By Peter Ward
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 large inland seaway of Western Washington, I was attracted to the sandy undersea bottoms 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 passing over countless clam necks, each marking the presence of a fat clam buried deeply 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. The treasure of clams in this sanctuary, like the tall firs and cedars lining the cool seaway, seemed inexhaustible. Sadly, that turned out not to be the case. A dive into the same cool waters today reveals sandy bottoms nearly devoid of life.
At first glance, one might think that overfishing of the clams must have occurred. But that is not what happened here. An extinction has occurred here—caused not by overharvesting but by the simple presence of numerous humans along the shores of Hood Canal. The clams have been killed by a new type of seawater in Hood Canal, a low-oxygen, nutrient-rich sea that is a product of the new 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 some future fossil record where clam-rich sediments formed from the bottom of Hood Canal are overlain by the sediments formed over the last few decades, bottom sediments nearly devoid of the telltale signs of life. The Hood Canal 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 worse than others. Five times over the past 500 million years, more than 50 percent of species have gone extinct. The Permian extinction 250 million years ago reduced species numbers on the planet by 90 percent. Because of its stupendous body count, its most ardent investigator, Douglas Erwin of the Smithsonian Institution, has nicknamed it "The Mother of All Mass Extinctions," a phrase alluding to the belief that this was the greatest diversity drop in Earth’s history.
But perhaps not. We now live in the midst of another mass extinction event. And there is a very good possibility that losses in diversity in the present may surpass anything in the past—in terms of number of species lost.
Suggesting that the modern mass extinction might become the deadliest mass extinction of all time seems, at first glance, ludicrous. Although no one disputes that species are disappearing due in large part to our wholesale transformation of the world’s ecosystems, there has never been a serious estimate of a greater-than-50-percent kill, the death toll of the Cretaceous, Triassic, Devonian, and Ordovician mass extinctions, and no one—no one—has ever uttered the sheer hyperbole of another mass extinction surpassing the 90-percent figure of the Permian event.
It turns out that a 90-percent extinction rate for the Permian may well be equivalent to a 25-percent extinction rate in the present. The discrepancy lies in our bookkeeping.
One of the major discoveries of the past two decades dealt with the number of species on the earth. John Phillips first showed that the history of life on this planet during the last 600 million years has been one of almost steady diversification, punctuated only by temporary setbacks imposed by mass extinctions. Prior to this discovery, it had long been thought that, early on, species diversity had reached some maximum level (thought to be imposed by some sort of evolutionary carrying capacity of the planet) and then remained constant. Work by people such as James Valentine and Jack Sepkoski showed this not to be true.
We now believe that the number of species in the present day is far higher than at any time during the Paleozoic or Mesozoic eras. But a totally unexpected offshoot of this research was the finding that diversification has been taking place at different rates among the various taxonomic categories. Although millions of new species have been produced over the past 100 million years, proportionately fewer higher taxonomic categories such as genera, families, orders, and classes have evolved. Evolution and diversification have occurred through the creation of new species among already existing body plans rather than by the invention of entirely new groups.
The enrichment of species among the higher taxonomic units was unexpected and has produced a bias in estimating the severity of modern extinctions compared to those of the past. The severity of an extinction is a function both of its extinction rate—the number of species becoming extinct per time unit—and of its percent extinction—the number of species suffering extinction divided by the total number of species on the earth at that time. But because most studies compare the losses of higher taxonomic categories, such as genera or families, rather than of species, they have consistently led to an underestimation of the current rates of extinction compared to the great events of the past.
An analogy can illustrate this process. Let us imagine that each car model being driven today is a 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 all can be placed into an even higher category, the Order Combustion-Engine Vehicle.
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 Model A, it now has the Taurus, Probe, Escort, Thunderbird, Tempo, Mustang, and so 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 offers far more models than it did in 1900; cars have thus diversified. But the number of car genera has increased only slightly in the same period (we now 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—the cars, trucks, motorcycles—soon appeared and have remained relatively stable in overall design ever since. This is not to say that evolution has not occurred, for cars have evolved enormously in details such as styling and engine type. Nevertheless, all car species still have four wheels, carry passengers in a cabin, and so on.
Let us now compare the diversity of vehicles on the road in 1920 with that of the present day. If we count only families, the numbers would be quite similar. If we count species, the numbers would be very different. Compared to 1920, the number of families has increased by three or four and the number of genera has increased by several tens, while species numbers have increased by 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 the number of species has increased enormously. And yet most measures of diversity of living creatures through time have depended not on counts of species but of families. In a similar fashion, most estimates of extinction levels during the various mass extinctions also have depended on rates of family extinction, not species extinction. Because of this, the current extinction looks far less severe than either the Permian or Cretaceous events. At the 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 or even 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 species per family now than at any previous time. But most people attempting to grapple with the problem of current and impending extinctions have missed these two salient points. All too often they argue that the current extinction is far less calamitous than either the end-Paleozoic or end-Mesozoic events (and thus not worth getting too upset about) because lower percentages of 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 extinct taxa compared to the total number of taxonomic units, whether they are families, genera, or species. Using this measure, scientists have argued that the extinctions occurring to date since the onset of the Ice Age have been trivial, compared to the earlier great extinctions, because the percentage of taxa becoming extinct is but a tiny fraction of the total diversity of the earth. What these scientists overlook, however, is the fact that the absolute—not relative—number of species (or other categories) that have already gone extinct in the last million years may be more than the total of the other mass extinctions combined.
The current extinction began 50,000 years ago with the loss of megamammal fauna on every continent save 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.
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). The survivors of such events in the past, such as the Permian extinction, are called a "recovery fauna," and they are generally weedy species. What might the recovery fauna of the current extinction be like? They are with us now, 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 dealing with 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 extended period of time—a span measured in the millions of years. And if humanity continues to exist and thrive (and I believe it will), the recovery biota may 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 Spring 2000 issue of Nature. The authors, James Kirchner and Ann Weil, posed the questions: 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 analyzing the fossil record of all recoverable organisms (which was compiled by the late Jack Sepkoski of the University of Chicago), Kirch-ner and Weil found that fully ten million years elapsed, on average, before the biodiversity of the world recovered to its pre-extinction values. Even more surprising than this long lag period between extinction and full recovery was their finding that this long period occurred whether the extinction was small or large. We paleontologists had assumed that the time to recovery would somehow correlate with the magnitude of the extinction. But to the surprise of us all, Kirchner and Weil found this not to be the case—ten million years was necessary, even after the smaller extinctions. They concluded their paper with the following passage:
"Our results suggest that there are intrinsic ‘speed limits’ that regulate recovery from small extinctions as well as large ones. Thus, today’s anthropogenic extinctions are likely to have long-lasting effects, whether or not they are comparable in scope to the major mass extinctions. Even if Homo sapiens survives several million more years, it is unlikely that any of our species will see biodiversity recover from today’s extinctions."
So what does this mean for conservation? Some years ago, I was lucky enough to meet and begin a correspondence with Oxford conservationist Norman Myers, one of the first, and still most effective, of what we might call biodiversity game wardens. Norman has thought deeply about the role of humanity on our planet and about our species’ effect on biodiversity. And, unlike many conservationists, he has taken a long view—a view both forward and backward, not through seasons but through the longer passages that we call geological time. From that vantage point, our conservation priorities might look very different.
For example, is it satisfactory to safeguard as much of the planetary stock of species as possible, or should not greater attention be paid to safeguarding evolutionary processes at risk? This is an entirely new way of looking at the world—not in 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 adaptations to persist, thereby leading to new species? For example, should two elephant species be maintained, or should we keep the option of elephant-like species in the distant future? Myers suggests that, when faced with choices, we opt for preserving species that might present the most potent evolutionary stocks or makeups to ensure future biodiversity and biodisparity (i.e., future diversity in body plans). Is there some minimum number of individuals necessary not just for survival of the species but also for survival of the capacity for future evolution of that species? Should the slow breeders (the mega-mammals) be given greater attention than, say, the rapidly breeding insects? Are we in a triage situation?
Perhaps the most difficult question of all is, should we let endemic taxa go—let species that seem to have little future evolutionary potential slide into extinction without a fight from the conservation community? This latter question is heresy according to the rules of modern conservation. It has long been argued that endemic centers are among the most important places to save. But the point is that endemic centers exist because they have not produced large numbers of successful species. Endemic centers are often living museums of 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 will be time for healing and recovery, just as there has been after all past mass extinctions. But how long will the clam beds of my native Puget Sound rest empty? How long before life returns in its diverse exuberance? It appears that the emergence of new biota will take a long time after this mass extinction is finished—time measured in epochs rather than in the few 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 and unproductive father.
About the Author:
Peter Ward is Professor of Biology, Professor of Earth and Space Sciences, and Adjunct Professor of Astronomy 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.
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