Feature

The Father of All Mass Extinctions

THERE IS A GOOD POSSIBILITY that losses in diversity in the present will surpass anything in the geological past. Facing that specter could shake the very tenets of conservation.

In the mid-1960s, when I first began scuba diving in Puget Sound, the largeinland seaway of Western Washington, I was attracted to the sandy underseabottoms that made up some of the richest subtidal clam beds in the world.On any shallow scuba dive over such bottoms, I would find myself passingover countless clam necks, each marking the presence of a fat clam burieddeeply in the sediment, a testament to a diverse and thriving community.One of the richest areas of all was a narrow fjord named Hood Canal. Thetreasure of clams in this sanctuary, like the tall firs and cedars liningthe cool seaway, seemed inexhaustible. Sadly, that turned out not to bethe case. A dive into the same cool waters today reveals sandy bottomsnearly devoid of life.

At first glance, one might think that overfishing of the clams must have occurred. But thatis not what happened here. An extinction has occurred here—caused not byoverharvesting but by the simple presence of numerous humans along theshores of Hood Canal. The clams have been killed by a new type of seawaterin Hood Canal, a low-oxygen, nutrient-rich sea that is a product of thenew septic tanks and fertilized lawns that now line the fjord.

This changeover, even during a period as short as forty years, will surely show up in somefuture fossil record where clam-rich sediments formed from the bottom ofHood Canal are overlain by the sediments formed over the last few decades,bottom sediments nearly devoid of the telltale signs of life. The HoodCanal is a microcosm for larger catastrophes that we find in the deep past.

Extinction has been a commonplace event throughout geological time, with some events worsethan others. Five times over the past 500 million years, more than 50 percentof species have gone extinct. The Permian extinction 250 million yearsago reduced species numbers on the planet by 90 percent. Because of itsstupendous body count, its most ardent investigator, Douglas Erwin of theSmithsonian Institution, has nicknamed it “The Mother of All Mass Extinctions,” aphrase alluding to the belief that this was the greatest diversity dropin Earth’s history.

But perhaps not.We now live in the midst of another mass extinction event. And there isa very good possibility that losses in diversity in the present may surpassanything in the past—in terms of number of species lost.

Suggesting that the modern mass extinction might become the deadliest mass extinction of alltime seems, at first glance, ludicrous. Although no one disputes that speciesare disappearing due in large part to our wholesale transformation of theworld’s ecosystems, there has never been a serious estimate of a greater-than-50-percentkill, the death toll of the Cretaceous, Triassic, Devonian, and Ordovicianmass extinctions, and no one—no one—has ever uttered the sheer hyperboleof another mass extinction surpassing the 90-percent figure of the Permianevent.Bookkeeping

It turns out that a 90-percent extinction rate for the Permianmay well be equivalent to a 25-percent extinction rate in the present. Thediscrepancy lies in our bookkeeping.

One of the major discoveries of the past two decades dealtwith the number of species on the earth. John Phillips first showed thatthe history of life on this planet during the last 600 million years hasbeen one of almost steady diversification, punctuated only by temporary setbacksimposed by mass extinctions. Prior to this discovery, it had long been thoughtthat, early on, species diversity had reached some maximum level (thoughtto be imposed by some sort of evolutionary carrying capacity of the planet)and then remained constant. Work by people such as James Valentine and JackSepkoski showed this not to be true.

We now believe that the number of species in the present day is far higher than at any timeduring the Paleozoic or Mesozoic eras. But a totally unexpected offshootof this research was the finding that diversification has been taking placeat different rates among the various taxonomic categories. Although millionsof new species have been produced over the past 100 million years, proportionatelyfewer higher taxonomic categories such as genera, families, orders,and classes have evolved. Evolution and diversification have occurred throughthe creation of new species among already existing body plans rather thanby the invention of entirely new groups.

The enrichment of species among the higher taxonomic units was unexpected and has produceda bias in estimating the severity of modern extinctions compared to thoseof the past. The severity of an extinction is a function both of its extinctionrate—the number of species becoming extinct per time unit—and of its percentextinction—the number of species suffering extinction divided by the totalnumber of species on the earth at that time. But because most studies comparethe losses of higher taxonomic categories, such as genera or families,rather than of species, they have consistently led to an underestimationof the current rates of extinction compared to the great events of thepast.

An analogy can illustrate his process. Let us imagine that each car model being driven today isa species and each company it came from a genus. All belong to one family,the family Cars. Other families are on the roads as well: the family Trucks,the family Motorcycles, the family Roadgraders, the family Ambulances,and so on. All these families first evolved in the early 1900s, and allcan be placed into an even higher category, the Order Combustion-EngineVehicle.

Since the time of the origin of the family Cars, the number of species has proliferated enormously;whereas the genus Ford once had only two species, the Model T and the ModelA, it now has the Taurus, Probe, Escort, Thunderbird, Tempo, Mustang, andso on, as well as many extinct species: the Galaxy, Fairlane, Pinto, Edsel,and so on.

The result of about ninety years of evolution among the cars is that each car company now offersfar more models than it did in 1900; cars have thus diversified. But thenumber of car genera has increased only slightly in the same period (wenow have Hondas, Toyotas, and Nissans to go along with the Fords, GMs,and Chryslers), and the number of families has barely increased at all.The passenger van is one of only a few new additions in two decades.

Early in the twentieth century, all the major families, which correspond to distinct body plans—thecars, trucks, motorcycles—soon appeared and have remained relatively stablein overall design ever since. This is not to say that evolution has notoccurred, for cars have evolved enormously in details such as styling andengine type. Nevertheless, all car species still have four wheels, carrypassengers in a cabin, and so on.

Let us now compare the diversity of vehicles on the road in 1920 with that of the presentday. If we count only families, the numbers would be quite similar. Ifwe count species, the numbers would be very different. Compared to 1920,the number of families has increased by three or four and the number ofgenera has increased by several tens, while species numbers have increasedby many hundreds.

The diversity of creatures has increased in similar fashion. Compared to the Paleozoic Era,the number of currently living families has increased slightly, but thenumber of species has increased enormously. And yet most measures of diversityof living creatures through time have depended not on counts of speciesbut of families. In a similar fashion, most estimates of extinction levelsduring the various mass extinctions also have depended on rates of familyextinction, not species extinction. Because of this, the current extinctionlooks far less severe than either the Permian or Cretaceous events. Atthe species level, however, just the opposite may be true.

During the Paleozoic Era, each genus might have been composed of but a handful of species and each family of but a few genera; because of this, during periods of increased extinction, the loss of even a moderate number of species could mean the loss of many families or other higher units as well. As time progressed, however, families became increasingly and disproportionately packed with new genera and species, each a slightly new variant on an already-established body design. Taxonomic groups such as genera and families are now composed of far more species than at any time in the past. These larger taxonomic groups are thus more extinction-resistant than during past eras, since today the extinction of hundreds oreven thousands of species may be needed to eliminate a given family. This form of bookkeeping—counting only the families going extinct—although useful in keeping track of the world’s creatures, masks the true calamity of the modern extinction.

Two points are indisputable: the number of species has increased through time, and there are more speciesper family now than at any previous time. But most people attempting tograpple with the problem of current and impending extinctions have missedthese two salient points. All too often they argue that the current extinctionis far less calamitous than either the end-Paleozoic or end-Mesozoic events(and thus not worth getting too upset about) because lower percentagesof families and genera are now going extinct than in the past.

Second, the severity of a given extinction is commonly tabulated as a percentage of extincttaxa compared to the total number of taxonomic units, whether they arefamilies, genera, or species. Using this measure, scientists have arguedthat the extinctions occurring to date since the onset of the Ice Age havebeen trivial, compared to the earlier great extinctions, because the percentageof taxa becoming extinct is but a tiny fraction of the total diversityof the earth. What these scientists overlook, however, is the fact thatthe absolute—not relative—number of species (or other categories) thathave already gone extinct in the last million years may be morethan the total of the other mass extinctions combined.

The current extinction began 50,000 years ago with the loss of megamammal fauna on every continentsave Africa. A new phase of this extinction may continue, as it did at the end of the Permian, by intense global warming. If so, this new extinction will surpass the Mother of All Mass Extinctions.Survivors

At the end of any mass extinction, the biota of Earth is of low diversity(number of species) as well as low disparity (number of body types). Thesurvivors of such events in the past, such as the Permian extinction, arecalled a “recovery fauna,” and they are generally weedy species. What mightthe recovery fauna of the current extinction be like? They are with usnow, of course, as is any recovery fauna in the midst of mass extinction.

They are not so difficult to identify. Chief among them are those species best pre-adapted for dealingwith humanity: flies, rats, raccoons, housecats, coyotes, fleas, ticks,crows, pigeons, starlings, English sparrows, and intestinal parasites,among others.

According to many seers, this group of new flora and fauna will be with us for an extendedperiod of time—a span measured in the millions of years. And if humanitycontinues to exist and thrive (and I believe it will), the recovery biotamay be the dominants of any new age of organisms on Earth.

How long the recovery fauna may last was estimated in a disturbing paper published in the Spring2000 issue of Nature. The authors, James Kirchner and Ann Weil, posed thequestions: How quickly does biodiversity rebound after a mass extinction?How long will the world exist at very low biodiversity?

The answer, it turned out, was far longer than anyone had heretofore estimated. By analyzingthe fossil record of all recoverable organisms (which was compiled by thelate Jack Sepkoski of the University of Chicago), Kirch-ner and Weil foundthat fully ten million years elapsed, on average, before the biodiversityof the world recovered to its pre-extinction values. Even more surprisingthan this long lag period between extinction and full recovery was theirfinding that this long period occurred whether the extinction was smallor large. We paleontologists had assumed that the time to recovery wouldsomehow correlate with the magnitude of the extinction. But to the surpriseof us all, Kirchner and Weil found this not to be the case—ten millionyears was necessary, even after the smaller extinctions. They concludedtheir paper with the following passage:

“Our results suggest that there are intrinsic ‘speed limits’ that regulate recovery from smallextinctions as well as large ones. Thus, today’s anthropogenic extinctionsare likely to have long-lasting effects, whether or not they are comparablein scope to the major mass extinctions. Even if Homo sapiens survives severalmillion more years, it is unlikely that any of our species will see biodiversityrecover from today’s extinctions.”Unsettling Questions

So what does this mean for conservation? Some years ago, I was lucky enoughto meet and begin a correspondence with Oxford conservationist Norman Myers,one of the first, and still most effective, of what we might call biodiversitygame wardens. Norman has thought deeply about the role of humanity on ourplanet and about our species’ effect on biodiversity. And, unlike manyconservationists, he has taken a long view—a view both forward and backward,not through seasons but through the longer passages that we call geologicaltime. From that vantage point, our conservation priorities might look verydifferent.

For example, is it satisfactory to safeguard as much of the planetary stock of species aspossible, or should not greater attention be paid to safeguarding evolutionaryprocesses at risk? This is an entirely new way of looking at the world—notin terms of losing species, but in terms of losing pathways of speciation.Perhaps the motto should be “save speciation” rather than “save species.”

By preserving precise phenotypes of particular species, could we enable evolutionary adaptationsto persist, thereby leading to new species? For example, should two elephantspecies be maintained, or should we keep the option of elephant-like speciesin the distant future? Myers suggests that, when faced with choices, weopt for preserving species that might present the most potent evolutionarystocks or makeups to ensure future biodiversity and biodisparity (i.e.,future diversity in body plans). Is there some minimum number of individualsnecessary not just for survival of the species but also for survival ofthe capacity for future evolution of that species? Should the slow breeders(the mega-mammals) be given greater attention than, say, the rapidly breedinginsects? Are we in a triage situation?

Perhaps the most difficult question of all is, should we let endemic taxa go—let speciesthat seem to have little future evolutionary potential slide into extinctionwithout a fight from the conservation community? This latter question isheresy according to the rules of modern conservation. It has long beenargued that endemic centers are among the most important places to save.But the point is that endemic centers exist because they have not producedlarge numbers of successful species. Endemic centers are often living museumsof very ancient species that do not have much potential for future evolution.

As we learn how the earth works and begin to ease our current,uncomfortable presence among the rest of our planet’s biota, there willbe time for healing and recovery, just as there has been after all pastmass extinctions. But how long will the clam beds of my native Puget Soundrest empty? How long before life returns in its diverse exuberance? Itappears that the emergence of new biota will take a long time after thismass extinction is finished—time measured in epochs rather than in thefew short years that we humans are bequeathed for the span of our lives.This extinction is no mother that breeds anew. It may be a vicious andunproductive father.

About the Author:

Peter Ward is Professor of Biology, Professor of Earth and Space Sciences, and Adjunct Professor ofAstronomy at the University of Washington, Seattle. His latest book, Gorgon: Paleontology, Obsession, and the Greatest Catastrophe in Earth’s History, was published by Viking Penguin in 2004.