Why should we care about extinctions?

Excerpt from Theoretical Ecology: Principles and Applications, by Lord Robert May with Angela McLean (Oxford University Press, 2007). Reprinted with permission from the authors.

Chapter 15: Unanswered questions and why they matter

The earlier chapters in this book could be thought of as travel notes from an intellectual journey across the landscape of ecological science. In particular, the previous five chapters, 10–14, implicitly or explicitly indicate some of the unintended consequences of the growth in numbers of people and in their environmental impacts. In this final chapter, I begin with a survey of some quantitative measures of the scale of human impacts. Emphasizing the many lamentable uncertainties in our knowledge base, I focus especially on the rising rates of extinction of plant and other animal species.

Why should we care about such impoverishment of our planet’s biological diversity? I outline three kinds of possible reasons, under the headings of narrowly utilitarian, broadly utilitarian, and ethical. Each of these is then discussed, with emphasis on ways in which current lack of knowledge—lack of data and/or lack of theoretical understanding—is a handicap. In places, this carries the discussion into areas not commonly found in ecology texts (ethical, economic, and political questions, for instance). In other places, there is the more familiar exhortation for more research on this or that topic.

The growth of human populations

Contrary to some impressions, human population growth has been far from simply exponential. Broadly speaking, humans have been around for a couple of hundred thousand years (Deevey, 1960; Cohen, 1995). For essentially all this time, they were small bands of hunter-gatherers, with the total human population being variously estimated at around 5–20 million people.

With the benefits of the invention of agriculture, roughly simultaneously in various parts of the world around 10 000 years ago, things started to change. Denser aggregations of people became possible, and villages began their journey to cities. Following the advent of this agricultural revolution, human populations arguably grew more rapidly in the first 5000 years than in the more recent 5000, up to the beginning of the ScientificIndustrial Revolution around the 1600s. This relative slowing of population growth is almost surely associated with infectious diseases which were not sustainable at the low population densities associated with hunter-gatherers. Reader (2004) summarizes it well: ‘Bacterial and viral diseases are the price humanity has paid to live in large and densely populated cities. Virtually all the familiar infectious diseases have evolved only since the advent of agriculture, permanent settlement and the growth of cities. Most were transferred to humans from animals—especially domestic animals. Measles, for instance, is akin to rinderpest in cattle; influenza came from pigs; smallpox is related to cowpox. Humans share 296 diseases with domestic animals.’

The next big upsurge in population growth resulted from agricultural and technological advances spurred by the Scientific-Industrial Revolution over the past few centuries. Human numbers reached their first billion around 1830. It took a century to double that.

Then arrived the third revolutionary upsurge, driven this time by advances in medical science—not CATscanners, but understanding of the transmission dynamics of infectious diseases, coupled with better hygiene and better nutrition (albeit still inequitably distributed). The next doubling took 40 years, to four billion around 1970. In 2006 we are about 6.5 billion.

Associated with population growth are great changes in patterns of urbanization. In 1700, about 10% of the world’s population lived in cities. By 1900 it was 25%. Some time in 2006 (or maybe 2005 or 2007; our knowledge is not that precise) a child will be born marking a hinge in history where, for the first time, more people will live in cities than in rural areas.

The present marks another tipping point in that overall fertility rates have just dropped below replacement levels: globally, although still with big regional variations, the average woman is producing less than one female offspring. However, the pyramidal shape of age profiles in most parts of the world gives momentum to population growth, such that even if fertility rates remain below unity the human population will continue to grow throughout the present century, reaching 9 billion around 2050. And essentially all these added people will live in cities, taking the urban fraction in 2050 to 67%.

(Credit: Julianza / Pixabay)
(Credit: Julianza / Pixabay)

Walt Whitman once evoked the feelings we have about the vast numbers of humans to have lived before us: ‘row upon row, rise the phantoms behind us’. But estimates of the total numbers of Homo sapiens ever to have lived run around 80 billion or so. So if they were regimented in tidy columns behind those alive today, each of us, looking over a shoulder, would see only a dozen or so phantoms. I find this a startling thought. It certainly underlines the singularity of our time.

The scale of human impacts

Our predecessors, even in hunter-gatherer times, had impacts on their environments. There are still debates about the extent to which early humans, as distinct from changing climate, contributed to the extinction of the Pleistocene megafauna in Europe, Africa, the Americas, and Australia, but there is no doubt that humans extinguished roughly half the bird species in Hawaii and New Zealand when they arrived less than 1000 years ago.

Significant though they were, these earlier impacts were not on the literally global scale of today’s. It is estimated that humans now take to their own use, directly or indirectly, between 25 and 50% of all net terrestrial primary productivity (the commonly quoted figure is 40%; see Vitousek et al., 1986; Daily, 1997). Perhaps even more striking, it has been estimated that more than half of all the atoms of nitrogen, and also of phosphorous, incorporated into green plants today come from artificial fertilizers (produced with fossil-fuel energy subsidies) rather than the natural biogeochemical cycles which built, and which struggle to maintain, the biosphere. These estimates are necessarily imprecise, but they accord with a very recent study, using satellite imagery, which found 40% of the Earth’s land surface being modified by human use, mainly for agriculture.

The Worldwide Fund for Nature (WWF, 2004) has presented estimates, country by country, of humanity’s ecological footprint (EF) at current levels of consumption. The EF for a given country is defined as the biologically productive area required to produce the food and wood people consume, to give room for infrastructure, and to absorb the carbon dioxide emitted from burning fossil fuels. Thus estimated, the EF is expressed in ‘area units’. Any such estimate is necessarily imprecise (the carbon dioxide bit arguably more so than other components), but on the other hand they are conservative in that other factors, such as requirements for natural ecosystem services to handle pollutants, are excluded. Having estimated individual countries’ EF, the WWF adds them up to get the overall global EF shown in Figure 15.1. The observed increase over time derives partly from population growth, and partly from increases in the average footprint per person.

The WWF also estimate the total EF that individual countries, and thence the planet, could satisfy sustainably (the biological capacity, BC). Here the figures depend, to a degree, on assumptions about the footprints of future crops and energy sources. Figure 15.1 suggests we passed the point where humanity’s actual EF exceeds the sustainable level—a milestone of milestones— around two decades ago. I again emphasize the ineluctable uncertainties in any such estimates of footprints. Even so, I believe Figure 15.1 is indicative.

The most unambiguous sign that human activities now are on a scale that rivals natural processes is, of course, climate change. The evidence for this, including the statement from the Science Academies of all G8 countries along with China, India, and Brazil, was set out at the start of the previous chapter.

Numbers of species

The remainder of this chapter will focus not on humans, but on consequences of our actions for the other species we share the planet with.

Seen through a wider-angle lens, the impending diminution of the Earth’s diversity of plant and animal species could be an even greater threat than climate change. Unfortunately, analysis of the causes and consequences of accelerating extinction rates is impeded by the rudimentary state of our knowledge, which in turn derives more from past intellectual fashions than dispassionate assessment of scientific priorities. It is worth reflecting that Newton’s Laws of Motion, and consequent explanation of how planetary motions derive from the inverse-square gravitational attraction of the sun, came a full century before Linnaeus began the task of codifying living things (the date of the canonical tenth edition of De Rerum Naturae being 1758). This legacy of the lag between Linnaeus and Newton lingers today.

In a review of Terborgh’s (1999) Requiem for Nature, McKibben (1999) has commented bitterly on these vagaries of human fashions and concerns: ‘You can follow the changes in the value of the Japanese yen second by second from your desktop; reporters by the dozen struggle valiantly to explain the particulars of Microsoft’s antitrust defense. But who can tell whether the tropical forest is disappearing more or less speedily than it was in the late 1980s when every singer worth her faded jeans was cutting a CD in its defense? This question is surely worth attention, since the equatorial jungles contain more examples of creation’s fabulous imagination than any other ecosystem, and since its trees are a key part of the earth’s system for cleansing excess carbon dioxide from the atmosphere. Perhaps you have a dim sense that some agreements have been signed to protect the rainforests, some programs put in place. But are they working? What strategies make the most sense to preserve what’s left? Far more money and attention is devoted to, say, searching for and describing the possible remains of microbial life in the dust of Mars.’

So what is the current state of knowledge about the planet’s organisms other than humans?

Currently around 1.6 million distinct species of plants and eukaryotic animals have been named and recorded (Hammond, 1995; May, 1999). Even this number—analogous to the number of books in the British Library or the US Library of Congress, which are precisely known—is uncertain to within around 10%, because the majority of species are invertebrate animals of one kind or another, for most of which the records are still on file cards in separate museums and other institutions. The consequence is a synonymy problem: the same species being separately identified and differently named in two or more places. Given, for example, that some 40% of all named beetle species are estimated to be known from only one geographic site (and sometimes from a single specimen), and that intercollated databases do not exist for many groups, this synonymy problem should not surprise us. Of course, a lot of taxonomic effort has gone, and today goes, into sorting out synonymies, especially in better-studied groups. But even as old synonyms are resolved, new ones are being added. In a seminal study, Solow et al. (1995) combined theoretical and empirical work to make a start on estimating true synonymy rates, with the aim of getting a better idea of just how many distinct species have been named and recorded. For thrip species recorded since 1901, Solow et al. found a directly observed synonymy rate of 22%, but estimated the true rate of synonymy in this group to be around 39%. Subsequent studies suggest comparable, although usually somewhat smaller, numbers for other groups (May, 1999). Overall, it could be argued that a discount factor of something like 20% might be applied to existing lists of known species. This seems to me to be an important area where more theoretical and empirical work is warranted.

Currently, new species are being identified at a rate of around 15 000–20 000 a year, while at the same time earlier synonyms are being resolved at around 3000–5000 each year, for a net addition of very roughly 15 000 species each year.

So much for what is known. But how many species may there be in total on Earth today? Recent estimates lie in the range 5 million–15 million (Hammond, 1995; May, 1999). Lower numbers, and also much higher ones, also have their advocates. Even if we take a low estimate of 3 million still to be identified, at the current rates just noted the job would take 200 years. Organizing better databases, and using molecular information about newly discovered species’ genomes (‘barcoding life’), promises to speed up this distressingly slow task (Godfray and Knapp, 2004; Savolainen et al., 2005). Even so, the craft of collecting material in the field will remain a seriously rate-limiting step.

Extinction rates

If we do not know how many species have been identified—much less their functional roles in ecosystems—to within 10%, nor the overall species total to within an order of magnitude, we clearly cannot say much about how many species are likely to become extinct this century. We can note that the IUCN Red Data Books in 2004, using specific and sensible criteria, estimate 20% of recorded mammal species are threatened with extinction, and likewise 12% of birds, 4% of reptiles, 31% of amphibians, 3% of fish, and 31% of the 980 known species of gymnosperms (IUCN, 2004). However, when these figures are re-expressed in terms of the number of species whose status has been evaluated (as distinct from dividing the number known to be threatened by the total number known—however slightly—to science), the corresponding numbers are 23, 12, 61, 31, 26, and 34% respectively. This says a lot about how much attention reptiles and fish have received.

The corresponding figures for the majority of plant species, dicotyledons and monocotyledons, are respectively 4 and 1% of those known, and 74 and 68% of those evaluated. Most telling are the two numbers for the most numerous group, insects: 0.06% of all known species are threatened, compared with 73% of those actually evaluated. The same pattern holds true for other invertebrate groups. For these small things, which arguably run the world, we know too little to make any rough estimate of the proportions that have either become extinct, or are threatened with it.

These disparities in our knowledge about different groups reflect differential attention from the research community. Rough estimates (and it would be good to have better ones) suggest the taxonomic workforce is roughly evenly divided between vertebrate, plant, and invertebrate species. Given that plant species are roughly 10 times as numerous as vertebrate ones, and invertebrates—by conservative estimates—at least 100 times more numerous, this reflects a gross mismatch between workforce and the task at hand (Gaston and May, 1992). Things get worse when we look at the conservation biology literature: a study of papers in the two leading conservation research journals from 1987 to 2001 showed roughly 70% dealing with vertebrates, 20% with plants, and 10% with invertebrates (of which half were butterflies and moths, enjoying the status of a kind of honorary bird; Clark and May, 2002). And when we turn to conservation-oriented nongovernmental organizations, we find an even greater preponderance of attention given not just to vertebrate species, but to the roughly one-third that are birds and mammals.

Perhaps surprisingly, we can nevertheless say some relatively precise things about current and likely future rates of extinction in relation to the average rates seen over the roughly 550-million-year sweep of the fossil record (May et al., 1995; May, 1999). For bird and mammal species (a total of approximately 14 000), there has been an average of about one certified extinction per year over the past century. This is a very conservative estimate of the true extinction rate, because many species receive little attention even in this unusually well-studied group. Such a rate, if continued, translates into an average ‘species life expectancy’ of the order of 10 000 years. By contrast, the average life expectancy—from origination to extinction—of a species in the fossil record lies in the general range 1–10 million years, albeit with great variation both within and among groups (May, 1999).

So, if birds and mammals are typical—and there is no good reason to assume they are not— extinction rates in the twentieth century were higher, by a factor of 100–1000, than the fossil record’s average background rates. And four different lines of argument suggest a further 10-fold speeding up over the coming century (May et al., 1995). Such an acceleration in extinction rates is of the magnitude which characterized the Big Five mass extinction events in the fossil record (Raup, 1998; Sepkoski, 1992). These Big Five are used to mark changes from one geological epoch to the next. Although there is much need for further work to refine estimates of this kind, it does seem likely that we are standing on the breaking tip of a sixth great wave of mass extinctions.

The crucial difference between the impending Sixth Wave of mass extinction and the previous Big Five is that the earlier ones stemmed from external environmental events. The sixth, set to unfold over the next several centuries—seemingly long to us, but a blink of the eye in geological terms—derives directly from human impacts.

Why should we care about extinctions?

What fraction of all eukaryotic species ever to have lived are alive on earth today? Following Raup (1998), Sepkoski (1992), and others, I can give a rough answer to this question. We saw above that the rough life expectancy of a species in the fossil record was typically a few million years. Juxtaposing this species lifetime against the roughly 550-million-year sweep of the fossil record leads to an estimate that the species extant at any one time represent roughly 1% of the total ever to have lived. The history of life on Earth, however, has been one of very approximately linear increase in diversification, so we might guess at approximately 2% for the proportion alive today.

Conversely, given that extinction has already been the fate of 98%, and possibly more, of all eukaryotes, why should the impending Sixth Mass Extinction concern us? I think the reasons can be brigaded under three broad headings, each posing an agenda for research.

Excerpted from Theoretical Ecology by Lord Robert May with Angela McLean. Copyright © R. M. May and A. R. McLean, 2007. All rights reserved.

Robert May is Professor of Zoology at the University of Oxford and Imperial College London, and a member of the British House of Lords. His research, first at Princeton University and since 1988 at Oxford University, has dealt with the ways in which plant and animal populations – either singly or in interacting communities – change over time, especially in response to natural or human-created disturbance. His work on chaos, on how infectious diseases can influence the numerical abundance or geographical distribution of populations (including applications to humans and HIV/AIDS), on estimating species’ numbers and rates of extinction, and more generally on conservation biology have been recognized by several major International Prizes (Crafoord, Balzan, Blue Planet). He has been Chief Scientific Adviser to the UK Government (1995-2000), and President of the Royal Society (2000-2005).

Angela McLean is Professor of Mathematical Biology in the Department of Zoology and Director of the Institute for Emergent Infections of Humans in the Oxford Martin School. Her research interests lie in the use of mathematical models to aid our understanding of the evolution and spread of infectious agents. This encompasses modelling of the dynamics of infections and immune responses within individual hosts as well as models of the spread of infections from one host to another.

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