In biology and ecology, extinction refers to the dying out of an organism or a group of organisms that constitute a species. 99% of the estimated four billion species to have evolved on the Earth over the last 3.5 billion years have gone extinct (Novacek, 2001). Throughout history, most extinction events have occurred naturally as life becomes outmoded in response to shifts in environmental contingencies that bring about unfavorable selection pressures. Such evolutionary pressures might include an increase in predators, a reduction in available prey, natural and anthropogenic habitat degradation, competition with other species (including humans), and intra-species competition.
The causes for extinction are as varied and unique as the species themselves. The ongoing extinction of an individual species due to environmental or ecological factors such as climate change, disease, loss of habitat, or competitive disadvantage in relation to other species is known as background extinction. It occurs at a fairly steady rate over geologic time and is the result of normal evolutionary processes, in which only a limited number of species in an ecosystem are affected at any one time (The American Heritage Science Dictionary, 2005a)
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By contrast, a mass extinction, also known as an extinction-level event (ELE), is characterized by a sharp decrease in the diversity and abundance of macroscopic life within a relatively brief geologic timespan. Mass extinctions are generally thought to be caused by a catastrophic global event or widespread environmental change that occurs too rapidly for most species to adapt (The American Heritage Science Dictionary, 2005b). These periodic breaks in the line of species represent marked deviations from the background extinction rate. They are the very breaks that led nineteenth century science to define the great geologic eras (Courtillot, 1999).
The current geologic era is known as the Holocene. Beginning at the end of the last Ice Age approximately 11,000 year ago, it is the younger of the two epochs of the Quaternary Period and is characterized by the development of human civilizations. Since the first anatomically modern Homo sapiens emerged in Africa about 200,000 year ago, humans have come to dominate the planet and, in the past three centuries, their effect on the global environment has escalated. In the words of Steffen, et al. (2005), "Human changes to the Earth System are multiple, complex, interacting, often exponential in rate and globally significant in magnitude. They affect every Earth System component - land, coastal zone, atmosphere and oceans . . . The magnitude, spatial scale, and pace of human-induced change are unprecedented" (p. 81).
Steffen et al. (2005) and a growing body of scientists believe that human impact on the global biosphere changed dramatically with the Industrial Revolution in the late eighteenth century, and that these human-driven changes-in terms of element cycles and climatic parameters-are pushing the Earth System beyond its normal operating range. The advent of fossil-fuel-based energy systems radically increased society's capacity to produce, distribute, and consume goods and services at a rate unmatched in pre-industrial human history. Nobel Prize winning atmospheric chemist, Paul Crutzen, coined the term "Anthropocene" to emphasize the extent to which human activities have come to dominate Earth's systems. (Geist, 2006). In an article in Nature, Crutzen contends that Anthropocene is a more apt descriptor for the present, human-dominated geologic epoch and he offers it as a supplement, if not a potential replacement, to Holocene. He argues that the Anthropocene could be said to have started in the late eighteenth century when, according to scientific analysis of air trapped in polar ice core samples, the atmosphere showed the beginning of growing global concentrations of carbon dioxide and methane. Crutzen points out that this date happens to coincide with James Watt's design of the steam engine in 1784, when the Industrial Revolution was already in full swing (Crutzen, 2002)
The history of species extinction provides a valuable ecological basis for understanding the present and forecasting the future. This is an area of much debate-one to which some of the world's brightest geologists, paleontologists, and evolutionary biologists have dedicated volumes of material. It is beyond the scope of this paper to rigorously scrutinize all of the various extinction theories and speculate as to their plausibility. Instead, the "Big Five" and their possible causations will provide a historical and scientific background for an analysis of the current geologic epoch and the assertion by some scientists that the next major extinction-level event-Earth's Sixth Mass Extinction-is already underway and that human activity is the driving force. Undoubtedly, humans are making a profound impact upon the Earth's interconnected bio-geo-chemical systems, leading some to speculate that the Holocene has already given way to the Anthropocene. This paper presents arguments for a new, human-induced geologic epoch and examines the possibility of a related anthropogenic mass extinction event and its impact on the future of humanity.
A Brief History of Mass Extinction
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In the late eighteenth century French anatomist, Baron Georges Cuvier, through extensive study of fossil deposits in the Paris Basin, identified what he thought were times of crisis in Earth's history. Cuvier's theory came to be known as catastrophism, in which he postulated that individual catastrophic events were responsible for wiping out a large number of species in a relatively short period of time, thereby setting the stage for new waves of creation. While Cuvier had effectively established the reality of mass extinction, catastrophism was still widely disputed in his time. Charles Lyell, Scottish geologists and author of the three-volume Principles of Geology, denied the existence of mass extinctions, arguing that scientists need not invoke "extraordinary agents" to explain large changes in the geological past. Known as uniformitarianism, his geological theory banished catastrophism from intellectual circles as an outdated concept that relied unscientifically on supernatural agents to explain sudden waves of extirpation and creation.
Charles Lyell was a close and influential friend of English naturalist, Charles Darwin, who had read Lyell's work while on a five-year survey expedition on the HMS Beagle. Uniformitarianism fit nicely with Darwin's gradualist theory of speciation, in which he argued that small changes accumulate over long periods of time, resulting in major evolutionary impacts. Darwin called this theory natural selection and-grounded in uniformitarianism-it too rejected Cuvier's catastrophe theory.
Despite staunch opposition and outright defeat in academia, catastrophism became more compelling as geologists and paleontologists continued to unveil historical remnants of mass expiration, filling in conspicuous gaps in the fossil record. Before long, catastrophism was vindicated and the theory of mass extinction took hold. While the gradualist progression of natural selection still had a role to play in speciation, it could not accurately account for sudden surges of biological diversity, such as the rapid appearance of most major phyla around 530 million years (Ma: mega-annum) ago in the Cambrian Period. Earth's history, as it turned out, was peppered with sporadic and cataclysmic upheavals of extermination and regeneration. Many of these events were relatively moderate in scale, in which 15 to 40 percent of marine species disappeared. Others were much larger biotic calamities that wiped out a significant portion of life in brief geologic instants. The most prominent of the latter, large-scale extinction events are known collectively as the Big Five (Leakey HYPERLINK "#_ENREF_11"&HYPERLINK "#_ENREF_11" Lewin, 1995)
The history of research in the area of mass extinction is long and complex, yet the problem of mass extinctions-that is, the search for adequate causal explanations for the disappearance of large numbers of fossil groups within a short geologic time span at specific stratigraphic intervals-has only within the last several decades become a focal topic in earth and life sciences, having experienced a rejuvenation in light of new empirical developments in the 1980s (Hoffman, 1989). Prior to the 1980s, mass extinctions were regarded as rather enigmatic events that happened occasionally in the geologic past and were difficult to interpret. Researchers were not explicitly focused on extinction level events and thus the literature was limited. This changed with the publication of two highly influential papers (S. K. Donovan, 1989) suggesting that such events may be produced by bolide impacts or other extraterrestrial causes (Alvarez, Alvarez, Asaro, HYPERLINK "#_ENREF_1"&HYPERLINK "#_ENREF_1" Michel, 1980), and that they could have occurred periodically (D M Raup HYPERLINK "#_ENREF_15"&HYPERLINK "#_ENREF_15" Sepkoski, 1984). In the wake of these new theories, there was a proliferation of research and literature as paleontologists and geo-chemists joined forces to search for the betraying signatures of extraterrestrial impacts, such as anomalous concentrations of platinum-group elements (e.g. iridium) at extinction boundaries (S. K. Donovan, 1989).
Identifying Mass Extinctions
Extinction is measured by two distinct, but intimately linked metrics: rate and magnitude. Rate refers to the number of extinctions divided by the time over which they occurred. Magnitude is the percentage of species that have gone extinct (Sengor, Atayman, HYPERLINK "#_ENREF_16"&HYPERLINK "#_ENREF_16" Ozeren, 2008). In terms of magnitude, Paleontologists characterize mass extinctions as times when the Earth loses more than 75% of its species in a short geologic interval, typically less than 2 million years.
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When analyzing mass extinctions to determine the causative processes, two important questions arise: (a) Are mass extinctions merely an extreme part of a continuum that includes background extinctions, or are they driven by separate and distinct environmental forces? (b) Are all mass extinctions driven by the same underlying mechanism or are their different causes for different events (Stephen K. Donovan, 2000)?
In their analysis of fossil data on invertebrate and vertebrate families in the marine realm, Raup and Sepkoski (1982) identified five times of particularly high extinction intensity when compared to background levels. These events occurred late in the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods. Newer data on marine genera supports the existence of these Big Five mass extinctions. However, the question as to whether mass extinctions are truly different from background extinctions has yet to be fully settled.
Some researchers support the notion of continuity in extinction magnitude, with mass extinctions simply representing higher concentrations of background levels. While continuity of intensity seems to be evident from the available data, there remains a lack of continuity of cause and effect, which may validate the separation of mass extinctions from background extinctions (Taylor, 2004). Although there is a continuum in magnitude of diversity loss between the smallest and largest biotic crisis, most paleontologists still refer to the largest five Phanerozoic events as mass extinctions. Nevertheless, many aspects of these events remain highly debatable and there is no common cause or single set of climatic or environmental variables common to all five events, although all are associated with evidence for climate change (Twitchett, 2006).
The Big Five
The most famous of the mass extinction events is the Cretaceous-Tertiary (K-T) event, which is best known for wiping out the dinosaurs approximately 65 Ma ago. In terms of sheer magnitude, the biggest event of the fossil record occurred at the end of the Permian, approximately 250 Ma ago, in which more than 90 percent of all plants and animals went extinct. The remaining three events making up the Big Five occurred at the End of the Ordovician, in the Late Devonian, and at the end of the Triassic. None of these were comparable in magnitude to the K-T and Permian events but all recorded severe biotic crises (Wignall, 2004). Table 1 plots the major mass extinction events relative to their geologic timeframe.
Table : Major Extinction Events
Adapted from (Leakey HYPERLINK "#_ENREF_11"&HYPERLINK "#_ENREF_11" Lewin, 1995; David M. Raup, 1991)
(millions of years before present)
Major Extinction Event
Present - 0.01
ß 6th Major Extinction?
0.01 - 1.6
1.6 - 5.6
5.3 - 24
24 - 37
37 - 58
58 - 65
65 - 144
ß End of Cretaceous: K-T Boundary
144 - 208
208 - 245
ß End of Triassic
245 - 286
ß End of Permian
286 - 325
325 - 360
360 - 408
ß Late Devonian
408 - 440
440 - 404
ß End of Ordovician
505 - 570
570 - 4500
There has been no shortage of proposed causal agents for mass extinctions, some of the more unlikely theories being showers of inner Oort Cloud comets and the lethal effects of nearby exploding supernovae. Theories that have been taken more seriously include climate change (especially cooling and drying), regression and transgression of sea levels, predation, epidemic disease, competition among species, and bolide impacts. Demonstrating the cause of a mass extinction event generally requires finding both a proximate cause and an ultimate kill mechanism. This approach is clearly demonstrated in the work of Alvarez, et al. (1980) where they proposed the ultimate cause of the K-T extinction to be a meteorite impact and the more immediate proximate cause to be a global dust cloud that shut down photosynthetic activity. These two theories are closely linked-the latter being a likely consequence of the former-but they are still separate causative agents. Other extinction events show less conclusive linkages. For example, marine anoxia has been strongly implicated as the proximate cause for the End Permian extinction in the seas, but the link with its ultimate cause, often thought to be volcanic activity, has only been weakly established. The point here is that it is extremely difficult to prove what caused an event that happened millions of years ago. It is important to remember that cause and effect can only be postulated and correlation does not always equate to causation (Wignall, 2004).
Mass extinction is a complex process and likely the end result of a confluence of detrimental forces, rather than the explicit consequence of a single causal factor. There is a solid body of evidence to suggest that the ultimate cause of the K-T extinction was a giant meteorite impact-one of the most compelling indicators being anomalously high levels of iridium found throughout the world in the thin layers of sediment at the K-T boundary. Concentrated levels of this transition metal are consistent with an influx of extraterrestrial material, such as meteorites, which contain a great deal of iridium compared to the vanishingly small amounts found in terrestrial rocks. Joining iridium are other impact indicators such as shock-metamorphosed minerals (quartz), stishovite (a high pressure variant of quartz), tektites (glassy melt rock thrown out from the impact), and various lines of geochemical evidence. The giant impact greater at Chicxulub is the 'smoking gun' that provides near irrefutable evidence of a major impact event (Wignall, 2004). The fallout from a bolide collision might include earthquakes, tsunamis, widespread fires, acid rain, and either global warming (from heat trapped in the debris-laden atmosphere) or global cooling (due to atmospheric particulates blocking out sunlight) (David M. Raup, 1991). The evidence for many of these proximate effects can be found in the stratigraphic record.
The causes of other major extinction events are not as clear. Of the remaining five, meteorite impacts have been suggested as a possible cause for three others (the Late Devonian, the End Permian, and the End Triassic), but in each case the evidence is tenuous. Table 2 below lists possible causes proposed for each of the Big Five extinction events.
Table : Possible Causes of Mass Extinctions
Adapted from (Barnosky et al., 2011; Stephen K. Donovan, 2000; Eldredge, 2001)
Onset of alternating glacial and interglacial episodes - growth and decay of the Gondwanan ice sheet following a sustained period of environmental stability associated with high sea level
Repeated marine transgressions and regressions.
Uplift and weathering of the Appalachians affecting atmospheric and ocean chemistry
Sequestration of CO2
Climate change (relatively severe and sudden global cooling)
Global cooling (followed by global warming), possibly tied to the diversification of land plants, with associated weathering, paedogenesis, and the drawdown of global CO2
Evidence for widespread deep-water anoxia and the spread of anoxic waters by transgressions.
Timing and importance of bolide impacts still debated
Complex amalgams of climate change (global warming) possibly rooted in plate tectonics
Spread of deep marine anoxic waters.
Elevated H2S and CO2 concentrations in both marine and terrestrial realms.
Gradual reduction in diversity produced by a sustained period of refrigeration associated with widespread regression and reduction in area of warm, shallow seas
Evidence for a bolide impact still debated.
Activity in the Central Atlantic Magmatic Province (CAMP) thought to have elevated atmospheric CO2 levels, which increased global temperatures and led to a calcification crisis in the world oceans
Increased rainfall with implied regression
A bolide impact in the Yucatan is thought to have led to a global cataclysm and caused rapid cooling.
Preceding the impact, biota may have been declining owing to a variety of causes: Deccan volcanism contemporaneous with global warming; tectonic uplift altering biogeography and accelerating erosion, potentially contributing to ocean eutrophication and anoxic episodes. CO2 spike just before extinction, drop during extinction.
Many scientists believe that the other mass extinctions, particularly the great marine extinctions of the Paleozoic era, were more likely caused by climatic or other environmental changes than by a catastrophic event such as a meteorite impact. Changes in global climate and sea level are by far the most popular non-impact causal agents, along with changes in salinity in ocean water and the depletion of oxygen (anoxia) in shallow, marine environments (David M. Raup, 1991).
In the book, Extinction: Bad Genes or Bad Luck?, author David Raup notes that extinction is a difficult research topic because no critical experiments can be performed and inferences are often influenced by preconceptions based on general theories. However, after extensive observation of fossils and living organisms, Raup proposes with confidence what he holds to be six fundamental principles relative to extinction:
Species are temporary. No species of complex life has existed for more than a small fraction of the history of life.
Species with very small populations are easy to kill. If a species has fewer individuals than its minimum viable population (MVP), extinction in a short time is probable, if not assured.
Widespread species are hard to kill. Species extinction can only be accomplished by the elimination of all breeding populations. The agent of extinction (physical or biological) must be active over the whole range. In other words, the killing condition must exist everywhere the species lives.
The extinction of widespread species is favored by a first strike. The extinction resilience of widespread species can be negated if extreme stress is applied suddenly over a large area.
The extinction of widespread species is favored by stresses not normally experienced by the species. Most plants and animals have adapted to survive and thrive in their respective environment, but a stress that has never been experienced by a species can cause extinction.
The simultaneous extinction of many species requires stresses that cut across ecological lines. Large scale extinctions are not limited to a single species, ecosystem, or habitat.
Raup answers the question posed in the title of his book by positing that most species go extinct because of bad luck. They die out because they are exposed to biological or physical stresses to which they have never been exposed and because they lack sufficient time for natural selection to help them adapt to novel environmental pressures. He concludes with a brief note on modern day extinctions in which he poses another question: If it's true that widespread species are hard to kill, as he suggest in point number 3 above, then why should we be concerned about things like habitat destruction and overhunting? He answers with reference to points 4 and 5. Human activities, he contends, provide the first strike necessary to reduce species' ranges so that extinction from other causes is likely. Moreover, humans are producing first strikes regularly-far more frequently than those supplied by nature at intervals of millions of years. Species are unable to adapt to these new, anthropogenic stresses that, on an evolutionary timescale, occur far too suddenly for traditional Darwinian survival mechanisms to be effective (David M. Raup, 1991). The following sections will take a closer look at human activity as a causal agent of extinction, and the possibility of a sixth mass extinction in what has been dubbed the human-dominated epoch of the Anthropocene.
The Anthropocene Epoch
The term "Anthropocene" is not the first of its kind to suggest that humankind is significantly altering the world. For more than a century, terms such as Psychozoic, Anthropozoic, and Noosphere have been proposed to denote the extent to which humans have become global forcing agents. Historically, terminology of this sort has failed to take root, falling by the wayside shortly after being introduced into the geologic lexicon. However, the term "Anthropocene," coined over a decade ago by Paul Crutzen, has yet to suffer the same fate. This may be attributed to the fact that by the turn of the millennium it was becoming increasingly clear to scientists that human activity was on par with the major disruptive events of the ancient past and was positioned to permanently alter the global ecosphere on a geologic time scale. Although informal and not officially defined, Anthropocene has been adopted by many practicing scientists to designate a new, human-dominated geologic interval (Zalasiewicz, Williams, Steffen, HYPERLINK "#_ENREF_24"&HYPERLINK "#_ENREF_24" Crutzen, 2010). In 2008 the Stratigraphy Commission of the Geological Society of London decided, by a large majority, that there was merit in considering the formalization of this term, meaning that it could potentially join the Cambrian, Jurassic, Pleistocene, and other such units on the Geological Time Scale (Zalasiewicz et al., 2008). The Geological Time Scale is fundamental to the work of geologists and is not easily amended. Nevertheless, formal steps are currently underway to evaluate the case for inaugurating the Anthropocene as a new era of geologic time. An Anthropocene Working Group has been initiated as part of the Subcommission on Quaternary Stratigraphy, itself part of the International Commission on Stratigraphy, which answers to the International Union of Geological Sciences. All of these groups will have to be convinced that there is overwhelming scientific evidence to support a transition from Holocene to Anthropocene (Zalasiewicz, et al., 2010).
Evidence and Indicators for a Sixth Mass Extinction
There is a growing consensus within the scientific community that humanity is either entering or in the midst of a sixth mass extinction event is currently underway and that human activity is largely to blame. Jeremy Jackson, Director of the Center for Marine Biodiversity and Conservation and William E. and Mary B. Ritter Professor of Oceanography at the Scripps Institution of Oceanography and a senior scientist at the Smithsonian Tropical Research Institute, is one of many scientists who are becoming increasingly alarmed by the rate at which humans are altering the environment. Jackson is well known for his ground breaking research documenting the historical consequences of humankind's exploitation of ocean resources. In 2008 he wrote:
The great mass extinctions of the fossil record were a major creative force that provided entirely new kinds of opportunities for the subsequent explosive evolution and diversification of surviving clades. Today, the synergistic effects of human impacts are laying the groundwork for a comparably great Anthropocene mass extinction in the oceans with unknown ecological and evolutionary consequences (Jackson, 2008, p. 11458).
The synergistic effects to which Jackson is referring are habitat destruction, overfishing, introduced species, warming, acidification, toxins, and massive runoff of nutrients, which are transforming once complex and productive ecosystems like coral reefs and kelp forests into anoxic dead zones.