Key Concepts: The Big Five mass extinctions represent intervals when environmental change overwhelmed the ability for life to adapt. Other types of environmental changes also happen, severe but not rising to the level of the Big Five. One such was the Paleocene-Eocene Thermal Maximum of 56 Ma, the closest Nature has had to the replicating the effects of anthropogenic global warming. Human activities, from Pleistocene overhunting to resource exploitation in the 19th and 20th Century have shown how we can reduce once-common species to the point of extinction. Alternatively, though, the historic record of conservation shows that when we so chose, we have been capable of bringing nearly-extinct species back from the brink.

A couple of lectures ago we looked at the concept of mass extinction. Let's take a look at each of the Big Five mass extinctions:

The Big Five
Ordovician/Silurian Mass Extinction (443.8 Ma [millions of years ago]): The oldest well-understood mass extinction. Occurs (possibly as a two phase pulse) as a byproduct of intense glaciation and drop of habitat range. The least understood of the Big Five at present. When the occurred, there were very few land organisms.

Devonian/Carboniferous Mass Extinction (358.9 Ma): Also less well-studied than the later three. Saw a collapse of a hugely-diverse reef community (even more widespread than today's coral reefs, and at least as diverse in terms of biodiversity). Saw the loss of many invertebrate groups, and also many important fish groups. As with the O/S extinction, the D/C may have been in two pulses.

Life had only colonized land during the Devonian: the first vascular plants and the first trees date back to this Period. Indeed, it appears as this this was the significant factor in the extinction. As plants began to move away from the water's edge they formed the first deep soils. Soils store some of the carbon from the plant matter, rather than releasing it back into the atmosphere: this burial of carbon brought carbon dioxide levels lower, and thus reduced the greenhouse effect, allowing the world to cool.

However, the more important aspect was the presence of abundant new nutrients (broken down plant matter and rocks broken up by the action of plants) washing into the shallow seas. This produced eutrophication (a problem we see today, too!). This is when the nutrients from land fertilize the shallow water, leading to a bloom in phytoplankton. When the phytoplankton dies, it settles to the sea floor and decays: that process exhausts the dissolved oxygen in the water, causing "dead zones" of anoxia (non-oxygen conditions). Today we see such dead zones offshore where there is a a lot of farming or development (and thus lots of excess nutrients in the water), and today such dead zones and cloudy water are two of the main threats to coral reefs (a third being coral bleaching due to warming seas, and a fourth being ocean acidification: more about this below).

Permo-Triassic Mass Extinction (252.17 Ma): The "Mother of all Mass Extinctions". If this is a single event, between 95-96% of species in the marine realm (and 70% of terrestrial species) died out. (And consider that a species can persist with just a couple of individuals, so we are looking at the death of >99.99% of the biosphere). Produced a MAJOR overall of marine communities: the single biggest change in the biosphere. The end of the Permian brought an end to the Paleozoic Era.

The primary trigger is the Siberian Traps, a massive series of eruption of lava in Siberia. This lava field covered an area of about 5 x 106 km² and represented a volume of 3 x 106 km³: if you took this amount and spread it evenly over North America, it would cover us to a depth of 121 m (almost 400 feet)!! All this material was erupted in less than 1 Myr time (possibly much less). But the lava itself isn't the main effect: it is the greenhouse gases (carbon dioxide and methane) that was released. Estimates show 12-18,000 Gt C were emitted: in comparison, the modern atmosphere has a mere 800 Gt C and the value before the Industrial Revolution was about 600, so all the global warming from human activity so far is a mere 200 Gt C.

The net result was a catastrophic rise in atmospheric CO2, leading to extreme global warming. This warmed the ocean floor, where methane clathrates (ice with methane trapped within it) was melted, releasing the methane into the atmosphere. Methane (CH4) is an even stronger greenhouse gas than carbon dioxide, so it added to the this warming.(There is even evidence that the magma which produced the eruption ignited buried coal fields in Siberia, so some of the carbon dioxide was from coal rather than the lava!)

Additionally, the oceans had extreme ocean acidification, greatly damaging to a wide variety of sea life. The mass loss of plants on land and algae at sea resulted in a tremendous drop in atmospheric and oceanic oxygen, producing anoxic effects. (A secondary contributor to anoxia was the raised global temperature: with the world pole-to-pole hot, there was little temperature differential, which reduced wind, which reduced the oceanic circulation, allowing the seas to stagnate.) Isotopic evidence shows that the oceans of the earliest Triassic were lethally hot in the tropics (about 40°C rather than today's 30°C; warm water holds very little oxygen.)

And there is evidence of bursts of hydrogen sulfide out of the ocean: this is poisonous to land and sea life, and damages the ozone layer, allowing harmful UV radiation to stream in. The sulfur from the eruptions and the ocean produced catastrophic acid rain levels.

Triassic-Jurassic Mass Extinction (201.3 Ma): Aka "Permo-Triassic, version 2.0". The environmental changes were from a very similar cause: a huge eruption of lava called the Central Atlantic Magmatic Province (CAMP), produced when North America + Eurasia ripped away from Africa + South America, splitting the supercontinent Pangaea and opening the Central Atlantic Basin. It clobbered life on land and in the sea, and opened up the terrestrial realm for dinosaur dominance. The environmental effects of CAMP were essentially the same as for the Siberian Traps, on a (just slightly) smaller scale.

Cretaceous-Paleogene Mass Extinction (66.0 Ma): Not the largest mass extinction, but easily the most famous. In older stratigraphic nomenclature, called the "Cretaceous-Tertiary (or K/T) Extinction". Essentially all land animals >5 kg died out (including all dinosaurs other than modern-style birds); in the marine realm, major extinctions of plankton and the part of the food chain that fed directly through plankton (rather than bottom-feeders).

There is a huge volcanic event associated with the Cretaceous-Paleogene, the Deccan Traps of western India. At least 2 x 106 km³ were erupted, producing effects similar to those of the Permo-Triassic and Triassic-Jurassic. The eruption began slightly more than 250 kyr before the Cretaceous-Paleogene boundary event. There is some evidence of global warming produced by the Deccan Traps in the latest Cretaceous fossil record.

However, the main event was first identified from chemical traces at Gubbio, Italy: the impact of a ~10 km diameter asteroid in the Yucatán Peninsula of Mexico. This produced a crater with a diameter of about 180 km (long since buried, but detectable by drilling, seismic scanner, etc.) The energy released was equivalent to 1.8 x 108 Mt (megatons) of TNT: this is about 1000-10,000 times the destructive power of the entire planetary nuclear arsenal at the height of the Cold War! This blasted out vast amounts of material high into the atmosphere, some of which rained down right away, and the rest of which stayed up for months.

The infalling debris burned up in reentry, causing a thermal pulse out of the skies with heat levels 8-10 times that at noon at the equator. Every spot on the Earth would have been baked as with a heat lamp for minutes to hours. The soot that stayed in the atmosphere blocked incoming solar radiation for weeks or months, chilling the surface of planet. And when the atmosphere cleared, the elevated carbon dioxide levels from the Deccan Traps and from the vaporization of carbonate rocks at the crater produced a global warming event.

The Paleocene-Eocene Thermal Maximum (PETM)
There are smaller scale environmental crises in the history of life which, although not as dramatic as the Big Five mass extinctions, nevertheless profoundly effected life on the planet. one of these is the Paleocene-Eocene Thermal Maximum (PETM). In this very short term global warming event 56.0 Ma in which for a geologically-short period of time (<10,000 years) the level of CO2 jumped to 3-4x the previous background level. (This jump is comparable to the upper end of anthropogenic greenhouse gas models, but from a higher starting point.) PETM saw a temperature increase of +4-5K at the tropics, +6-8K at the poles, and even +4-5 K in the deep sea.

Some consequences of this event:

The likely cause of PETM was due to a massive release of methane clathrates from the sea floor. (This matches the isotopic signature of the carbon increase.) In turn, this is likely due to a sudden burst of submarine volcanism in the northeastern Atlantic (between Greenland and Europe).

Mass Extinction & Other Crises as the "Game of Thrones"
Why do we have mass extinctions at certain times in Earth History? It isn't necessarily that the environment changes, because they change all the time. The main reason is the intensity and rate of the change: Thus, the shared cause for mass extinctions and other crises is: environmental change too severe in intensity to allow adaptation.

The Pleistocene Megafaunal Extinction
The end of the Pleistocene (2.6 million to 11.8 thousand years ago) saw the end of glacial-interglacial cycles. But it also saw the end of giant mammals (and to a lesser extent, reptiles and birds) around the world:

Africa: Essentially modern composition of the animal life in the Pleistocene (indeed, nearly modern in the Pliocene). So very little evidence for a Quaternary Period extinction among carnivores and herbivores from the continent of humanity's birth.

Boreal Eurasia: In addition to Mammuthus primigenius (the woolly mammoth: also in North America) and Coelodonta antiquitatus (the woolly rhino), there was the giant Irish elk (Megaloceros giganteus), the cave lion (Panthera spelea), the cave bear (Ursus speleus), and Homo neanderthalensis.

East Asia, Southeast Asia, and Indonesia: Less well studied than many other regions, there were local mammoths and other proboscideans (such as species of Stegodon), rhinos, and deer which died out, as well as the giant orangutan relative Gigantopithecus and the tiny hobbit Homo floresiensis. (Indeed, on Flores Island there were giant storks, giant rats, giant lizards, and dwarf Stegodon: an odd community). Also, the "mystery" Denisovans and Maludong peoples disappear as well.

Beringia: (that is, the Siberia-Alaska corridor) Many fauna shared in common between Boreal Eurasia and Continental North America.

Continental North America: Prior to the megafaunal extinctions, continental North America south of the Ice was one of the most diverse assemblages of large bodied mammals the world has ever seen; rivaling or surpassing the modern Africa savannas. Among the notable victims were the giant short-faced bear Arctodus simus, the giant American lion (once thought to be a jaguar-like cat, but now found to be the sister taxon to the cave lion and close to the extant lion) Panthera leo atrox, the dire wolf Canis dirus, the North American sabretooth cat Smilodon fatalis, and the giant condor-like Teratornis merriami among carnivores; and the extremely tall Columbian plains mammoth Mammuthus columbi, the more woodland American mastodon Mammut americanum, the giant black bear-sized beaver Castoroides ohoiensis, Yesterday's camel Camelops hesternus, the stagmoose Cervalces scotti, the huge broadheaded bison Bison latifrons, several species of native horses, various species of ground sloth such as elephant-sized Eremotherium laurillardi, rhino-sized Megalonyx jeffersoni, and bear-sized Nothrotheriops shastensis, and the giant super-armadillo glyptodont Glyptotherium arizonae among the herbivores. The sloths and glyptodonts were recent immigrants from South America.

South America: For much of the Cenozoic, South America was an island continent with a unique fauna of marsupials, endemic placentals, giant birds, and so forth, but during the Neogene an interchange of faunas with North America over the young Isthmus of Panama mixed the assemblages. Some South American animals did well in the North: opossums, armadillos, porcupines, ground sloths, glyptodonts. But far more North American animals did well in the South: proboscideans, horses, camels, dogs, cats, rodents, raccoon-relatives, peccaries, tapirs, deer, squirrels, rabbits, and so on. There was a major extinction of the native South American animals at this time. But some of the native forms and new immigrants thrived and diversified up to the Pleistocene, including the largest gylptodonts such as Doedicurus clavicaudatus, the largest ground sloths such as Megatherium americanum and nearly as large Eremotherium laurillardi (which ranged into North America as well), as well as a diversity of smaller sloths, camel-like Macrauchenia patachonica and rhino-like Toxodon platensis (last survivors of once-diverse radiations of South American hoofed mammals), the sabrecat Smilodon populator, various elephants such as Cuvieronius hyodon, Stegomastodon platensis and S. waringi (both genera were also present in North America earlier), as well as other diverse forms.

Sahul: (that is, Australia, New Guinea, and Tasmania) In addition to the giant wombat Diprotodon opatum, other notable marsupials which died out in the late Pleistocene were the marsupial "lion" (probably more like a jaguar ecologically) Thylacoleo carnifex and the giant kangaroos Procoptodon goliah and Simosthenurus occidentalis. But in the Sahul some giant sauropsids were present as part of the megafauna: the possibly-carnivorous giant duck relative Genyornis newtoni; a giant constricting snake Wonambi naracoortensis (not a boa or python, but last survivor of a Cretaceous lineage of big snakes); the giant club-tailed horned turtle Meiolania platyceps; and enormous Varanus priscus (once called "Megalania"), a huge predatory lizard closely related to the Komodo dragon (Varanus komodoensis).

NOTE: Properly speaking, this is NOT a "mass extinction" as that term is used by paleontologists who work in the rest of the geologic record. We see no impact on the marine invertebrate fauna (which are the key aspects of ALL true mass extinctions), nor plants, nor smaller terrestrial fauna. It is limited to larger-bodied land mammals and reptiles (including birds). That is not so say there are no extinctions among smaller land animals; however, these do not show elevated rates above background (as opposed to the massive die off of large land beasts).

What sort of patterns do we see in the extinction event? In particular, are there common ecological factors? Common factors of intensity? Common factors of timing?

Size Pattern:Overall, there is an important thread to all these extinctions: they strongly bias towards size. That is to say, large animals tend to die out, small ones tend to survive. In contrast, the general ecology of the victims (carnivores vs. herbivores; browsers vs. grazers; woodlands vs. grasslands; uplands vs. lowlands; etc.) seem to be far less important. Hence, this has been called the Pleistocene megafaunal extinction as it really only affects the big land animals.

NOTE: Properly speaking, this is NOT a "mass extinction" as that term is used by paleontologists who work in the rest of the geologic record. We see no impact on the marine invertebrate fauna (which are the key aspects of ALL true mass extinctions), nor plants, nor smaller terrestrial fauna. It is limited to larger-bodied land mammals and reptiles (including birds). That is not so say there are no extinctions among smaller land animals; however, these do not show elevated rates above background (as opposed to the massive die off of large land beasts).

Regional Patterns: Why No African Megafaunal Extinction?: If we look at the differences in regional intensity of extinction, we notice a pattern among land mammals greater than 40 kg (88 lbs) adult body mass:

Why this pattern? Africa essentially doesn't participate in the extinction (it is essentially background rates), Europe has moderate rates, the Americas very high rates, and the Sahul catastrophic ones.

It is not a difference in environment: much of Africa is similar to either the Sahul or South America in terms of environment, while Europe and North America have largely similar environments.

The pattern does match a particular aspect, though: regions in which no close human relative had ever lived prior to the arrival of H. sapiens saw truly tremendous extinctions; Europe (and Asia, not shown)--where earlier species of Homo had lived by and hunted the local megafauna for hundreds of thousands of years--shows intermediate levels; and Africa--in which human relatives, and indeed Homo sapiens, first evolved and spent most of its/our evolutionary history--shows essentially none. The megafauna of Africa co-evolved with us, and so had much longer time to adapt to our escalating ability to hunt; the Eurasian forms had at least some experience with human hunters of some species; but the Brave New Worlds first encounters were with fully modern humans.

Timing?: This remains a contentious issue. As we will see there are a handful of late survivors of some species in isolated environments, so what do we mean when we refer to the timing of the extinction? Furthermore, not all the taxa die simultaneously on the scale of centuries.

That said, the signal is very clear on a broad scale: Australian forms undergo the strongest intensity of extinction in the 45-40 ka range; Boreal Eurasian forms tend to die out in the 30-25 ka range (with some late survivors); American ones in the 13-11.5 ka range. Intriguingly, these match pretty closely the arrival time of H. sapiens (or at least continued presence of them) to these respective regions.

So what are the potential triggers for the megafaunal extinction? Four main contenders exist:

Fellow Travelers: If you look at the list of the largest native North American mammals today, it turns out four of the most common (the moose Alces alces, the elk Cervus elaphus, the brown bear Ursus arctos, and the wolf Canis lupus) are all very recent immigrants. Indeed, they may have arrived in North America with the Paleo-indians! These species also live in Eurasia, and so are much better adapted to living alongside humans and our effects.

The Holocene Extinctions
The Holocene Epoch of the Quaternary Period of the Cenozoic Era is our name for "modern" times: that is, the last 11,784 years. This is the time since the end of a 1000-year cool snap (the Younger Dryas) during the deglaciation from the Last Glacial Maximum. During this time humans discovered agriculture, developed cities, started using metals, and created writing (and thus a written history).

During the Holocene humans spread to many oceanic islands in the Pacific, Indian Ocean, and elsewhere. In the wake of new arrivals, many species became extinct. For example, the total number of bird species at 1 CE was probably 3000 more than today, largely do to losses of island avian species.

Let's take a look at some of the patterns of extinction.

Historic Cases Recognized at the Time:

But there are many other larger-scale or more dramatic extinctions during the Holocene:

Madagascar: Humans (settlers from Borneo, rather than nearby southeastern Africa) reached the island of Madagascar between 350 and 550 CE. They arrived with an agricultural system based on slash-and-burn. Through a combination of habitat destruction and direct hunting, they wiped out such as:

New Zealand: These islands remained uninhabited until 1280 CE, when Polynesians first arrived. By 1400 (possibly before) they had wiped out many taxa, including:

In the case of the moa there is direct evidence of hunting and feeding by humans, and this seems likely in the case of the adzebills, too. The Haast's eagles were probably victims of trophic collapse: with the moa gone, there was no targets left (other than men) large enough to serve as prey.

Tasmania: Part of Sahul, it was isolated from mainland Australia as the sea rose in the last deglaciation. Its inhabitants lost a substantial part of earlier technology, such as bone tools, boomerangs, hooks, sewing, and the ability to start fires. In many ways, they had reverted to Homo erectus-grade technologies. In Tasmania some of the larger animals that were wiped out on mainland Australia survived, most famously the thylacine or Tasmanian tiger or Tasmanian wolf (Thylacinus cynocephalus. This animal was a marsupial predator strongly convergent on placental wolves. With the settlement by Europeans in 1803, a systematic campaign of extermination of both the Tasmanian people and Tasmanian tiger began. Sadly, both were successful. The native population of Tasmania was hunted down to the last individual and their language and culture lost for all time: the last full-blooded Tasmanians were the women Truganini who died on 8 May 1876 and Fanny Cochrane Smith who died in 1905. The thylacines actually survived longer, with the last specimen "Benjamin" dying in captivity on 7 September 1933. Below is a compilation of all surviving film footage of this species (all of it in zoos, rather than in the wild):

The mainland Old World: But these are all small, isolated lands. Surely extinction hasn't happened in recent times on the continents? Sadly, here to. For example, the quagga (Equus quagga quagga, an extinct southern African subspecies of plains zebra: 12 August 1883), the aurochs of Eurasia (Bos primigenius, although technically a pseudoextinction as the domestic cow Bos taurus is a descendant: 1637), the tarpan of Eurasia (Equus ferus ferus, the ancestor of the domestic horse Equus caballus: 1879 for the last scientifically verifiable non-hybrid), and others. And the extinctions in North America were particularly striking.

Continental North America: Here the extinctions included not only rare forms, but some of the most common species to inhabit the continent:

A very, very close call was Bison bison, the plains bison, largest and by far most common large mammal of North America after the megafaunal extinction. Their huge herds made them essentially hunt-proof to the Folsom points--and later bows and arrows--of Native Americans. But arrival of advanced rifle technology, the expansion westward of American farming and railroads, and a tremendous market for bones for fertilizer and hides for many numerous uses led to commercial hunting on a phenomenal scale, bringing species dangerous close to extinction at the end of the 19th Century. Once hundreds of millions formed vast herds, but by 1890 less than 1000 individuals remained. Political and social action from the grassroots on up to Congress and the White House led to protection of this symbol of the American West, and the species was saved and once again roams the West.

Another close call was the eastern blue bird Sialia sialis. It was a common bird in America, but some birds introduced in the 20th Century (in particular the Eurasian starling Sturnus vulgaris and the house sparrow Passer domesticus) outcompeted it for nesting space. In fact, today starlings and house sparrows are probably the birds you are most likely to see on campus!! But there is some good news! Conservation action (generally as simple as putting out bluebird nest boxes on your own property) has allowed the eastern bluebird to rebound in population. This again shows that human agency on behalf of endangered species CAN bring them back in some cases.

It is worth noting that slower paced environmental change is something with which species are better at dealing. For instance, they can migrate in their habitats if the climates don't change too quickly and if there is viable habitat for them to pass through. And given sufficient time, some populations might evolve into new species adapting to the new conditions.

But the big lesson of the history of the planet is that although "Life" may find a way, most individuals and many species DO NOT! So as we produce more profound and more quick environmental changes, a "natural" response if the disappearance of the species around us. If we want to avoid this, we need to find ways of reducing our impact on the environment.

But what about people? Does history show any evidence that environmental change can affect human societies? That is next week's topic.