To study rare, mysterious animals, some biologists climb into submersibles and plunge two miles down to the bottom of the sea. Michael Hoggarth has only to leave his office at Otterbein College in Westerville, Ohio, and climb into a pair of waders. He slides his feet into rubber boots welded to waterproof canvas overalls that come up to his armpits, and steps slowly into Killbuck Creek, a soupy green river that winds through rolling cropland in north central Ohio. Under a low lid of clouds the countryside can remind him of his native Washington State, although when the sky clears there are no snowy mountains in the distance. But what the Pacific Northwest has in mountains, the Mississippi River watershed has in freshwater mussels, and they are what Hoggarth is looking for. The entire West Coast has only eight native freshwater mussel species. The Mississippi valley has virtually all the nearly 300 other North American species--the richest collection of freshwater mussels on Earth--including Hoggarth’s quarry today: the elusive purple catspaw.
A few years ago Hoggarth would have considered this a fool’s errand. Only two populations of the purple catspaw were known, one in Tennessee and one in Kentucky. The species hadn’t been seen in Ohio since the Civil War. But in 1991, Hoggarth turned over a recently dead shell in the Walhonding River (of which the Killbuck is a tributary) and was startled by a distinctive flash of purple. For two years Hoggarth and his students scoured the Walhonding for the endangered purple catspaw without luck. A year later, while looking for rare fish in Killbuck Creek, Hoggarth stumbled onto a live population of the mussels. Now he visits the Killbuck regularly in the hopes of finding more.
In the telling, Hoggarth’s mission might be mistaken for a pleasant little episode in malacology. It is not. The remarkable diversity of mussels like the purple catspaw in this region and their equally remarkable elusiveness are significant ingredients of the most important ecological questions of the day: Is the diversity of life diminishing at a catastrophic rate? And if it is, just how fast is it disappearing?
When asked to think of the biodiversity crisis, people tend to envision distant locales like the Amazon or the forests of Sumatra. In fact the crisis is just as real here in Ohio. The purple catspaw belongs to a vast tribe of freshwater mussels known as the unionoids, which once flourished in the great North American inland rivers. Pioneers might have preferred the word infested; they named one species heelsplitters for the menace they posed to livestock and barefoot humans. The unionoids were so numerous that they accounted for more than half the world’s species of freshwater mussel. But they were also remarkable in that they were all endemic species--that is, they were localized species that adapted to a particular ecosystem and thus were limited to one range. The purple catspaw, for example, was a big river mussel, a lover of strong currents and hard river bottoms.
Today, Hoggarth wades straight into the Killbuck and falls on his knees as if in prayer. The water swells up to his waist. He eases his bare arms into the water and begins noodling, feeling through the substrate of coarse sand and fine gravel for the half-buried mussels. His companions in waders, naturalist Dan Rice of the Ohio Department of Natural Resources and Otterbein biology student Bill Gates, take up stations alongside. The three kneel and noodle. Soon Gates pulls up one mussel after another, which Rice and Hoggarth glance back to identify: Fusconaia flava, Quadrula pustulosa, Lasmigona complanata. The Latin comes easily to them, but they fumble for the common names, gaudy homespun labels like Wabash pigtoe, pimpleback, white heelsplitter. Yet an hour passes, and the expedition can’t find a single Epioblasma obliquata obliquata, the purple catspaw.
I said we’d find a catspaw and I guarantee it, gentlemen. It’s still early, Hoggarth declares. After ten more minutes of noodling he has one. It’s a male, and judging by the dark green growth rings on its shell, Hoggarth says it is 18 years old. He gently presses it back into the sand to get on with its bivalvular life. It’s Hoggarth’s fifty-ninth living purple catspaw. By day’s end, he has found three more. Every one of them counts, desperately. Before Hoggarth’s discovery, the purple catspaw was officially endangered, but it was, in reality, doomed. Neither of the two previously known populations was breeding, so even though an individual catspaw can live 30 years, the creature’s days as a species seemed numbered. Hoggarth’s population is breeding, however, and in a single day he has raised its estimated number by 7 percent. His discovery raises the catspaw’s chances of survival from nil to merely bleak.
The story of the purple catspaw is not unusual for unionoid mussels. Humans have trashed them unwittingly but with a terrifying thoroughness. These filter feeders were buried in agricultural silt, drowned by dams, and poisoned by pollution. Their beds were dredged up for the river gravel beneath their feet. These same disturbances also wiped out populations of the fish on which the larvae of unionoids must mature. The mussels became a commodity in a ruinous shell trade, first for pearl buttons, then, in recent decades, for seeds--plugs cut from the shell that are used for culturing artificial pearls. They survived one exotic invasion by the Asian clam in 1859, only to face a new and possibly more damaging alien European competitor, the infamous zebra mussel.
The damage to the North American bivalves is not limited to a few species--the entire order of unionoids may be in jeopardy. Extinction of freshwater bivalves has been happening since at least 1900 but has only recently begun to be recognized, says Arthur Bogan, a curator at North Carolina State Museum of Natural Sciences in Raleigh. We are now, however, poised on the brink of a major and widespread extinction event.
Bogan does not make such remarks lightly. A lot of my work has been based on the archeological record. I examined tens of thousands of specimens covering the last 6,000 years in the Cumberland, Tennessee, Wabash, Mississippi, and Yazoo River systems. The species diversity and relative abundance are constant through time. It’s only when you get changes from European settlement that you see problems. You can see the impact of plowing, the impact from tanneries and paper mills, from sewage, from heavy industry, and from dams. By the end of the last century, experts were already reporting the ‘deaths,’ as far as the bivalves were concerned, of the Monongahela in Pennsylvania, the Pigeon River in Tennessee, and others. The problems we’re seeing are not just something that we’re seeing in the last 30 years. This has been coming for a long time.
If the 300 or so species of unionoid, the American Malacological Union considers 35 as possibly extinct and another 56 as endangered or threatened. Bogan thinks another 70 species should be considered threatened. The North American Unionoida have lost 10 percent of their species since Europeans arrived; if all the endangered mussels disappear in the next century, 54 percent will have gone extinct.
That’s a startling figure. What’s even more startling is that these days such figures don’t seem to startle many malacologists at all. Similar extinction rates scarcely raise an eyebrow among ornithologists, freshwater ichthyologists, and herpetologists. Arthur Bogan’s revised figures on the unionoid mussels certainly don’t surprise Stuart Pimm, an ecologist at the University of Tennessee at Knoxville. The depressing thing about those numbers is that they are so broadly typical of lots of other things that we look at, he says.
Pimm came to Knoxville 15 years ago, and it is a fitting place for a biologist who thinks about global patterns of extinction. His laboratory at the University of Tennessee is just up the hill from what was once the finest freshwater mussel river in America. Officially, the Tennessee River begins just east of Knoxville at the conjunction of the Holston and the French Broad Rivers. But today the Tennessee is no longer a river at Knoxville. Its tributaries flow into Fort Loudoun Lake, which transformed the upper stretch of the Tennessee from a swift-flowing river into a long pool. The lake was a boon for hydroelectric turbines and water- skiers but a disaster for mussels, since they depend on vigorous currents to supply them with food.
Pimm is a high-energy Englishman, an Oxford graduate who acquired a passion at the University of New Mexico at Las Cruces for applying mathematics to ecological problems and who became an expert on the diversity of birds in Hawaii. Now in Tennessee, he is trying to calculate nothing less than the global progress of extinction. Whenever biologists try to assess the current extinction rate, they face a daunting yet simple fact: we know only a fraction of the species alive today. The total number of species scientifically described--that is, collected, characterized, and assigned a scientific name--is only around 1.5 million. Taxonomists have estimated the undiscovered species in their various fields, and Pimm has assembled a range of them, coming up with a planetary number of 10 to 100 million species. Skeptics jump on this ignorance as evidence that the claims of a biodiversity crisis are unsupported. After all, if we have no reliable idea of how many species there are, why should we worry about losing a few owls or mussels or lilies of the field?
This line of argument infuriates Pimm. These are not just dry numbers, he says. These are real and happening things. I work in areas of the Pacific where I’ve seen species that are probably extinct now. They are declining so fast that species I could have seen easily 15 years ago, I would have a hell of a time finding now. There’s also stuff out there that I’ve spent months and months wandering around in the pouring rain looking for, and they’re not there. They’re probably gone.
But Pimm also knows that local anecdotes can’t illuminate a global phenomenon. So he recently made new estimates of extinction rates that don’t depend on first knowing the planet’s complete biodiversity. First he turned to fossils, drawing on 11 studies of fossilized marine invertebrates for a kind of actuarial table of ancient life. He found that during normal times--in other words, any time except for the five mass extinctions that have punctuated the history of life over the past 600 million years--a species typically lasts between 1 and 10 million years. Another way of saying this is that the rate of extinction is .1 to 1 species per year per million species. Pimm concedes that his estimate of the background rate of extinction is not perfect, since common creatures are more likely to be found in rocks than rare ones, and those rare ones might well be more likely to go extinct. But there is a way to double-check rates of extinction: look at the other end of the process, at the rate at which species are born. Whatever the background rates of extinction must be, they can’t be very different from the speciation rate, Pimm says. They certainly can’t be more than a factor of two or three different either way, or else we’d see dramatic losses of species or dramatic speciations. And the speciation rates agree fairly well with the extinction rate that Pimm calculated.
With an established background extinction rate, Pimm turned to recent history. He reviewed the extinctions of well-studied groups such as birds and mammals, whose total number of species is known relatively well. The most rigorous work has been done in a few extinction hot spots, areas where human impact has been relatively recent and devastating. For Pimm the dilemma is dramatized most vividly in Hawaii, where his own research has helped show that at least 100 of the 135 native birds have gone extinct since humans arrived, with another 24 in dire straits. Only 11 species have a good chance to survive into the next century, according to Pimm. Thus, if his prediction holds, some 90 percent of Hawaiian birds will have gone extinct by 2100.
Dividing the recorded extinctions of birds in hot spots like Hawaii by the 10,000 species of bird worldwide, he finds that birds are going extinct at a rate equivalent to 100 species per year per million species. The true rate is probably higher because many birds may have gone extinct during human history without our noticing, but his number serves as a good lower boundary. Surveying research on other animals in other parts of the world, Pimm discovered that this rate was not unusual. Mammals, birds, reptiles, frogs, and freshwater clams have all been suffering rates of 20 to 200 extinctions per million species per year.
Although these are only glimpses of extinction, Pimm believes they argue for a consistent global crisis. We all understand that to make calculations, we have to take samples, he explains. Sampling is an integral part of almost everything we do in science. What I’m doing in this study is taking samples, samples of birds and mussels and amphibians and reptiles and butterflies and mammals. If you take a sufficiently diverse sample from lots of different places, you can come up with a fairly consistent estimate of the rate of extinction. And that rate is terribly high: species are dying out 100 to 1,000 times faster today than in prehistoric times. Moreover, if all the animals that are currently designated as threatened go extinct in the next 100 years, that rate will multiply by a factor of ten. To Pimm, we are in mass extinction number six.
For years scientists have been trying to figure out the laws that determine why extinctions happen where they do. In the 1960s, for example, ecologists
E. O. Wilson of Harvard and Robert MacArthur of Princeton discovered a predictable ratio between the size of a habitat and the number of species it supports. Reversing the relationship, they proposed that if you destroyed part of a habitat, a certain number of species would go extinct. While this relationship has proved to be a powerful one, it does not fully capture the patterns of extinction. It suggests, for example, that small territories, such as islands, are by definition most vulnerable to extinction. But consider that while Hawaii’s island birds have suffered greatly, the islands of Great Britain have lost only three species since being inhabited by humans. The forests of the eastern United States are another puzzle: European settlers cut down 95 percent of the primeval woods, yet only four species of bird have gone extinct. The MacArthur- Wilson formula says the number should be 26.
In his tour of global extinctions, Pimm realized that the purple catspaw and other threatened species are more often than not endemics--and that endemics held the key to explaining the diversity crisis. A lot of people start to say, ‘It’s just Hawaii. It’s just Hawaiian birds. You’re just looking at something special,’ says Pimm. And I quickly began to realize that wasn’t true. What unites all these extinction hot spots, whether it’s fish in these rivers, clams in these rivers, or flowering plants in South Africa, is not that they are all growing on islands--the Mississippi valley is manifestly not an island--but the high levels of endemism. They happen to be species with small ranges, and they are uniquely vulnerable.
Endemics like the purple catspaw and other freshwater mussels inhabit small islands within the environment. When such an island-- whether an actual island in the Pacific or the rapids along a stretch of a big North American river--is drastically disturbed or destroyed, the endemics disappear. The North American mussels that have suffered most have been species, like the purple catspaw, that are adapted to big, fast- flowing rivers. Thanks to the construction of dams, that big-river habitat has almost vanished from America. The Mississippi basin may sound like a lot of territory, but big-river channels are, in truth, a narrow range from the mussel’s point of view. If you say the freshwater mussels are endemic to a third of North America, that’s a big area--certainly bigger than the level of endemicity of the salamanders that occur only in one valley in the Smoky Mountains, says Pimm. But relative to some other species--I can think of many species of vertebrate--just being found in the Mississippi drainage is quite a restricted distribution.
The vulnerability of endemics also explains the missing extinctions of the eastern United States. Of the 200 or so bird species that were living there when the first colonist swung an ax, all but 28 also live in other parts of the country. As the pioneers slowly made their way through the old-growth forests, local populations died out but could be replaced by immigrating birds as farms were abandoned and some of the forest grew back. In fact, if one considers only the 28 bird species that are endemic to the eastern United States, the predictions from the destruction of forests--4 extinctions--is spot on. In the past two years Pimm has traveled to fragmented forests with high levels of endemic species--in Indonesia, eastern Brazil, and western Kenya--and his revisions of MacArthur and Wilson’s formula are working nicely there as well. You come up with depressingly accurate predictions, he says.
Yet Pimm admits his methodology can’t predict how far or how fast this extinction crisis will go. The forces driving species into oblivion-- population growth, deforestation, pollution, invasive alien species--are too complex to be reduced to simple equations. Nor can Pimm or any other biologist offer a theory that makes sense of why endemic species show up in the places they do, like Hawaii or the Mississippi basin. The problem is not that we don’t have any explanations. The problem is that we have tons of explanations, he says. Someone once compiled them and came up with, I think, 30 different hypotheses.
And if Pimm is right, biologists may not have much time left to find the right one. Before malacologists can explain why freshwater mussels have evolved into the racehorses of endemism, for example, they first have to understand mussels’ simplest behaviors, like reproduction. After mating, a female releases millions of larvae, called glochidia, near enough to a fish that some latch onto its fins and gills. The fish themselves coat the glochidia in a capsule in which they can mature into tiny juveniles. Weeks later the larvae drop off the fish and rain down onto the river bottom. If they land on good ground--good, that is, for that species of mussel--the juveniles dig in and prosper. Yet the relationship between a particular mussel and a particular fish host is one of the hot questions in musseldom right now, given that many of the fish in these rivers have disappeared. For example, Hoggarth wants to know what kind of fish his rediscovered purple catspaw can use as a nursery.
Nor do malacologists have a good idea of how mussels draw fish into their reproductive triangle. Among some species, it appears that the females lure their fish hosts to them. Some of the things they come up with are truly remarkable, says Bogan. Three species have just been recorded that produce a meter-long gelatinous tube that comes out of the mussel’s excurrent siphon. They stick all their glochidia in a little package at the end that looks like a minnow--down to having an eyespot. This thing flops around in the water column and looks like a wounded minnow. We’ve been working on freshwater mussels for 175 years in North America and this was just reported last June.
In a world of 100 million species, the possible loss of 161 North American freshwater mussels may not seem large. But that loss--more than half the known species on the continent gone in only 300 years--is a regional catastrophe, and one that is duplicated by other animals and plants many times around the world. That’s a staggering thought, almost like stepping backward from the Killbuck Creek’s shallows into deeper water and having the strong current unexpectedly grab you around the middle. Even in this placid farmers’ river, you can’t help shuddering in a moment of fear.