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Extinction and the Future of Biodiversity
Monthly Feature, July, 2017.
By Frederick S. Rogers
Extinction and the Future of Biodiversity
Monthly Feature, July, 2017.
By Frederick S. Rogers

What is Extinction?
To those of us living in the modern world, especially those of us concerned with the many pressing issues of environmental concern, extinction is an unfortunately all-to-familiar word, the meaning of which is clearly understood: it is the complete disappearance of a taxon (for example, a species) of organism. Perhaps surprisingly, however, the concept of extinction is a very recent one in human history. It was the great French stratigrapher, paleontologist, and comparative anatomist Georges Cuvier, who, around the beginning of the 19th century, demonstrated to the scientific community and to the world at large the reality of extinction. We now know that the vast majority (>90%) of the organisms that have lived on Earth are forever gone - extinct!

Today, we differentiate between background extinction and mass extinction. Background extinction refers to the normal course of events, the normal rate of 0.1 to 1.0 species per million species per year suffering extinction. Indeed, it is the normal course of evolution that species originate (evolve from ancestral species), persist for a time (about 12 million years, or so, for invertebrate species; about 2 million years, or so, for vertebrate species), then become extinct. This fact that species have a point of origination, and a subsequent point of extinction, gives them their time value in reading the geologic record: the physical stratigraphic range of a species in the rock record also delineates a finite span of time from the evolutionary origin of a species to its demise, or extinction. This, in turn, allows us to tell time in the rock record, and to correlate, in a time sense, rock units in even widely separated localities. This relative time scale based on fossils (established by the middle of the 19th century) coupled with the absolute time scale based on radiometric dating of igneous rocks - such as ancient lava flows interbedded with fossiliferous sedimentary rocks - (established by the middle of the 20th century) gives us our modern geologic time scale.

            Diversity of life over 600 million yrs.
            with mass extinctions indicated.
On the other hand, mass extinction refers to those periods of time when the rate of extinction is accelerated, dramatically so, perhaps as much as 1000 to 10,000 times the background extinction rate. It is these mass extinction events that are of particular interest to us. As is dramatically apparent in the rock record, mass extinction events literally reset the evolutionary clock. Across a stratigraphic boundary marking a mass extinction event, the post-extinction biota of the planet is markedly different from the pre-extinction biota. Whether in the marine realm or in the terrestrial realm, once-dominant groups of organisms are drastically reduced in diversity, or become extinct altogether, and once-subsidiary groups of organisms now become the dominant organisms.
(Click here to hide this section about mass extinctions)

A Precambrian Mass Extinction Event?
In terms of intervals of time, the geologic time scale is subdivided into eons (from oldest to youngest, Hadean, Archean, Proterozoic, and Phanerozoic), the eons into eras (for example, the Paleozoic, Mesozoic, and Cenozoic Eras of the Phanerozoic Eon), the eras into periods (for example, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods of the Paleozoic Era), and the periods, in turn, are subdivided into epochs and on down into biostratigraphic zones. In terms of absolute time, the Hadean Eon extends from 4.5 to 3.9 billion years ago, the Archean from 3.9 to 2.5 billion years ago, the Proterozoic from 2.5 to about 0.5 billion years ago, and the Phanerozoic from 542 million years ago to present. There is no record of life on Earth from the Hadean Eon, and the fossil record of the Archean and Proterozoic Eons, at least until the Ediacaran Period, the final period of the Proterozoic Eon, largely is microbial. It is only the Phanerozoic Eon that has the abundant and familiar fossil record of multi-cellular plant and animal life – and so it is “only” in this last half-billion years, this last 11% of Earth history, that we can read the history of life with a high degree of resolution. In those earlier eons – Hadean, Archean, and Proterozoic, collectively known as the Precambrian – the history of life is more difficult to discern. Nevertheless, we can make at least one informed speculation about a Precambrian mass extinction event. During the Early Proterozoic interval of time, some 2.3 billion years ago, at the time of the “Great Oxygenation Event,” when the Earth’s oceans and atmosphere began to be infused with molecular oxygen, it is likely that a major mass extinction event severely hitting the anaerobic microbes of the time occurred. However, given the relative lack of resolution in Precambrian rocks, this is about all we presently can say about Precambrian mass extinction events.

The Phanerozoic Mass Extinction Events
The excellent fossil record of the Phanerozoic Eon allows excellent time resolution of important biological trends and events. We now know that the species-level biodiversity of Earth has increased dramatically over the 542 million year course of the Phanerozoic Eon to its present highest level in Earth history, but this record of increasing diversity was severely punctuated along the way by five major mass extinction events, known today as “The Big Five.” In chronological succession, these were the Late Ordovician Event, ending 443 million years ago; the Late Devonian Event, ending 359 million years ago; the Late Permian Event, ending 251 million years ago; the Late Triassic Event, ending 200 million years ago; and the Late Cretaceous Event, ending 65 million years ago. With the exception of the Late Ordovician Event, largely affecting only the marine realm because life on land was just becoming established at that time, each of the other four events affected both the marine realm and terrestrial realm. In each case, over the course of the duration of a few million years, at least 75% of the known species of the time were lost - 95% in the case of “The Mother of All Extinctions,” the Late Permian Event – and the recovery of the Earth’s previous species-level biodiversity took several million years to several tens of millions of years before exceeding the pre-extinction level of biodiversity. In addition, the very trajectory of evolution was dramatically altered: think about how the extinction of the dinosaurs paved the way for the great adaptive radiation, the great evolutionary expansion, of our own taxonomic order, the mammals – it is entirely possible that we would not be here today if not for the demise of the dinosaurs!

The obvious question about each of these five major mass extinction events is, “What caused it?” Over the last 100 years, or so, there has been much speculation as to causes – many hypotheses, ranging from the outlandish to the reasonable, have been put forward – for each of these events. However, just in the last decade or so, evidence has grown for a common “suspect” for four of the five events. With the exception, again, of the Late Ordovician Event, caused by the severe climatic and sea level fluctuations that occurred throughout the Late Ordovician Ice Age, the other four events are associated with the activities of Large Igneous Provinces (LIPs), themselves associated with the long period of assembly and ultimate break-up of the supercontinent known as Pangaea. LIPs are mega-eruptions of flood basalts, vast outpourings of very hot, fluid, iron-rich (mafic, in geological parlance) lava along with equally staggering outpourings of toxic halogen, sulfur dioxide, and carbon dioxide gases. The outpourings of these gases were such that the chemistry and temperature of the oceans and the atmosphere were, geologically speaking, rapidly and profoundly altered. (At the end of the Cretaceous Period, the impact of the asteroid may have been “merely” the coup-de-grace for the dinosaurs!) The result, in each case, was the death of at least 75% of the Earth’s species!

Up to this point, I have written about “The Big Five” mass extinction events, all of them occurring long geologic ages before Homo sapiens was on the planet, and, therefore, all of them entirely natural as to their causes. But what about that other, geologically much more recent, and indeed on-going extinction event, the one that often is in the news, increasingly so in recent years?

The Pleistocene (“Ice Age”) and Holocene (“Recent”) Extinctions
The period of time from 2.6 million years ago to 11,700 years ago is known as the Pleistocene, or, more popularly, the “Ice Age.” The last 11,700 years up to the present is known as the Holocene, or “Recent.” The period of time from the beginning of the recession of the last continental ice sheets to the present, that is, the last 20,000 years (Late Pleistocene through Holocene), has been notable for its extinctions, most famously the extinctions of the mammoths, mastodons, wooly rhinoceroses, long-horn bison, giant ground sloths, cave bears, saber-toothed cats, dire wolves, etc. – the list goes on. Collectively, these spectacular, but now extinct, animals dominated the ecosystems of the northern hemisphere continents during the Ice Age, and are known as the “Pleistocene Megafauna.” However, the extinctions did not stop, about 11,000 years ago, with the demise of the megafauna, but rather have continued into the present day. The totality of these individual extinction events is popularly referred to as the “Sixth Extinction,” alluding to the “The Big Five,” and suggesting that our present time is the time of the Phanerozoic Eon’s sixth major mass extinction event. How does this on-going extinction event compare to its predecessors?

Although it is tempting to attribute these extinctions to the dramatic shifts in climate and floral distributions of the last 20,000 years – in other words, to attribute these extinctions to natural causes – natural causes are only part of the story. The other part of the story is entirely related to human activities – the impacts caused by hunting pressure, reduction of habitat area, appropriation of resources and energy, pollution, and climate change, beginning with the rise of agriculture about 11,000 years ago, and accelerating in the 200 years since the start of the Industrial Revolution. In short, this ongoing extinction event is not like “The Big Five” extinction events of the geologic past, for two reasons. The first reason is that, although it is a popular perception that we are in the midst of the sixth major mass extinction event of the Phanerozoic Eon, and although we do seem to be rapidly heading toward that horrific “goal,” we really are not quite there yet. Although there are many alarming estimates of present rates of species extinction, and these estimates probably are largely accurate for oceanic islands – the bellwethers of the biodiversity crisis – we really do not have direct empirical evidence that current global rates of extinction are yet at the rates estimated for “The Big Five.” This in no way means that there is no serious issue before us – emphatically, there is a serious issue before us. The second reason is that this largely is a human-induced extinction event, and, therefore, is an extinction event whose effects, with appropriate will and action on the part of humankind, can be ameliorated.


The Condition of Modern Ecosystems
Fred Rogers on the Block Island
ferry contemplating the current
state of coastal ecosystems.
So what is the actual state of affairs that we now find ourselves in? I think our current situation is well – and alarmingly – illustrated in what are two important publications in the areas of ecology and conservation biology, Jackson et al. (2001) and Yeakel and Dunne (2015). The research presented, and the conclusions reached, in each of these studies is based on excellent paleontological, archeological, and historical data sets. Jackson and colleagues focus on estuarine, coastal, and continental shelf marine ecosystems around the globe, while Yeakel and Dunne review a variety of marine and terrestrial ecosystems spanning the Phanerozoic Eon, from the half billion-year-old, Middle Cambrian-age Burgess Shale to the modern Egyptian terrestrial ecosystem and the modern Adriatic Sea marine ecosystem. In each study, the authors make a convincing case that modern marine and terrestrial ecosystems, by and large, and with few exceptions, are mere shadows of their former selves. What is alarming about our modern ecosystems is not so much the extinction of species per se – although that certainly already has occurred in some instances – but that for most of the species in these ecosystems the number of individuals typically is very low in comparison to the often fantastically large number of individuals prior to the modern era, especially prior to the Industrial Revolution. These small populations matter because they make a species susceptible to extinction, and, when a species is critical to maintaining the structure and function of the ecosystem (more likely for species high in the food web), its extinction can cause the ecosystem as a whole to collapse. This appears to have happened in a number of marine ecosystems studied by Jackson and colleagues and in the terrestrial ecosystems of Egypt studied by Yeakel and Dunne. It is my opinion that a thorough review of the technical ecology and conservation biology literature will reveal dozens of similar, depressing examples. We may not yet be at the level of a “Sixth Extinction,” but because we are “insulting” and overexploiting our already reduced and fragile ecosystems, we may be nearing a tipping point for many of them, and thus may be poised to fall in that direction very soon. To directly quote Yeakel and Dunne (2015, p. 195), “From a perspective of self-preservation, biological diversity is the raw material from which agriculture, medicine, and all natural resources – including the air we breathe – derive! Maintaining intact communities of interacting species harbors that diversity, and is thus a requirement for the survival of human societies.”

Koyaanisqatsi: Life Out of Balance
Some readers of this essay, those of my “vintage,” may remember the visionary environmental film “Koyaanisqatsi,” the title being the Hopi word for “life out of balance,” a title perhaps now even more appropriate than it was then, given the acceleration we now are experiencing with human population growth, habitat loss, hunting and fishing pressure, climate change, ecosystem disruption, and outright extinction. As mentioned above, we may not yet be in the midst of the “Sixth Extinction,” but we certainly have in place all the prerequisites for falling into it in the near future! The question of how to proceed in the face of this comes to mind.

In my lifetime, I have actually heard some people express the opinion that we should not be worrying about extinction because, as “The Big Five” mass extinction events show us, extinction is an entirely natural process – even if 95% of all species go extinct, as in the end Permian event, life will go on, and it will rebuild back to the level of biodiversity that was there before the mass extinction event, and then go on to exceed that previous level of biodiversity. In the meantime, so goes this argument, humanity will find ingenious solutions to living on the post-mass extinction Earth. To this argument I would say that no, the present extinctions are occurring against a radically different background than those earlier events occurred against. First of all, our current situation is not “natural,” it is the result of the activities of human beings, and so we do have the power to change the direction of events. Second, if we do not change that direction, we may end up having altered the Earth’s surface to such a degree that life cannot rebound to its previous level of biodiversity even in the usual several tens of millions of years – for all practical purposes, extinction really is forever! Third, in the meantime, an Earth largely populated by plants we normally consider to be “weeds” and resilient animals like cockroaches and rats, along with the loss of the services that intact ecosystems provide, may not be as pleasant an Earth to live on as the people who make this argument think it will be!

I have also heard people express the opinion that we now have the possibility, in the near future, of escaping the confines of a dying Earth, if necessary. We will terraform and colonize Mars and other bodies in our Solar System, even travel on to planets orbiting other stars. In short, we will be able to escape whatever ecological catastrophe that may befall us by moving on just as earlier, less technologically-advanced civilizations here on Earth – civilizations that overshot their natural resource bases – moved on to new lands, even to new continents. So to this argument I also would say no – no, I do not believe that terraforming Mars and other bodies in our Solar System will ever meaningfully replicate Earth, and no, for some very good reasons, we cannot just move out into the galaxy, either.
(Click here to hide section this section about extra-solar planets)

To elaborate on that last point, the Kepler telescope, at latest count, has revealed about 3000 confirmed extra-solar planets, but, by far, most of these planets are not Earth-like, and the few that may truly be Earth-like will not meaningfully replicate Earth, either. While I certainly expect that there is life, even complex, intelligent life, on other, Earth-like planets orbiting other stars, I personally am sympathetic to the thought behind Enrico Fermi’s famous question “Where is everybody?” with regard to the frequency of alien visitations. I also am sympathetic to Ward and Brownlee’s (2000) “Rare Earth Hypothesis” predicting that complex life – especially intelligent life – is rare in the galaxy and in the universe as a whole. Furthermore, let us for a moment imagine that one of the Kepler telescope’s current “objects of interest,” the star KIC8462852, also known as “Tabby’s Star,” or “Boyajian’s Star,” lying 1280 light years distant in the constellation of Cygnus really does have a truly Earth-like planet orbiting it. The star is of interest because of the unusual nature of the pattern of brightening and dimming of its light as seen and recorded by Kepler. Remarkably enough, one of the hypotheses seeking to explain the star’s unusual light curve, one of the few hypotheses about it that no one can yet dismiss, is that the star is being eclipsed by an orbiting “Alien Megastructure” of the kind posited by the physicist Freeman Dyson – the star is being orbited by a “Dyson Swarm,” a megastructure designed to steal energy from the star in order to power the civilization that built it! While I believe that this rather extraordinary hypothesis ultimately will be rejected, my point is that 1280 light years probably represents something of a minimum distance in terms of the distances out to stars with Earth-like, habitable planets orbiting them. Even at the speed of light, which we can never reach, potential colonizers would be traveling for over a millennium – a very long, multi-generational, lonely journey to an Earth-like, yet, nevertheless, very alien world! (Yes, I know that because of the relativistic effect of time dilation at near-light speeds, the journey would be much shorter for those intrepid colonists, but still ultimately would be a lonely journey!) No, neither the journey to distant planets nor those planets themselves will be meaningful substitutes for our Earth!

Because I have a daughter of a certain age, over the last decade, I have become very familiar with many of the Pixar and similar cartoons, and I have noticed something interesting about most of these animations. It has struck me that the land use typically depicted in these cartoons is very different from our actual practices, more like land use from an earlier time in human history. In these cartoons, people (or ogres, animals, monsters, cars, etc.!) typically live in dense compact settlements, whether small villages or major cities, settlements that are not surrounded by miles of urban sprawl and suburbs. Beyond the sharp boundaries of these urban areas are rolling farmlands, and beyond these agricultural areas are truly wild areas abounding with plants and animals. Does this perhaps indicate, on the parts of the writers and animators of these cartoons, and of the many fans of these cartoons, both children and adults, an underlying longing for a different human aesthetic with regard to the land, and a different human relationship to nature, something that no longer is “koyaanisqatsi”? I am beginning to think that just might be the case!


Hope for 2100
We have this one Earth, and we all will be on this one Earth together for some time into the foreseeable future, and our children and grandchildren will all be on it together after we are gone. We owe it to ourselves and to those future generations to change this current course that we are on, and to become stewards of the Earth. Earlier essays in this series have illustrated humanity’s deep spiritual connections to nature, the philosophical and ethical foundations for stewardship of nature, and examples of, and reasons for, hope for the future. There really are many individual instances of species averting extinction that give hope for the future, the comeback of bald eagles, peregrine falcons, and, more recently, nenes (Hawaiian geese), come to mind – but now, more than ever, we need to build on these and other individual conservation successes with comprehensive, ecosystem-scale initiatives, indeed, global-scale initiatives, in order to permanently avert a human-caused “Sixth Extinction,” to rebuild our fragile ecosystems before they reach that tipping point and their species slide into extinction. Even if ultimately unrealistic, we need to strive to realize E. O. Wilson’s vision of saving 50% of the Earth entirely for nature. Planning for both the rural and urban environments, at the local, regional, national, and international levels, should be “biodiversity-based planning,” always with the goal of maximizing habitat for nature, maximizing species connectivity, the coherence of ecosystems, and ecosystem services. We need a new “Green Revolution,” as it were, not only in agriculture, but in economics, industry, transportation, housing, recreation, and energy that not only protects the biodiversity of the Earth, but that also meaningfully employs the Earth’s human population in ways that help support that biodiversity!

I will close with a verse from a song by a seminal rock band of the 1960s, a verse written before that first Earth Day on April 22, 1970, but still as current as it was then:

Falling into your passing hands
Please don’t destroy these lands
Don’t make them desert sands

- The Yardbirds, "Shapes of Things"               

Image Frederick Rogers is a Professor of Geology and Environmental Science in the Division of Natural Sciences at Franklin Pierce University in Rindge, New Hampshire, USA. He is a paleontologist specializing in micropaleontology and biostratigraphy, and with a strong interest in the history and diversity of life and in the effects of mass extinctions on the trajectory of the evolution of life.


Jackson, J. B. C., M. X. Kirby, W. H. Berger, K. A. Bjorndal, L. W. Botsford, B. J. Bourque, R. H. Bradbury, R. Cooke, J. Erlandson, J. A. Estes, T. P. Hughes, S. Kidwell, C. B. Lange, H. S. Lenihan, J. M. Pandolfi, C. H. Peterson, R. S. Steneck, M. J. Tegner, and R. R. Warner. (2001) Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science, volume 293, issue 5530, pages 629 – 637.
Ward, P. D., and D. Brownlee. (2000) Rare Earth: Why Complex Life Is Uncommon in the Universe. Copernicus Springer–Verlag, New York, 333 pages.
Yeakel, J. D., and J. A. Dunne. (2015) Modern Lessons from Ancient Food Webs. American Scientist, volume 103, number 3, pages 188 – 195.