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Mass extinction going on, scientists say

Aug. 12, 2008
Courtesy PNAS
and World Science staff

Dev­as­tat­ing die-offs of am­phib­ians are a sign that a “mass ex­tinc­tion” is un­der­way on our plan­et—brought on by us, two sci­en­tists say.

“Many sci­en­tists ar­gue that we are ei­ther en­ter­ing or in the midst of [Earth’s] sixth great mass ex­tinc­tion,” wrote the re­search­ers in a pa­per pub­lished on­line this week in the jour­nal Pro­ceed­ings of the Na­tional Acad­e­my of Sci­ences.

Dead Yellow-legged Frogs in Six­ty Lake Ba­sin in Si­er­ra Ne­vada, Cal­i­for­nia. The frogs died of chytrid­iomy­co­si, an am­phib­i­an dis­ease caused by a vir­u­lent fun­gus. (Im­age: Vance Vre­den­burg )


The die-offs of am­phib­ians and oth­er plant and an­i­mal spe­cies sup­port that claim, they added.

“The­re’s no ques­tion that we are in a mass ex­tinc­tion spasm,” said Da­vid Wake, a bi­olo­g­ist at the Uni­ver­s­ity of Cal­i­for­nia, Berke­ley and a co-author of the stu­dy. “Am­phib­ians have been around for about 250 mil­lion years. They made it through when the di­no­saurs didn’t. The fact that they’re cut­ting out now should be a les­son.”

New spe­cies arise and old spe­cies die off all the time, but some­times the ex­tinc­tion num­bers far out­weigh the emer­gence of new spe­cies. Ex­treme cases of this are called mass ex­tinc­tions. There have been five in our plan­et’s his­to­ry be­fore now.

The new one is dif­fer­ent—it’s ap­par­ently caused by us, Wake said. The study is co-authored by Wake and bi­olo­g­ist Vance Vre­den­burg of the uni­ver­s­ity at Berke­ley and San Fran­cis­co State Uni­ver­s­ity.

When the cur­rent ex­tinc­tion started is de­bat­a­ble, Wake said. It may have been 10,000 years ago, when hu­mans first came from Asia to the Amer­i­cas and hunt­ed many of the large mam­mals to ex­tinc­tion. It may have started af­ter the In­dus­t­ri­al Rev­o­lu­tion, when the hu­man popula­t­ion ex­plod­ed. Or, we might be see­ing the start right now, Wake said. But no mat­ter what the start date, ex­tinc­tion rates have un­de­niably dra­mat­ic­ally in­creased over the last few dec­ades, Wake de­clared.

The glob­al am­phib­i­an ex­tinc­tion is a par­tic­u­larly bleak ex­am­ple, he added. In 2004, re­search­ers found that nearly one-third of am­phib­i­an spe­cies are threat­ened, and many of the non-threat­ened spe­cies are on the wane. Wake studies am­phib­ians in the Si­er­ra Ne­vada in the Uni­ted States. The pic­ture is as grim there as else­where, he said. “We have these great na­tional parks here that are about as close as you can get to ab­so­lute pre­serves, and there have been really startling drops in am­phib­i­an popula­t­ions the­re, too,” Wake said.

Of the sev­en am­phib­i­an spe­cies in­hab­iting the Si­er­ra Ne­vada peaks, five are con­sidered threat­ened. Wake and his col­leagues ob­served that, for two of these spe­cies, the Si­er­ra Ne­vada Yellow-legged Frog and the South­ern Yellow-legged Frog, popula­t­ions over the last few years de­clined by 95 to 98 per­cent, even in highly pro­tected ar­eas such as Yo­sem­i­te Na­tional Park. Orig­i­nal­ly, frogs liv­ing atop the high­est peaks seemed to thrive, but re­cent­ly they al­so suc­cumbed he said.

There are sev­er­al frog killers in the Si­er­ra Ne­vada, Wake said. The first hint of frog de­cline in this ar­ea came in the 1990s, and re­search­ers orig­i­nally thought that rain­bow trout in­tro­duced to this ar­ea were the cul­prit­s—they like to snack on tad­poles and frog eggs. Wake’s team tried re­mov­ing trout from some ar­eas, and frog popula­t­ions started to reco­ver.

“But then they dis­ap­peared again, and this time there were car­cass­es,” Wake said. The cul­prit is a nas­ty path­o­gen­ic fun­gus that causes the dis­ease chy­trid­iomy­co­sis. Re­search­ers disco­vered the fun­gus in Si­er­ra Ne­vada frogs in 2001. Sci­en­tists have doc­u­mented over the last five years mass die-offs and popula­t­ion col­lapses due to the fun­gus in the moun­tain range.

But the fun­gus is not un­ique to Cal­i­for­nia. It has been wip­ing out am­phib­ians around the world, in­clud­ing in the trop­ics, where am­phib­i­an bio­divers­ity is par­tic­u­larly high. “It’s been called the most dev­as­tat­ing wild­life dis­ease ev­er record­ed,” Wake said.

Glob­al warm­ing and hab­i­tat con­stric­tion are two oth­er ma­jor killers of frogs around the world, Wake said. And the Si­er­ra Ne­vada am­phib­ians are al­so sus­cep­ti­ble to poi­son­ous winds car­ry­ing pes­ti­cides from Cen­tral Val­ley crop­lands. “The frogs have really been hit by a one-two punch,” Wake said, “although it’s more like a one-two-three-four punch.”

The frogs are not the only vic­tims in this mass ex­tinc­tion, Wake added. Sci­en­tists stu­dying oth­er or­gan­isms have seen si­m­i­larly dra­mat­ic ef­fects. “Our work needs to be seen in the con­text of all this oth­er work, and the news is very, very grim,” Wake said. The Na­tional Sci­ence Founda­t­ion and Na­tional In­sti­tutes of Health helped sup­port the stu­dy.


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Devastating die-offs of amphibians are a sign that a “mass exinction” is underway on our planet—brought on by us, two scientists say. “Many scientists argue that we are either entering or in the midst of [Earth’s] sixth great mass extinction,” wrote the researchers in a paper published online this week in the journal Proceedings of the National Academy of Sciences. The die-offs of amphibians and other plant and animal species support that claim, they added. “There’s no question that we are in a mass extinction spasm,” said David Wake, a biologist at the University of California, Berkeley and a co-author of the study. “Amphibians have been around for about 250 million years. They made it through when the dinosaurs didn’t. The fact that they’re cutting out now should be a lesson.” New species arise and old species die off all the time, but sometimes the extinction numbers far outweigh the emergence of new species. Extreme cases of this are called mass extinction events, and there have been only five in our planet’s history, until now. The new one is different—it’s apparently caused by us, Wake said. The study is co-authored by Wake and biologist Vance Vredenburg of the university at Berkeley and San Francisco State University. When the current extinction started is debatable, Wake said. It may have been 10,000 years ago, when humans first came from Asia to the Americas and hunted many of the large mammals to extinction. It may have started after the Industrial Revolution, when the human population exploded. Or, we might be seeing the start right now, Wake said. But no matter what the start date, extinction rates have undeniably dramatically increased over the last few decades, Wake declared. The global amphibian extinction is a particularly bleak example, he added. In 2004, researchers found that nearly one-third of amphibian species are threatened, and many of the non-threatened species are on the wane. Our own backyard provides a striking example, Wake said. He and his colleagues study amphibians in the Sierra Nevada, and the picture is grim there, as well. “We have these great national parks here that are about as close as you can get to absolute preserves, and there have been really startling drops in amphibian populations there, too,” Wake said. Of the seven amphibian species that inhabit the peaks of the Sierra Nevada, five are threatened. Wake and his colleagues observed that, for two of these species, the Sierra Nevada Yellow-legged Frog and the Southern Yellow-legged Frog, populations over the last few years declined by 95 to 98 percent, even in highly protected areas such as Yosemite National Park. This means that each local frog population has dwindled to 2 to 5 percent of its former size. Originally, frogs living atop the highest, most remote peaks seemed to thrive, but recently, they also succumbed. There are several frog killers in the Sierra Nevada, Wake said. The first hint of frog decline in this area came in the 1990s, and researchers originally thought that rainbow trout introduced to this area were the culprits—they like to snack on tadpoles and frog eggs. The UC Berkeley team did experiments in which it physically removed trout from some areas, and the result was that frog populations started to recover. “But then they disappeared again, and this time there were carcasses,” Wake said. The culprit is a nasty pathogenic fungus that causes the disease chytridiomycosis. Researchers discovered the fungus in Sierra Nevada frogs in 2001. Scientists have documented over the last five years mass die-offs and population collapses due to the fungus in the mountain range. But the fungus is not unique to California. It has been wiping out amphibians around the world, including in the tropics, where amphibian biodiversity is particularly high. “It’s been called the most devastating wildlife disease ever recorded,” Wake said. Global warming and habitat constriction are two other major killers of frogs around the world, Wake said. And the Sierra Nevada amphibians are also susceptible to poisonous winds carrying pesticides from Central Valley croplands. “The frogs have really been hit by a one-two punch,” Wake said, “although it’s more like a one-two-three-four punch.” The frogs are not the only victims in this mass extinction, Wake added. Scientists studying other organisms have seen similarly dramatic effects. “Our work needs to be seen in the context of all this other work, and the news is very, very grim,” Wake said. The National Science Foundation and National Institutes of Health helped support the study.