiped out. Scientists believe two major events resulted in this extinction: glaciation and falling sea levels. Some theories suggest that the Earth was covered in such a vast quantity of plants that they removed too much carbon dioxide from the air which drastically reduced the temperature. Falling sea levels were possibly a result of the Appalachian mountain range forming. The majority of the animal life lived in the ocean. Trilobites, brachiopods, and graptolites died off in large numbers but interestingly, this did not lead to any major species changes during the next era.

Late Devonian Extinction

Estimates propose that around 75% of species were lost around 364 million years ago. Information is unclear as to whether the late Devonian extinction was one single major event or spread over hundreds of thousands of years. Trilobites, which survived the Ordovician-Silurian extinction due to their hard exoskeletons, were nearly exterminated during this extinction. Giant land plants are thought to be responsible as their deep roots released nutrients into the oceans. The nutrient rich waters resulted in mass amounts of algal blooms which depleted the seas of oxygen and therefore, animal life. Volcanic ash is thought to be responsible for cooling earth’s temperatures which killed off the spiders and scorpion-type creatures that had made it on land by this time. A distant amphibian cousin, elpistostegalians, had also ventured onto land but became extinct. Vertebrae did not appear on land again until 10 million years later, the ichthyostegalians from which we all evolved. If the late Devonian extinction had not occurred, humans might not exist today.

Permian–Triassic extinction

This mass extinction, which occurred 251 million years ago, is considered the worst in all history because around 96% of species were lost. Ancient coral species were completely lost. “The Great Dying” was caused by an enormous volcanic eruption that filled the air with carbon dioxide which fed different kinds of bacteria that began emitting large amounts of methane. The Earth warmed, and the oceans became acidic. Life today descended from the 4% of surviving species. After this event, marine life developed a complexity not seen before and snails, urchins, and crabs emerged as new species.

Triassic–Jurassic extinction

The Triassic-Jurassic extinction happened between 199 million and 214 million years ago and as in other mass extinctions, it is believed there were several phases of species loss. The blame has been placed on an asteroid impact, climate change, and flood basalt eruptions. During the beginning of this era, mammals outnumbered dinosaurs. By the end, dinosaurs’ ancestors (archosaurs) reigned the earth’s surface. This extinction laid the path that allowed for the evolution of dinosaurs which later existed for around 135 million years.

Cretaceous–Paleogene extinction

Perhaps the most well-known of the Big 5, the end of the Cretaceous-Paleogene brought on the extinction of dinosaurs. A combination of volcanic activity, asteroid impact, and climate change effectively ended 76% of life on earth 65 million years ago. This extinction period allowed for the evolution of mammals on land and sharks in the sea.


In the Light of Evolution: Volume II: Biodiversity and Extinction (2008)

Unfortunately, this book can't be printed from the OpenBook. If you need to print pages from this book, we recommend downloading it as a PDF.

Visit NAP.edu/10766 to get more information about this book, to buy it in print, or to download it as a free PDF.

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

12 Megafauna Biomass Tradeoff as a Driver of Quaternary and Future Extinctions ANTHONY D. BARNOSKY Earth’s most recent major extinction episode, the Quaternary Megafauna Extinction, claimed two-thirds of mammal genera and one-half of species that weighed >44 kg between ≈50,000 and 3,000 years ago. Estimates of megafauna biomass (includ- ing humans as a megafauna species) for before, during, and after the extinction episode suggest that growth of human bio- mass largely matched the loss of non-human megafauna bio- mass until ≈12,000 years ago. Then, total megafauna biomass crashed, because many non-human megafauna species sud- denly disappeared, whereas human biomass continued to rise. After the crash, the global ecosystem gradually recovered into a new state where megafauna biomass was concentrated around one species, humans, instead of being distributed across many species. Precrash biomass levels were finally reached just before the Industrial Revolution began, then skyrocketed above the pre- crash baseline as humans augmented the energy available to the   global ecosystem by mining fossil fuels. Implications include (i ) an increase in human biomass (with attendant hunting and other impacts) intersected with climate change to cause the Quaternary Megafauna Extinction and an ecological threshold event, after   which humans became dominant in the global ecosystem (ii ) with continued growth of human biomass and today’s unprecedented global warming, only extraordinary and stepped-up conservation Department of Integrative Biology and Museums of Paleontology and Vertebrate Zoology, University of California, Berkeley, CA 94720. 227

228  /  Anthony D. Barnosky efforts will prevent a new round of extinctions in most body-size   and taxonomic spectra and (iii ) a near-future biomass crash that will unfavorably impact humans and their domesticates and other species is unavoidable unless alternative energy sources are developed to replace dwindling supplies of fossil fuels. T he Quaternary Megafauna Extinction (QME) killed >178 species of the world’s largest mammals, those weighing at least 44 kg (roughly the size of sheep to elephants). More than 101 genera perished. Beginning ≈50,000 years (kyr) B.P. and largely completed by 7 kyr B.P., it was Earth’s latest great extinction event. The QME was the only major extinction that took place when humans were on the planet, and it also occurred at a time when human populations were rapidly expanding during a global warming episode. Thus, the QME takes on special significance in understanding the potential outcomes of a similar but scaled-up natural experiment that is underway today: the exponential growth of human populations at exactly the same time the Earth is warm- ing at unprecedented rates. Causes of the QME have been explored primarily through analyzing the chronology of extinction, geographic differences in extinction intensity, timing of human arrival vs. timing of climate change, and simulations that explore effects of humans hunting megafauna (Martin, 1967 Martin and Wright, 1967 Martin and Klein, 1984 MacPhee, 1999 Alroy, 2001 Roberts et al., 2001 Grayson and Meltzer, 2003 Barnosky et al., 2004 Trueman et al., 2005 Koch and Barnosky, 2006 Wroe and Field, 2006). Results of past studies indicate that human impacts such as hunting and habitat alteration contributed to the QME in many places, and that climate change exacerbated it. Potentially added to those megafauna stressors was the explosion of a comet over central North America, which may have helped to initiate the Younger Dryas (YD) climatic event, and which may have caused widespread wildfires, although those ideas are still being tested (Firestone et al., 2007). Whatever the cause of the QME, one thing is clear: there was a dra- matic change in the way energy flowed through the global ecosystem. The energy that powers ecosystems is derived from solar radiation, which is converted to biomass. Before the extinction, the energy needed to build megafauna biomass was divided among many species. After the extinc- tion, increasing amounts and proportions of energy began to flow toward a single megafauna species, humans. Humans are, by definition, a megafauna species, with an average body weight of ≈67 kg for modern Homo sapiens and 50 kg for Stone-Age people, placing us at the lower end of the body-size distribution for megafauna

Megafauna Biomass Tradeoff  /  229 FIGURE 12.1  Body-size distribution of mammals in North America. The black bars illustrate the distribution of species that went extinct in the QME. Note that humans are at the lower end of the distribution for species that went extinct. Illus- tration modified1from Lyons et al. (2004) 1 column.tif Fig Barnosky PNAS Book BW see that source for similar distributions of fauna from other continents. as a whole (Fig. 12.1). Previous work has demonstrated that, as human biomass grows, the amount of solar energy and net primary productivity (NPP) available for use by other species shrinks, ultimately shrinking the amount of the world’s biomass accounted for by those non-human species (Vitousek et al., 1986 Maurer, 1996 Vitousek et al., 1997 McDaniel and Borton, 2002). Therefore, growth of human biomass should be inversely related to biomass of other species in general and to other megafauna spe- cies in particular, given that large body size itself to a large extent depends on available NPP. Such energetically driven biomass tradeoffs provide a new way to explore the QME and have the potential of extracting general principles relevant to understanding the future. That is the approach I take here, one that necessarily has many caveats (see Methods), but that nevertheless leads to some interesting observations. Details of the QME and debates about its causes are summarized in recent reviews (Barnosky et al., 2004 Lyons et al., 2004 Koch and Barnosky, 2006 Wroe and Field, 2006). Salient points include the following. It was a time-transgressive extinction, beginning by 50 kyr B.P. in Australia and largely ending there by 32 kyr B.P., possibly concentrated in an interval between 50 and 40 kyr B.P. (Roberts et al., 2001 Trueman et al., 2005 Wroe and Field, 2006). In northern Eurasia and Beringia, extinctions were later and occurred in two pulses, the first between 48 and 23 kyr B.P. and the second mainly between 14 and 10 kyr B.P. (Koch and Barnosky, 2006), although some species lingered later in isolated regions (Irish elk until 7 kyr B.P. in central Siberia and mammoths until 3 kyr B.P. on Wrangel

230  /  Anthony D. Barnosky and St. Paul Island) (Guthrie, 2004 AJ Stuart et al., 2004). In central North America, extinctions corresponded with the second Eurasia–Beringia pulse, starting at 15.6 kyr B.P. and concentrating between 13.5 and 11.5 kyr B.P. (Koch and Barnosky, 2006 Waters and Stafford, 2007). In South America, the extinction chronology is not well worked out, but growing evidence points to a slightly younger extinction episode, between 12 and 8 kyr B.P. (Hubbe et al., 2007). Extinction intensity varied by continent, with Australia, South Amer- ica, and North America hard-hit, losing 88% (14 extinct, 2 surviving), 83% (48 globally extinct, 2 extinct on the continent, 10 surviving), and 72% (28 globally extinct, 6 extinct on the continent, 13 surviving), respectively, of their megafauna mammal genera. Eurasia lost only 35% of its genera (4 globally extinct, 5 extinct on the continent, 17 surviving). Africa was little affected, with only 21% loss (7 globally extinct, 3 extinct on the continent, 38 surviving), including at least three Holocene extinctions. Humans evolved in Africa, and hominins have been interacting there with megafauna longer than anywhere else. Insofar as they are dated, there is no correlation between human arrival or climate change for the few African extinctions. In general, extinctions in Australia intensified within a few thousand years of human arrival ≈50 kyr B.P. but did not correspond with unusual climate change. Extinctions in northern Eurasia corresponded in time with the first arrival and population expansions of H. sapiens, but both pulses also were concentrated in a time of dramatic climate change, the first pulse at the cooling into the Late Glacial Maxi- mum (LGM) and the second pulse at the rapid fluctuation of YD cooling followed by Holocene warming (Barnosky et al., 2004 Koch and Barnosky, 2006). Other species of Homo had been interacting with the megafauna for at least 400,000 years without significant extinctions before H. sapiens arrived. In Alaska and the Yukon, the first pulse of extinctions corre- sponded with LGM cooling but in the absence of significant human pres- ence the second pulse coincided with humans crossing the Bering Land Bridge and with the YD and Holocene climatic events. In central North America, extinction was sudden and fast, coinciding with the first entry of Clovis hunters, the YD–Holocene climatic transition, and the purported comet explosion. In South America, humans were already present by 14.6 kyr B.P., megafauna did not start going extinct until Holocene warming commenced some 11 kyr B.P., and species of ground sloths, saber cats, glyptodonts, and horses have seemingly reliable radiocarbon dates as young as 8 kyr B.P. (Hubbe et al., 2007). Few islands ever had non-human megafauna sensu stricto. That, and the fact that even human biomass of islands is very small compared with the continents, caused me not to consider them in this analysis. However, it is important to note that, on nearly every island where humans have

Megafauna Biomass Tradeoff  /  231 landed, extinctions (especially of birds) and wholesale habitat destruction have shortly followed. RESULTS AND DISCUSSION Species Loss vs. Human Population Growth The numbers of megafauna species lost were modest until the human growth curve began its rapid exponential rise between 15.5 and 11.5 kyr B.P. (Fig. 12.2). Then, species losses accelerated, primarily in the Americas, until the non-human megafauna baseline leveled off at 183 species, where it more or less remains today. However, human population continued to rise dramatically even after the counts of non-human megafauna species stabilized. Biomass Crash When converted to biomass, the inverse relationship between humans and non-human megafauna is evident (Fig. 12.3). Non-human megafauna biomass fell dramatically between 15.5 and 11.5 kyr B.P., concomitant with the initial steep rise in human biomass. Summing the biomass calculated for humans and nonmegafauna species provides a way to track changes in overall megafauna biomass through time (Fig. 12.4). The results suggest that biomass loss from the early megafauna extinctions in Australia and the first pulse of extinc- FIGURE 12.2  Number of non-human megafauna species that went extinct through time plotted against estimated population growth of humans.

232  /  Anthony D. Barnosky FIGURE 12.3  Estimated biomass of humans plotted against the estimated biomass of non-human megafauna. See Methods for parameters used. tions in Eurasia and Beringia were almost exactly balanced by the gain in human biomass. However, global megafauna biomass crashed dramatically between 15 and 11.5 kyr B.P. The crash reflects the second pulse of extinction in Eurasia–Beringia and the major extinction pulse in North and South Amer- ica. This crash is evident in everyPNAS BW 600dpi 1 tests, so it does not Fig 3 Barnosky one of the sensitivity appear to be a computation (20p6) The crash also remains evident when column.tif artifact. the biomass added by domestic species that support humans—pigs, sheep, goats, cattle—are included beginning 11 kyr B.P. Even using unreason- ably high proportions of domesticates to humans (i.e., assuming today’s FIGURE 12.4  Change in the sum of human and non-human wild megafauna bio- mass through time. The brackets indicate when extinction pulses hit the respective geographic areas. See Methods for parameters used. Fig 4 Barnosky PNAS BW 600dpi 1 column.tif (20p6)

Megafauna Biomass Tradeoff  /  233 proportions even at the dawn of animal domestication) fails to make the crash disappear. Significantly, even though human biomass was rising dramatically at the time, that rise was not enough to balance the biomass lost from the megafauna that were going extinct. Therefore, more was at work than a simple biomass tradeoff among megafauna. The suddenness of the crash, its magnitude, and its distribution across three continents suggest a global threshold event (Scheffer et al., 2001 Scheffer and Carpenter, 2003). Threshold events by definition are sudden changes to alternative ecosystem states induced either by some gradual change reaching a critical value or by abnormally strong perturbations. In either case, the net effect is to push the system from one ‘‘basin of attraction’’ (in this case, a world where megafauna body mass is distributed across many species) into a different one (a world where most megafauna body mass is concentrated around humans). The cause of the QME threshold event may well reflect a synergy of reaching a critical value of human biomass at the same time that ecologically unusual perturbations came into play. The unusual perturba- tions included increasingly sophisticated hunting of megafauna by people, habitat alteration by growing human populations, climate changes that would have decreased total global ecological energy at least temporarily, and possibly a comet impact. The potentially dramatic effects of hunting (so-called overkill) have been most convincingly demonstrated by simulations of the effects of Clovis hunters first entering North America (Alroy, 2001), which occurred just as global human biomass began to steeply rise. Those simulations sug- gest that hunting alone would result in many extinctions, because humans killed megafauna for food (Alroy, 2001). Similarly comprehensive simulations have not yet been done for other continents, but at least indirect human impacts (including habitat altera- tion and fragmentation) seem likely, given the coincidence of the mega- fauna biomass crash after first entry or markedly increasing population sizes of humans into various regions. These coincidences include first entry of humans into South America (near 14.6 kyr B.P.) and the entry and population growth of H. sapiens into Eurasia (from ≈40 kyr B.P.). Entry of humans also precedes the QME in Australia, although there, both were earlier than the worldwide biomass crash. In parts of Australia (Miller et al., 2005), North America (Burney et al., 2005), and South America (Moreno, 2000), the evidence for indirect human impacts includes sedi- mentary records of increasing fire frequency, potentially indicating wide- spread habitat alteration through human-set fires. The indirect or direct role of humans in the QME also is suggested by the observation that the main megafauna survivors had habitat preferences that would have kept them farthest from humans (Johnson, 2002).

234  /  Anthony D. Barnosky Also coincident with the megafauna biomass crash was rapid cli- matic cooling, then warming as the YD gave way to the Holocene. In the Americas, where most of the extinction was concentrated, the tail end of the LGM, then the YD cooling, depressed NPP in at least the Northern Hemisphere. A slightly earlier YD-like cooling did the same in South America, just as humans began to interact with the non-human mega- fauna. Likewise, YD cooling was pronounced in northern Eurasia at the time of the world biomass crash. If the evidence for a comet explosion over North America stands the test of time, NPP available to megafauna would have been further depressed at the time of the big extinction pulse. Large tracts of land are thought to have burned, and the explosion itself may have triggered the YD cooling in the Northern Hemisphere through opening the way for massive amounts of cold glacial meltwater to flood into the North Atlantic (Firestone et al., 2007). Biomass Recovery At the crash, megafauna biomass fell below its previous baseline value (Fig. 12.4). Then, beginning ≈10 kyr B.P., it began to build back up. By that time, the energy bottleneck that accompanied the crash was over. Global NPP was increasing as Holocene temperatures warmed, more land area was being exposed as glaciers melted, and there were fewer megafauna species on Earth among which to split the energy allocation. Even so, it took thousands of years for megafauna biomass to build back to precrash levels. The way it built back up was fundamentally different from the way it had been before, because virtually the entire recovery was by adding human biomass the biomass of non-human megafauna remained virtu- ally unchanged. In terms of ecosystem dynamics under threshold models (Scheffer et al., 2001 Scheffer and Carpenter, 2003), the biomass trajectory suggests that the global ecosystem crossed a threshold when the crash occurred. In the precrash state, megafauna biomass was distributed among many megafauna species, each with a relatively narrow ecological niche. In the postcrash alternative state, megafauna biomass concentrated in one species, humans, which has a very broad ecological niche. That means that ultimately humans were successful in coopting energy previously shared among other species with big bodies. It also means that not only are those extinct megafauna gone forever, but also there is no potential for new megafauna species to evolve into the ‘‘megafauna space’’ as long as humans are so abundant. In that respect, we have decreased biodiversity for as long as we remain abundant on Earth.

Megafauna Biomass Tradeoff  /  235 Recognizing the length of time it took the global ecosystem to recover to the precrash baseline depends on assumptions that were explored in the sensitivity tests. What I regard as the most reasonable input parameters result in the data illustrated in Fig. 12.5. That scenario includes domestic livestock, humans, and wild species as megafauna biomass and leads to two important observations. First, the buildup of human-associated megafauna biomass, even in the absence of the extinct megafauna, took ≈9,700 years to reach precrash levels. That indicates that recovering from global ecosystem shifts takes much longer than the shift itself. Even the sensitivity test that gives the fastest recovery time (unreasonably using large carnivore density equa- tions for all species) requires 8,000 years to reach precrash megafauna biomass. The lesson is that if another threshold causes changes as dramatic as the QME, Earth’s recovery will be far in the future, and not something the next few generations would see. Second, the point at which biomass recovery is reached is very close to the beginning of the Industrial Revolution (Fig. 12.5) or at most 700 years before that (the sensitivity test noted above). This suggests that humans were unable to exceed the normal, precrash, solar-energy-limited FIGURE 12.5  Semilog plot of the sum of human and non-human wild megafauna (dots) and the sum of human, wild, and domestic megafauna (triangles connected by line). Light gray bar indicates the timing of the YD-Holocene climatic event that led into the current interglacial. See Methods for parameters used. Fig 5 Barnosky PNAS BW 600 dpi 1 column.tif (20p6)

236  /  Anthony D. Barnosky baseline until we began to add to the global energy budget through min- ing fossil energy out of coal, oil, natural gas, and related sources. As soon as we began to augment the global energy budget, megafauna biomass skyrocketed, such that we are orders of magnitude above the normal baseline today. CONCLUSIONS When examined in the light of megafauna biomass tradeoffs, the cause of the QME becomes clearer, and implications for the future emerge. In essence, the QME begins to stand out not just as a major extinction event but also as an example of how threshold effects change the global ecosys- tem, and what new threshold events may be in sight. In the specific case of the QME, a global crash in megafauna biomass resulted when the coincidence of at least two events constricted the share of ecological energy allotted to each non-human megafauna species. One event was a time of rapid growth in human biomass, which meant an inordinate supply of NPP began to be consumed by a single megafauna species. The other was a probable temporary reduction of NPP as the YD cooling hit both of the Americas and northern Eurasia (Hajdas et al., 2003). Exacerbating the global energetic constraints were the first entry of humans into the Americas, increasingly sophisticated hunting strategies and wider disruption of habitats, and possibly a comet explosion over North America. In the general sense, the QME has four lessons. First, the global eco- system is in a fundamentally different state than before the megafauna bio- mass crash. In contrast to the distribution of resources among the 350-plus megafauna species that were alive before the QME, most of the energy available to megafauna species in the post-QME world was coopted by humans. What is left after that is being subdivided among only 183 (plus or minus) other non-human megafauna species. It is perhaps comforting from a biodiversity standpoint that those other 183 species have remained on Earth since the crash. That may speak to a reasonable amount of sta- bility in the alternative state the global ecosystem reached after the QME threshold event, at least in pre-Industrial times. It is also consistent with the expectations of ecological threshold theory. Second, the Industrial Revolution elevated Earth’s carrying capac- ity for megafauna biomass. However, despite that increase in carrying capacity, ≈50% (>90 species) of those megafauna species that persisted so well for the previous 10,000 years have become extinct, critically endan- gered, endangered, or vulnerable to extinction in the past few decades, including >40% of the megafauna species of mammals in Africa, the only continent that made it through the QME largely unscathed. For mammals

Megafauna Biomass Tradeoff  /  237 as a whole, 25% of the 4,629 species known on Earth fall in the critically endangered through vulnerable categories. This suggests that not only has all of the ‘‘extra’’ carrying capacity been used by humans, but also we are beginning, as happened during the QME crash, to steal from the part of the global energy budget allotted to other megafauna species. We are also going farther and using energy previously allotted to species in even smaller body-size classes. Under business-as-usual scenarios, the inevi- table result will be another biomass crash that moves down the body-size classes relative to the QME event. Third, that the normal biomass baseline was exceeded only after the Industrial Revolution indicates the current abnormally high level of mega- fauna biomass is sustained solely by fossil fuels. If biodiversity is actually a tradeoff between human biomass and other species’ biomass, as both the QME and theoretical considerations indicate (Vitousek et al., 1986, 1997 Maurer, 1996 McDaniel and Borton, 2002), then depletion of fossil fuels without replacement by alternative energy sources would mean that a biomass crash is imminent, this one depleting human biomass and caus- ing extinction in a wide spectrum of other species. Reliable projections on the number of years into the future that fossil fuels can sustain the global ecosystem at current levels vary, but generally are in the area of 50 more years for oil, 200 more years for natural gas, and 2,000 more years for coal (Galoppini, 2006). Thus, without technological breakthroughs, the next biomass crash could be in as little as a few human generations. Fourth, it may be no coincidence that the QME did not occur until the intersection of growing human biomass and climate change that ultimately manifested as global warming. Climate change, either cooling or warming, itself produces adjustments in geographic range distribution and popula- tion size that can lead to extinction (Barnosky, 1986 Parmesan and Yohe, 2003 Root et al., 2003 Thomas et al., 2004 Parmesan, 2006 Pounds et al., 2006). Add to that the overall reduction of NPP that must have occurred with YD cooling, the indirect co-opting of energy by rapidly growing human biomass, and direct human displacement of megafauna by killing and habitat alteration, and the combination becomes particularly lethal. Today, we stand at a similar crossroads, because growth of human biomass in the past few decades has moved us to the point where we are beginning to co-opt resources from, further displace, and cause extinctions of species with whom we have been coexisting for 10,000 years. At the same time, Earth’s climate is warming even faster than the rates of climate change that characterized the QME. Recognizing the tradeoff between human biomass, non-human mega- fauna biomass, and non-human biomass in general highlights the need for extraordinary efforts to conserve the world’s remaining biodiversity (McDaniel and Borton, 2002). Business as usual will not stave off severe


Abstract

Since the late Pleistocene, large-bodied mammals have been extirpated from much of Earth. Although all habitable continents once harbored giant mammals, the few remaining species are largely confined to Africa. This decline is coincident with the global expansion of hominins over the late Quaternary. Here, we quantify mammalian extinction selectivity, continental body size distributions, and taxonomic diversity over five time periods spanning the past 125,000 years and stretching approximately 200 years into the future. We demonstrate that size-selective extinction was already under way in the oldest interval and occurred on all continents, within all trophic modes, and across all time intervals. Moreover, the degree of selectivity was unprecedented in 65 million years of mammalian evolution. The distinctive selectivity signature implicates hominin activity as a primary driver of taxonomic losses and ecosystem homogenization. Because megafauna have a disproportionate influence on ecosystem structure and function, past and present body size downgrading is reshaping Earth’s biosphere.

Wild mammals are in decline globally because of a lethal combination of human-mediated threats, including hunting, introduced predators, and habitat modification (15). Extinction risk is particularly acute for the largest mammals, which are more frequently in conflict with humans (1, 6). The ongoing extirpation of large-bodied mammals is a major conservation concern because their decline can lead to the loss of ecological function within communities (3, 5, 7). Megafauna have crucial direct and indirect impacts on vegetation structure, biogeochemical cycling, ecological interactions, and climate (710). Although the current extinction rate is higher than earlier in the Cenozoic (4), the ongoing biodiversity crisis may be an acceleration of a long-term trend over the late Quaternary. For example, a striking feature of the Pleistocene was the abundance and diversity of extremely large mammals such as the mammoth, giant ground sloth, wooly rhinoceros, and sabretooth tiger on all habitable continents. The debate about the causes of the terminal Pleistocene megafauna extinction has been long and acrimonious, with particular controversy surrounding the role of humans (1113).

Multiple hominins—including at a minimum Neandertals, Denisovans, and archaic/modern humans—have been part of ecosystems throughout the late Pleistocene. Genetic analyses reveal a complicated history, with substantial admixture between populations (14). Anthropologists remain divided about the routes, exact timing, and number of early migrations from Africa (1418), but several hominin species were probably widespread across Africa and Eurasia around 80 thousand to 60 thousand years (ka) ago (1517). Further expansion followed, with modern Homo sapiens reaching Australia

60 to 50 ka ago and crossing into the Americas

15 to 13 ka ago (15). Migrations were likely driven or facilitated by climatic factors (17, 18) and were followed by rapid increases in population sizes (17, 19). For example, hominin populations in western Europe increased 10-fold by the Neandertal-to-Modern human transition

40 ka ago (19). Middle to Upper Paleolithic hominins were hunters who lived in groups and used both tools and fire (20) thus, it is plausible that their activities and rapid population growth influenced mammal biodiversity well before the terminal Pleistocene.

We investigated the influence of these emerging and increasingly sophisticated hominin predators on continental and global mammalian biodiversity over the late Quaternary (21). Ongoing biodiversity loss is robustly linked to human activities (15) and previous work linked extinction risk over the Holocene, terminal Pleistocene, and end-Pleistocene to human activities (4, 6, 1113, 2225) but earlier influences remain poorly characterized. Although recent work on paleodemography exists for H. sapiens over the late Pleistocene and Holocene (17), a lack of data for other hominins precludes direct comparison of mammalian extinction risk over time against hominin population density. However, should we find significant differences between the pattern of late Quaternary extinction selectivity and the rest of the Cenozoic mammal record, this would strongly suggest a role of hominin activity (13, 24, 25).

We used two data sets to test the potential role of hominin activity on extinction selectivity, mammalian body size distributions, and patterns of biodiversity over time and into the future (21). First, we updated a spatially explicit global record of body size and trophic mode for nonvolant, terrestrial mammals for the late Quaternary (MOM). Second, we constructed a global data set of Cenozoic mammals with associated stratigraphic duration, body mass, and trophic mode. We categorized late Quaternary extinctions into five temporal bins: late Pleistocene (125 to 70 ka ago), which corresponded with the initial waves of migration of hominins out of Africa end Pleistocene (70 to 20 ka ago), which represented the continued expansion of hominins into Eurasia and the colonization of Australasia terminal Pleistocene (20 to 10 ka ago), which encompassed the migration of humans into the Americas Holocene (10 to 0 ka ago), which represented further expansion of humans throughout the globe and future (

+0.2 ka), where we assumed that all currently threatened mammals become extinct (21). We binned the Cenozoic fossil data set into intervals of 1 million years (Ma) as a reference standard and computed temperature metrics for each bin (21). For each time interval, we characterized the size selectivity of extinction using logistic regression and examined overall body size distribution and trophic guild structure (tables S1 to S7) (21). For the late Quaternary, we also characterized size selectivity by continent and trophic level.

Our analyses demonstrated a striking and significantly size-biased pattern of mammalian extinction over the late Quaternary, distinct in the Cenozoic record (Figs. 1 to 3 and fig. S1). We found a mass difference of two to three orders of magnitude between victims and survivors of late Quaternary extinction intervals (Fig. 2A and table S1), reflecting a significant association between size and extinction probability (Fig. 2B and table S5). This size bias occurred on each continent (Fig. 2, C and D) and within each major trophic group (Fig. 2, E and F), with the magnitude of the size difference and the statistical measure of size selectivity decreasing between the Pleistocene and Holocene (Fig. 2, A to F). The reduced selectivity of the Holocene and future extinctions likely reflects changes in the nature of threats. Today, many smaller-bodied animals are vulnerable because of habitat alteration, introduced predators, or urbanization (57, 11, 26).

All selectivity coefficients reflect change in the natural logarithm of the odds of extinction associated with a one-log10-unit change in body mass. Values of zero indicate no bias, positive values indicate bias against larger size, and negative values indicate bias against smaller size. LQ, average of all late Quaternary (LP to H) extinctions LP, late Pleistocene EP, end Pleistocene TP, terminal Pleistocene H, Holocene and F, future extinctions.

(A and B) Global patterns of extinction. (A) Difference in the mean of log-transformed sizes of victims versus survivors for intervals across the late Quaternary. (B) Selectivity coefficients measuring the association between body size and extinction probability derived from logistic regression of extinction status as a function of body mass. Multiple regressions controlling for the additive contributions of continental location and trophic guild yield even stronger associations between extinction status and body mass (table S5) (21). (C and D) Extinction patterns on each continent. (C) Size differences. (D) Size selectivity coefficients. (E and F) Influence of trophic guild on extinction risk. (E) Size differences. (F) Size selectivity coefficients. Bars indicate 95% confidence interval (CI).

(A) Global mean body size over the Cenozoic (65 to 1 Ma ago). (B) Mean body size by continent over the late Quaternary (past 125 ka). (C) Maximum body size across the Cenozoic by continent. (D) Maximum body size over the late Quaternary and into the future. (E) Size selectivity coefficients across the entire Cenozoic fossil record. (F) Size selectivity of late Quaternary extinctions. Bars indicate 95% CI. All masses are in kilograms. Light blue shading indicates late Pleistocene, white shading indicates Holocene, and gray shading indicates the future (+200 years). Ages here and elsewhere are plotted as midpoint of time interval.

Comparison of extinctions across the entirety of the Cenozoic demonstrated that body mass was rarely significantly associated with the probability of extinction before the late Pleistocene (Figs. 1 and 3, E and F), and further, size differences between victims and survivors never approached those observed in the Pleistocene (tables S1 and S3). There was a preferential loss of small-bodied species in the Oligocene that is perhaps linked to expansion of grasslands and prairies (

29 Ma ago) (Fig. 3E), although this value had high uncertainty. However, no interval over the past 65 Ma was as selective as the late Quaternary. Moreover, climate change did not increase extinction risk for large-bodied mammals before the spread of hominins. We found no relationship between temperature change over the Cenozoic and size bias of extinction neither small nor large mammals were more vulnerable to extinction during times of high climate variability (table S3 and fig. S4). The probability that the late Pleistocene and Cenozoic selectivity coefficients came from the same distribution was very low (P < 0.001), given either log likelihood or nonparametric tests (fig. S3). Moreover, grouped as a single extinction event (as they would appear to a future paleontologist), the Quaternary extinction pulse was by far the most selective episode of extinction in the Cenozoic (Fig. 1). Such pronounced size selectivity is highly unusual in other fossil records larger-bodied vertebrates and mollusks did not experience increased extinction risk over the Cenozoic or during the five mass extinction events (27). Because a reported signature of human hunting is size selectivity (24, 25), our results are consistent with the hypothesis that hominin activities contributed to extinctions long before the terminal Pleistocene.

The late Quaternary biodiversity losses led to dramatic, time-transgressive shifts in both mean and maximum body mass on each continent (Fig. 3), which followed hominin dispersal patterns (15) and began much earlier than previously suspected. Because body size distributions are related to the size of the landmass (28), the largest average or maximum body mass would be expected on Eurasia, followed by Africa, then North and South America, and the smallest on Australia. This expectation was largely met in the late Pleistocene (Fig. 3), but Africa was a notable outlier, with a mean body mass

50% less than that of Eurasia or the Americas before 125 ka ago (table S1). We hypothesize that the late Pleistocene size distribution in Africa reflects the long prehistory of hominin-mammal interactions (29). This finding suggests that the homogenization of natural ecosystems was a consequence of hominin behavior in general and not specific to H. sapiens. Over the following

100 ka, mean body mass dropped dramatically—first by 50% in Eurasia, and then by an order of magnitude in Australia—while remaining largely unchanged in the Americas until the terminal Pleistocene. Thus, for most of the late Quaternary, mean and maximum body masses were larger in the Americas than elsewhere—a pattern largely exceptional in the mammalian fossil record (Fig. 3 and table S1) (28). By the terminal Pleistocene, other hominin species were extinct, and the remaining H. sapiens had developed efficient long-range weapons (11). The latter likely contributed to the severity of the extinction in the New World (Fig. 1), with 11.5 and 9.7% of nonvolant terrestrial species lost in North and South America, respectively (tables S1 and S2). The loss of biodiversity resulted in a greater than 10-fold drop in both mean and maximum body mass, which was a steeper decline than elsewhere (Fig. 3, B and D). For example, mean mass of nonvolant terrestrial mammals in North America fell from 98.0 to 7.6 kg (table S1).

Future extinctions will continue the pattern of biodiversity loss and body size downgrading (fig. S1). If all species currently at risk are eventually driven extinct,

22.4 to 53.7% of mammals will be lost relative to 125 ka ago (table S2). This will further decrease mean body mass in North America from 7.7 to 4.9 kg (Fig. 3B and table S1) similar declines are predicted for other continents. Thus, the largest mammal on earth in a few hundred years may well be a domestic cow (Bos taurus) at

900 kg. Furthermore, the loss of currently endangered species would reduce terrestrial mammal body mass to the lowest values in the past 45 Ma (Fig. 3, A and C, as compared with Fig. 3, B and D). The last time the body size distribution of terrestrial vertebrates was similarly disrupted was

66 Ma ago, during the end-Cretaceous mass extinction.

Because body size is strongly linked to most biological rates and processes (30), the extirpation of large mammals led to a fundamental restructuring of energy flow through mammal communities over the late Quaternary. The severe body size downgrading—a truncation of more than two orders of magnitude—resulted in substantial shifts from bimodal toward unimodal size distributions (Fig. 4 and fig. S1). Homogenization of distributions continued through the Holocene and is predicted to continue into the future (Fig. 4 and table S4). Extinctions also led to changes in the proportional representation of trophic guilds, especially herbivores (fig. S2). In the future, continental distributions will be severely skewed toward smaller mammals (Fig. 4)—in particular, rodents (fig. S2). Ecological principles suggest that changes in energy flow over the Pleistocene likely led to compensatory changes, potentially numerical responses by surviving smaller-bodied mammals to maintain ecosystem homeostasis (31). By the Holocene, however, humans were a strong influence on energy flow within ecosystems. Global expansion was accompanied by increased human densities (17) and animal domestication (10). By historical time, the terrestrial biosphere was transformed from one dominated by wild animals into one dominated by humans and their livestock, many provisioned with domesticated crops (2, 5, 10). Today, the biomass of the >4.5 billion domesticated animals on Earth exceeds estimates for wild mammals at the terminal Pleistocene (10).

(A) Africa. (B) Eurasia. (C) Australia. (D) North America. (E) South America. Body sizes for each temporal interval are plotted distributions are overlaid from oldest to youngest. Yellow shading indicates the predicted distribution in the future, if vulnerable species go extinct.

Our study highlights the long and sustained influence of humans and other hominins on terrestrial ecosystems. As Neandertals, Denisovans, and humans spread across the globe over the late Quaternary, a highly size-biased extinction followed, a pattern distinct in the Cenozoic mammal record. The subsequent downgrading of body size was severe and differentially targeted herbivores. Thus, contemporary biodiversity loss is part of a trend spanning more than 125 ka, with expected future extinctions of greater magnitude, but reduced size selectivity, than in the past. The homogenization of ecosystems has dramatically influenced the past, present, and future role of wild mammals in the terrestrial biosphere.


Betrayals of trust: Human nature's dark side may have helped us spread across the world

New research by an archaeologist at the University of York suggests that betrayals of trust were the missing link in understanding the rapid spread of our own species around the world.

Dr Penny Spikins, of the University's Department of Archaeology, says that the speed and character of human dispersals changed significantly around 100,000 years ago.

Before then, movement of archaic humans were slow and largely governed by environmental events due to population increases or ecological changes. Afterwards populations spread with remarkable speed and across major environmental barriers.

But Dr Spikins, a senior lecturer in the Archaeology of Human Origins, relates this change to changes in human emotional relationships. In research published in Open Quaternary, she says that neither population increase nor ecological changes provide an adequate explanation for patterns of human movement into new regions which began around 100,000 years ago.

She suggests that as commitments to others became more essential to survival, and human groups ever more motivated to identify and punish those who cheat, the 'dark' side of human nature also developed. Moral disputes motivated by broken trust and a sense of betrayal became more frequent and motivated early humans to put distance between them and their rivals.

According to Dr Spikins, the emotional bonds which held populations together in crisis had a darker side in heartfelt reactions to betrayal which we still feel today. Larger social networks made it easier to find distant allies with whom to start new colonies, and more efficient hunting technology meant that anyone with a grudge was a danger but it was human emotions which provided the force of repulsion from existing occupied areas which we do not see in other animals.

Early species of hominin were limited in distribution to specific environments such as grasslands and open woodland. The expansion of Homo erectus out of Africa into Asia around 1.6 million years ago appears to have been caused by the need to find more large scale grasslands. By contrast, Neanderthals occupied cold and arid parts of Europe. All archaic species adapted slowly to new opportunities for settlement and were often deterred by environmental and climatic barriers.

After 100,000 years ago, however, dispersal into distant, risky and inhospitable areas became relatively more common compared with movements into already occupied regions. Most notably, the spread of modern human populations was not inhibited by biogeographical barriers. Populations moved into cold regions of Northern Europe, crossed significant deltas such as the Indus and the Ganges, deserts, tundra and jungle environment and even made significant sea crossings to reach Australia and the Pacific islands.

Dr Spikins argues that betrayals of trust resulting from moral disputes were a significant reason for such risky dispersals into apparently unwelcoming environments with a desire to avoid physical harm from disgruntled former friends and allies being a key motivation. Offenders and any allies within their social network would feel driven to get out of harm's way.

She says: "Active colonisations of and through hazardous terrain are difficult to explain through immediate pragmatic choices. But they become easier to explain through the rise of the strong motivations to harm others even at one's own expense which widespread emotional commitments bring.

"Moral conflicts provoke substantial mobility -- the furious ex ally, mate or whole group, with a poisoned spear or projectile intent on seeking revenge or justice, are a strong motivation to get away, and to take almost any risk to do so.

"While we view the global dispersal of our species as a symbol of our success, part of the motivations for such movements reflect a darker, though no less 'collaborative', side to human nature."


Watch the video: Extinction of Species. Evolution. Biology. FuseSchool (January 2023).