Mass extinctions: what can the past tell us about the present and the future?

Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 82 (1990) 175-185 175 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Mass extinctions: what can the past tell us about the present and the future?

NORMAN MYERS

Upper Meadow, Old Road, Oxford OX3 8SZ (U.K.)

(Received March 15, 1989; revised and accepted September 1, 1989)

Introduct ion

The biosphere is undergoing a series of environmental perturbations whose impacts are collectively exceeding all but the greatest geologic upheavals of the prehistoric past. Among these current impacts is a mass extinction that portends the elimination of a large proportion, perhaps as many as half, of all species now extant.

The predominant driving force behind the present-day perturbations is made up of two factors: growth in human numbers and growth in human consumerism. Since the start of this century human numbers have increased three- fold, growth in human consumption of fossil-fuel energy has increased 12-fold, and growth in the global economy has increased 29-fold (Brown et al., 1989; World Resources Institute, 1989). The most rapid growth has occurred in just the past few decades. Of this century's economic growth, four-fifths has taken place since 1950; and the expansion of the global economy in 1988 and 1989, $600 billion, has been as great as was the entire economy in the year 1900.

As a result of these various forms of growth in the human enterprise, along with inap- propriate technologies and deficient economic policies, the biosphere now suffers from a lengthening list of environmental assaults: widespread pollution such as acid rain, soil ero- sion, tropical deforestation, spread of deserts and decline of water supplies, to name but the main forms of environmental degradation and

biospheric impoverishment. Also underway are depletion of the ozone layer and the greenhouse effect.

To comprehend the full scope of our over- burdening of the biosphere, consider the capacity of humankind to feed its present total of over 5 billion people. If we all lived off a basi- cally vegetarian diet and shared our food supplies equally around the world, the biosphere could support roughly 6 billion people. If the diet were upgraded until 15% of calories were derived from animal products, making it the equivalent of what many South Americans consume today, the global total would be only 4 billion. Were the diet to be further improved to a "full but healthy" level with 25% of calories derived from animal products, being the equivalent of what many people in southern Europe consume today, the total would fall to 3 billion; and if the level included 35% of calories from animal products, or the equivalent of today's average North American diet, the total would be a mere 2.5 billion (Kates et al., 1989). Of course we can hope that better agro-technologies will improve the prospect. But despite the exceptional technological feats of the past few decades, the 1988 harvest worldwide was about 5% smaller than the 1985 harvest--while the global population was 5% percent larger (Ornstein and Ehrlich, 1989).

Hence the present throngs of humankind press ever-harder upon the planetary ecosystem. To summarize the situation through an alternative analysis, humankind now co-opts some 40% of

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all net primary productivity (Vitousek et al., 1986)--with all that implies for the life-support systems of Earth's species.

As a result of the environmental assaults of increasing numbers of people with increasing demands on a declining natural-resource base, there is widespread elimination of wildlife habitats. This is the principal proximate factor in fast increasing extinctions of species.

Yet this is only the start of biospheric disruptions. The main population explosion is still to come. Within the next 35-40 years we must expect that human numbers will double. Even more to the point, we must anticipate that con- snmption of food and fibre will triple, demand for energy will quadruple, and economic activity will quintuple (World Commission on Environ- ment and Development, 1987). So the problem lies not only with sheer growth in human num- be r s - though since that takes place mostly in the tropics, where the great bulk of species occur, population growth must rank as a prime factor in the mass extinction underway. Let us remember that an average American utilizes 150 times as much energy as an average Bang- ladeshi, Ethiopian or Bolivian, and a similarly disproportionate amount of other key raw materials.

Before concluding this introductory section, let us note a further factor of the phenomenon of mass extinction now underway. While the phenomenon is closely tied in with the other forms of biospheric depletion, it constitutes a distinctive category of its own in that it is in- trinsically irreversible. Tropical deforestation, spread of deserts and the like must rank as reversible processes: were humankind to take the necessary corrective measures with due vigour and urgency (albeit at sizeable cost), we could start to restore tropical forests, soil cover, etc. But within time scales of interest to humankind, the present spasm of species extinctions is essentially a once-and-for-all affair.

It is the purpose of this exploratory paper to examine the various ways in which the current mass extinction will resemble those of the past, and the ways in which it will probably diverge --especially the ways in which it will prove to

be a unique phenomenon. Despite its impor- tance, the overall issue has been hardly touched upon in the professional literature. So this paper's analyses and findings are preliminary to a degree: they are no more (and no less!) than an effort to define the issue's nature and dimen- sions. Far from seeking to supply many correct answers, the author concentrates on asking most of the right questions. Indeed parts of the paper amount to no more than "creative speculation" - - a valid scientific exercise when one comes to probe a new field of enquiry.

Mass ext inct ions; nature and scope

There is much evidence emerging (Ehrlich and Ehrlich, 1981; Club of Earth, 1986; Myers, 1987 and 1988; Raven, 1987; Wilson, 1988; Western and Pearl, 1989) that we are into the opening stages of a mass-extinction episode, de- fineable as an unusual decline in biodiversity that is substantial in size and global in extent, and affecting a broad range of taxonomic groups over a relatively short period of time (Jablonski, 1986a; Sepkoski, 1988). Within the foreseeable future of the next few decades, and given a continuation of present environmental trends, we shall surely lose one quarter of the 250,000 higher plant species on Earth, and during the remainder of the next century we may well lose a further one quarter. Because it is due to gross-scale destruction of natural environments and wildlife habitats, this mass demise of plant species will be accompanied by the elimination of a similar proportion of animal species. In terms of the compressed time-scale of the episode, the numbers and types of species involved, and the impoverishing impact on the future course of evolution, this extinction spasm will likely surpass most if not all of the mass-extinction episodes of the prehistoric past.

Still another aspect of the current phenomenon is not shared by any of the mass extinctions of the past. The present episode is caused by the "non-natural" influence of a single species, Homo sapiens. This has significant implications for the episode's character, since it will not be, as has been the usual case in the past, caused

MASS E X T I N C T I O N S : W H A T CAN PAST T E L L ABOUT P R E S E N T AND F U T U R E ? 177

primarily by physico-chemical factors such as paroxysmic vulcanism and asteroid impact. Rather it will derive from the essentially biological factor of humankind's impacts on the biosphere. Precisely because these impacts will prove pervasive and profound, affecting all parts of the planetary ecosystem, species losses will surely be more widespread, in terms of both geographic exent and species' categories, than has occured with mass extinctions in the geological past.

In certain other respects, however, there may well be similarities between the present episode and those of the past. These similarities are likely to manifest themselves through such factors as the preferential loss of endemic species and higher taxa, plus the vulnerability of tropical communities (Jablonski, 1986a). To this extent, the present phenomenon could well be illuminated by comparative evaluation of past phenomena: "Without consideration of the time perspective available from the geological record, a full evaluation of the contemporary extinction problem may prove as difficult as would be the case if a land-use planner were to attempt pro- jections without benefit of historical experience or if an epidemiologist were to treat an infec- tious disease without medical records" (Raup, 1988).

It is curious that more attempts have not been made to compare and contrast the past with the present in light of the many insights and lessons of the geological record to elucidate the present. For instance, what can we learn about survival strategies--whether ecological, biogeographic or evolutionary strategies--that have best enabled species to come through extinction crises in the past? How have mass extinctions affected the subsequent course of evolution? When the planetary biota loses a good part of its species complement in short order, how fast does it recover, in both quantitative and qualitative terms, before it eventually features a fresh spectrum of species with abundance and variety to parallel what was there before? (for some recent reviews of evolution from this standpoint, see Mayr, 1982; Milkman, 1982; Berggren and Van Couvering, 1984;

Eldredge, 1986; Jablonski and Raup, 1986 and Stanley, 1986).

Past mass-extinction episodes

There have been five major mass-extinction events in the past 600 million years, when a "goodly share"- -a t least one quarter, generally one half and occasionally an even greater proportion--of all animal species extant have disappeared within the space of probably one million years or less (Jablonski and Raup, 1986; see also Elliott, 1986; Kauffman and Walliser, 1989; Nitecki, 1984; and Stanley, 1987). To put this in perspective, suppose there have been four billion animal species during the last 600 million years, and today there are only a few tens of millions, say 50 million at most. This argues an average background extinction rate of roughly seven animal species per year (the actual average rate is presumably less, since sizeable numbers of species have disappeared during mass-extinction events).

But mass extinction in the past has been much more than an intensification of normal extinction pat terns--a "turning up of the dial", to cite Jablonski's graphic phrase (1986a). Rather it has entailed a change in categories of species that become extinct. Certain species that would normally resist extinction pressures during "background circumstances" have often found their adaptive capacities useless at a time of mass extinction; and certain species that would normally have appeared to be "extinction prone" have often come through a biotic crisis un- scathed. So a change-over in the rules has meant that during a mass extinction there has been not only an increase in the numbers of species disap- pearing. There has also been a basic switch in the kinds of species that disappeared (Jablonski, 1986b; Raup, 1986 and 1987). "Mass extinction can break the hegemony of groups of species honed by millions of years of selection, and thereby give other, less favoured groups a change" (Jablonski, 1986b). Or, to change the metaphor, mass extinction throws a wrench into the works of evolution: as an important compo- nent of macroevoloution, it becomes a special

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controlling event in life's history (Jablonski, 1986a; see also Chaloner and Hallam, 1989).

In short, and despite their broad-brush sweep, mass extinctions of the past have exerted selective effects: certain taxa and ecological groups have proven more prone to mass extinction than others. This has been due to biological traits that have varied within as well as among higher taxa, e.g. geographic range, trophic level and population density. In turn, this means that differential survivorship patterns have tended to emerge at relatively low taxonomic levels (Hal- lam, 1987; Jablonski, pers. comm., 1988).

Of course the selectivity pressures have not necessarily been "constructive" in a Darwinian sense (Buffetaut, 1984 and 1987; Jablonski, 1986a and b; Raup, 1986; McLaren, 1989). They have ostensibly been indifferent to the many adaptations that emerged and flourished under the background regime. At the sam~e time, however, they have frequently led to the emergence of new groups of species that have rapidly risen to dominance in the post-mass-extinction world. The most notable instance of this phenomenon has been the mammals, that, with the dinosaurs out of the way, soon established themselves as the predominant group during the Tertiary.

Drawing on our understanding of the past, there appear to have been several species attributes that have militated against survival during biotic crises (though whether that will apply again during the present episode, is another question--see below). They include; large body size, especially among terrestrial animals; tropical location; and capacity for high background rates of speciation and extinction (Raup, 1986). Endemic species too seem to have been hard hit at times of biotic crisis in the past: during the K / T mass extinction, 50% of endemic species were eliminated, and of the survivors only 10% were endemics, meaning that endemics suffered worse than one would expect on the basis of the numbers (Jablonski, 1986a, b; see also Janzen, 1986).

Following each mass extinction there has en- sued a recovery period during which evolutionary processes, notably natural selection, have generated new species at an unusually rapid

rate. Eventually there has emerged an array to match what was there before in terms of species abundance and variety. This "bounce back" period has usually been protracted (e.g. Sepko- ski, 1984; Raup and Sepkoski, 1986; Erwin et al., 1987). After the late Cretaceous crash, for instance, there was a gradual establishment of more specialized and diversified biotas until there emerged multiple assemblages of mammal life some 5 to 10 million years later (Jablonski, 1986a). In the case of coral reefs, which suffered more severely than most other biomes, there was a 10-million-year hiatus at the end of the Creta- ceous before reef fresh community of coral-reef building species became established. Similarly the end of the Permian was followed by a 5-million-year period when marine assemblages re- mained depauperate (Raup, 1988).

The present mass-extinction episode

As already noted, there is much evidence emerging to the effect that we are into the opening phase of a mass extinction episode. Con- sider, for instance, the situation in just ten "hot spot" localities in tropical forests, these being areas that are characterized by (a) exceptional concentrations of species with exceptional levels of endemism, and (b) exceptional threats of im- minent destruction. In these hot-spot areas alone, comprising only 300,000 km 2 or less than 3.5% of remaining undisturbed forests (and only 0.2% of Earth's land surface), there could well be eliminated by the year 2000 some 17,000 endemic plant species (13.5% of all plant species in tropical forests and 6.5% of all plant species worldwide), plus a minimum of 350,000 endemic animal species (Myers, 1988).

A similar analysis is underway for another 40 hot-spot areas, not just in tropical forests but coral reefs, wetlands and Mediterranean-type zones. An exploratory appraisal suggests that at least 40% of Earth's biodiversity is at risk of early extinction in less than 10% of Earth's land and near-shore surface (Myers, 1989).

A parallel calculation (Raven, 1987) indicates that one quarter of all plant species face early extinction within a handful of tropical-forest

MASS E X T I N C T I O N S : W H A T CAN PAST T E L L A B O U T P R E S E N T AND F U T U R E ? 179

countries--and for each plant species that becomes extinct, we can expect on average the related demise of between 20 and 60 animal species.

Note, moreover, that these analyses take no account of the greenhouse effect, a climatic up- heaval that may eliminate a large proportion of surviving species during the middle part of next century (Peters, 1990). The fossil fuel burners may ultimately account for a good number of species, even as many as the tropical forest burners.

On top of the immediate extinctions, viz. those that will stem directly and promptly from man's activities, we must reckon with those many additional extinctions--indirect and longer-term extinctions--that will arise when wildland habitats of the future are reduced for the most part to ecological islands. Due to this islandizing effect and processes of "ecological equilibration", there will ensue a delayed fall-out of species (MacArthur and Wilson, 1967; Janzen, 1986; Sepkowski, 1987). Consider, by way of illustration, the case of Amazonia (Simberloff, 1986). Were deforestation to continue at present rates until the year 2000 (it is likely to accelerate) and then halt completely, we should anticipate an eventual loss of about 15% of the region's plant species. But were the forest to be ultimately reduced to those areas now set aside as parks and reserves, we should anticipate that 66% of plant species will eventually disappear, together with almost 69% of bird species and similar proportions of other major categories of animal species.

All in all, then, it is likely that we are wit- nessing the onset of a mass-extinction episode that, if present environmental trends persist (but see below), will eliminate a full one half of all present species by the end of next century or shortly thereafter--during a time span of, say, 200 years at most.

Moreover, the analysis set out here focuses almost entirely on species, as if they represent the most significant of all taxa. But there is an additional dimension to be addressed: the pro- spective demise of higher taxonomic categories - - a qualitative aspect deserving equal consider-

ation, In addition to species, many genera and even families will surely be eliminated. This makes the mass extinction ahead a yet more impoverishing phenomenon than is suggested by a simple count-up of species.

Of course all this pre-supposes that we fail to do a better job of preserving species at risk. Whatever the dismal character of the prognosis presented above, it is not altogether inevitable that we face an extinction spasm of the full scope anticipated. After all, prediction is not destiny. Of course our best efforts will fail to safeguard sizeable numbers of species at risk, since the processes of habitat destruction have worked up too much momentum to be halted in short order. But there is still time, though only just time, for us to save animal and plant species in their millions--provided we get to urgent grips with the challenge, and deploy a sufficient information base illuminated by our understanding of the past.

At the same time, we must be careful not to over-state the scope for optimism. Let us remember that all too much degradation has already been imposed upon the biosphere, and many depletive processes will persist through dynamic inertia for a good way into the future, no matter how vigorously we may try to resist them (if indeed any such efforts were mounted with sufficient urgency). Suppose, by way of illustration, that in the year 2000 the whole of humankind were to be removed from the face of the Earth in one fell swoop. Because of the many environmental perturbations already imposed, with their impacts persisting for many subsequent decades, gross biopheric depletion would continue and thus serve to eliminate further large numbers of species. On top of this would be the delayed fallout effects of ecological equilibration, eliminating still further hosts of species. As a result, there would likely be as many extinctions during the first few centuries in a post-2000 world relieved of humankind's continuing disruptions, as would have occurred during the period while humankind was still present.

Since humankind is not about to disappear, but will continue to engage in tropical deforesta-

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tion, etc., with progressive impacts, together with new environmental assaults such as the greenhouse effect [plus synergistic compounding of interactive effects (Myers, 1986)], the real- world outcome will be a marked acceleration of extinction rates. Indeed the mass extinction will continue unabated until such time as humankind learns to live in ecological accord with the biosphere.

An outburst of speciation possible?

It is theoretically plausible that while the current mega-extinction episode is taking place, some of the same disruptive processes causing it could lead to an outburst of speciation. Among categories of disruption with potential to foster speciation are: (a) splitting off of species' populations, as in the case of Lake Nabugabo, a recent "offshoot" of Lake Victoria, which has led to the development of six species of Haplochromis; (b) introduction of new food resources and other materials into species' habitats, as occurred with bananas in Hawaii, lead- ing to the emergence of several moth species of the genus Hydylepta, all of which are obligate feeders on banana plants; and (c) the introduction of species into new habitats, e.g. the housesparrow in North America, now manifest- ing several distinctive races, even subspecies.

But a speeding-up of speciation along these lines will not remotely match the accelerated rate of extinctions. Whereas an extinction can occur in just a few decades, and sometimes in just a year or two (a valley in a tropical forest with a pocket of endemic species can be con- verted into pastureland within a single season), the time required to produce a new species is much longer. It takes decades for outstandingly capable contenders such as certain insect species; centuries if not millennia for many other in- vertebrates; and hundreds of thousands or even millions of years for most vertebrates.

There is a fourth factor that possibly encour- ages speciation. While it is currently a controversial factor, it could eventually prove to be the most important of all. It is the prospect of large numbers of niches vacated in the wake of a

mass extinction, allowing new species to appear more rapidly than when there are diverse and abundant numbers of species (as is the case right now). Regrettably we know too little about this process in principle and it will take too long to reveal itself in practice, for us to suppose there could be an outburst of speciations which might remotely compensate for the ultra-accelerated rate of extinctions.

Equally possible is the prospect that a speciation outburst will first be preceded by an extended period of little differentiated biotas. Dur- ing the geological past the survivors of extinction episodes have not been the most highly evolved species. They have been the most widespread species--ecological generalists rather than specialists (e.g. Wallisher, 1986). This is no more than we might expect. We know from present-day experience that destablized environments often lead to species communities with steeply graded distributions of species abundance, a few species being predominant--by contrast with little-disturbed environments that usually feature many species (May, 1988).

Were this kind of post-mass-extinction scenario to work out within the foreseeable future, the reduced stock of species that survives the present mass extinction will likely contain a disproportionate number of opportunistic species. Such species rapidly exploit newly vacated niches are usually short-lived (with relatively brief gaps between generations and rapid generation turnover rates), feature high rates of population increase, and are adaptable to a wide range of environments--all traits that enable them to exploit new environments and to make excellent use of "boom seasons." These are precisely the attributes that enable opportunistic species to prosper in a man-disrupted world, where they often become pests. Examples include the English sparrow, the European star- ling, the housefly, the rabbit and the rat, plus many "weed" plants.

At the same time, this trend toward op- portunism carries a sizeable cost for less gener- alized, more specialized, species. The sparrow, for example, is adept at usurping the food stocks and nesting sites of many native North Ameri-


can birds, notably bluebirds, wrens and swal- lows. Entire populations of these species are being displaced and supplanted, leaving them with less promising prospects of survival (Kendergh, 1973). The same process is overtaking other regions of the world that the sparrow has colonized with human help, notably South America, South Africa, southeastern Australia and New Zealand.

If generalist species are to profit from the coming crash, the specialists, notably predators and parasites, will probably suffer disproper- tionately high losses. This is because their lifes- tyles are much more refined than the opportunists'; and their numbers are generally much smaller anyway. Since the specialists are often the creatures that keep down the populations of opportunists, there may be little to hold the pests in check. Today probably less than 5% of all insect species rank as pests. But if extinction patterns tend to favour generalist species, the upshot could soon be a situation where these species increase until their natural enemies can no longer control them. In short, our descendants could shortly find themselves living in a world with a "pest and weed" ecology.

Relationships between the past and the present, and the foreseeable future

What else can we learn from the past in order to illuminate the present and the future? How far, in fact, will the present mass-extinction event resemble events of the past?

First, let us note the compressed timescale of the present episode, putatively reckoned at a mere 200 years at most. This contrasts with the duration of mass exinctions in the past, which, while virtually instantaneous in geological terms, appears to have lasted for tens of thousands if not hundreds of thousands of years (Raup and Sepkoski, 1984; Hallam, 1987; Larwood, 1988). Even if the dinosaurs' demise was ultimately precipitated by an asteroid impact (and the final crash took little longer than, so to speak, a weekend), the dinosaurs had ostensibly been in decline for many thousands of years (e.g. Hsu, 1986, 1989)--and the same applied to associated

assemblages of species. To this (controversial) extent, the present mass extinction is occurring in a space of time that is only a tiny fraction as long as the swiftest of such episodes in the past.

Secondly, the number of species in question is surely greater than on any occasion in the past. We cannot be sure about this, since it is less than clear that there are more species extant now than before. But the evidence seems com- pelling (e.g. Valentine, 1969; Sepkoski et al., 1981; Stanley, 1981; Benton, 1987). According to Raup and Sepkoski (1982), the number of marine families, both invertebrate and vertebrate, "has increased substantially since the Cambrian." Some 125 million years ago flowering plants numbered only around 25,000 species, whereas by the late Cretaceous they had increased to at least 100,000 species, and today they total 200,000 species (Knoll, 1984). In addition there were generally far fewer members of higher taxa (Niklas et al., 1985); at the time of the late Cretacous, for instance, there were only 60 vertebrate families, by comparison with today's 325 (Benton, 1985). Throughout the subsequent Tertiary, moreover, insect faunas radiated widely (Knoll, 1986).

To the extent that there are more species extant today than at any time in the past, and if half disappear within the next 200 years, this will constitute a greater quantitative decline in biodiversity than has ever occurred before (ex- cept on those putative occasions in the first one billion years of life's history when, it is sometimes hypothesized (Mayr, 1982), life was eliminated altogether, before starting up afresh). Whereas the late Permian witnessed the disappearance of perhaps 77-96% of all species (Raup, 1979), the total complement of species then presumably amounted to far fewer than are extant today. The late Cretaceous crash may have eliminated 60-70% of all species, but again there were surely fewer species than are extant today. During each of the three other mass extinctions of the Phanerozoic it seems that at least half of all species disappeared; but each time there must have been fewer, probably far fewer, species extant than is the case today.

A third reason for supposing this could be the

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greatest single setback to life on Earth is that today's array of life forms may be qualitatively superior to those of the past. That is to say, it could be more "advanced", reflecting the adap- tively progressive nature of life's history (Stan- ley, 1979; Van Valen, 1985; Vermey, 1986; but see Benton, 1987; Nitecki, 1989). According to this view, certain features of evolutionary trends reveal the steady development in animals of ever-more sophisticated attributes (backbone, jaws, lungs, amniote egg, internal thermoregu- lation, large brain), all of which permit a grow- ing degree of control over, and independence from, the environment. Moreover these adaptive advances all appear to foster "survivorship capacity" during normal circumstances. Of course this is a somewhat contentious interpre- tation, and is subject anyway to many qualifica- tions. But to the extent that there is substance to this viewpoint, we have a third reason for supposing that the extinction spasm impending could represent the most depletive process that has ever overtaken Earth's biotas.

A fourth factor, surely the most significant of all, lies with the impoverishing impact on the future course of evolution. There are two salient aspects to this putative outcome. As already noted, there could shortly be a "pest and weed" ecology overtaking the biosphere, with all that means for human welfare within the immediate future. The repercussions are difficult to discern with substantive detail, but for purposes of humankind's fortunes within just a short-term perspective, the outcome could be deleterious indeed.

The second aspect relates to a much more protracted time frame, extending for millions of years into the future. While this may be deemed of little consequence to human generations of the next few centuries, it is worth considering if only from an ethical standpoint: do we, with our current activities of environmental perturbations, have any "right" to assume unwitting control over an exceptionally extended period of evolution's course during which there will be a gross impoverishment of biotic abundance and diversity?

To gain perspective of this aspect, let us re-

call that processes of biotic recovery in the past have generally needed some 5 to 10 million years. This time, by contrast, the bounceback period required could prove far longer (even after Homo sapiens finds ways to live in ecological accord with the biosphere). The forces of natural selection can work only with the "resource base" of species available. If that base is drastically reduced, the result could be evolutionary disruption persisting much further into the future.

The critical factor lies with the likely loss of key environments. We appear set to lose most if not virtually all tropical forests, also coral reefs and other tropical biotopes with exceptional abundance of species and ecological complexity. Yet it is precisely these environments that have served in the past as pre-eminent "powerhouses" of evolution, having thrown up more species than other environments. It is often considered (e.g. Mayr, 1976, 1988) that virtually every major group of vertebrates and many other large categories of animals, together with angiosperm plants, originated in zones with warm, equable climates, notably tropical forests, coral reefs and other tropical biotopes. It has likewise been sup- posed that the rate of evolutionary diversifica- t i o n - w h e t h e r through proliferation of species or through emergence of major new adaptations --has been greatest in the tropics, especially in tropical forests (Stenseth, 1984). In addition tropical species, especially tropical-forest species, appear to persist for only brief periods of geological time, which implies a high rate of evolution.

Of course tropical forests have been severely depleted in the past. During drier phases of the late Pleistocene they have been repeatedly reduced to only a small fraction, occasionally as little as one tenth, of their former expanse. Moreover tropical biotas seem to have been un- duly prone to extinction (Jablonski, 1986a; Jablonski and Raup, 1986; Raup, 1986). But the remnant forest "refugia" usually contained sufficient stocks of surviving species to recolonize suitable territories when moister conditions re- turned (Prance, 1982). Within the foreseeable future, by contrast, it seems all too possible that most tropical forests will be reduced to much


less than one tenth of their former expanse, and their pockets of "holdout" species will be so much less stocked with potential colonizers. This loss of key environments will result in enduring damage, to the extent that it will not permit a compensatory outburst of speciation until after possibly twice as long a delay as on past occasions.

Also likely to be eliminated during the present crash, and probably for the first time, is a good part of the detritivore community. At the end of the Cretaceous this community was somewhat buffered against extinction (Sheehan and Hansen, 1986), whereas during today's episode a large number of detritivores, such as freshwater fishes and other members of lake communities, are suffering widespread extinctions (Lowe-McConnell, 1987).

Even more important is the unprecedented reduction of the plant kingdom. So far as we can discern, during mass extinctions of the past most plant diversity survived (Hickey, 1984; Knoll, 1984; Traverse, 1988), whereas this time it is mostly being depleted. Already we are losing at least one higher-plant species per day in tropical forests alone, and by the middle of next century we may well have lost at least one quarter of all such species, to be followed by a further one quarter within another hundred years or so (as ecological equilibration and relaxation processes take their toll). This will strongly contrast with mass-extinction episodes of the past when, in the aftermath of mass extinction, plants sup- plied a resource base on which evolutionary processes could start to generate replacement species forthwith. If this biotic substrate is markedly depleted this time, the restorative capacities of evolution will be diminished all the more.

All this postulates a dire prognosis. The bounce-back period this time could take a lot longer than on occasions in the past, conceivably as long as 15 to 25 million years. Equally significant, and again in contrast with occasions in the past, there could be no emergence of a striking evolutionary innovation for a still more protracted period, insofar as the "raw materials" on which natural selection could work (as was the

case with the mammal stock at the start of the Tertiary) will have been widely eliminated. In the past, salient categories of species have escaped mass extinctions with few losses--as occurred at the K / T boundary not only with placental mammals but with a whole list of other major groups such as non-dinosaurian re- ptiles (crocodilians, turtles and squamates), amphibians and birds. This time, and as is axiomatic given the scope and scale of human destruction of species' habitats, virtually every category of life forms will suffer severe depletion; and there may be scant scope for an evolutionary radiation of innovative groups--con- trary to the situation in the aftermath of almost all mass extinctions in the past (cf. Holland and Trandell, 1984).

Overall, then, the prospect is that in the wake of the present extinction spasm there will not be a mere hiatus in evolutionary processes. Rather we- -or rather, our distant descendants--may well find that many evolutionary developments that have persisted throughout the Phanerozoic could be suspended if not terminated. To cite the vivid phrasing of Soule and Wilcox (1980), "Death is one thing, an end to birth is some- thing else" (for further elaboration of this key factor, see Frankel and Soule, 1981).

Conclus ion

By way of wrap-up, let us reiterate that the greatest divergence of the present mass extinction with those of the past is that it is entirely due to the activities of a single species, Homo sapiens. Ironically it is this same species that possesses the unique capacity to stem and even halt the exceptionally destructive tide of extinctions that is washing over Earth's biotas.

A c k n o w l e d g e m e n t s

It is a pleasure to acknowledge the helpful comments of two colleagues who have kindly :hecked through a working draft of this manuscript, and have discussed past and present extinction issues with me on numerous occasions, whether in Oxford or Chicago or at vari-

184 N. MYERS

ous points in between, namely Professors David Jablonski and David M. Raup of the University of Chicago. In addition I appreciate the constructive comments of a referee, Dr William G. Chaloner. I have also benefitted from the many incisive comments of colleagues at the IUGS/IGCP Workshop on Global Changes at Interlaken, Switzerland, April 20-24, 1989.

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Mass extinctions: what can the past tell us about the present and the future?