Smithsonian Science » biodiversity

Heliconius butterfly genome explains wing pattern diversity

John Barrat — Thu, 17 May 2012 01:25:58 +0000

Pooling funds and putting their heads together, more than 70 scientists from 9 institutions including the Smithsonian Tropical Research Institute, sequenced the entire genome of the butterfly genus Heliconius, a brightly colored favorite of collectors and scientists since the Victorian era. Their results are published in the prestigious journal, Nature.

Image right: Based on the new sequence, scientists found that different species copy each other’s wing patterns by exchanging genes, a process thought to be very rare, especially in animals. (Photo by Mathieu Joron)

The genome of the Postman butterfly, Panama’s Heliconius melpomene, helps scientists understand how the stunning diversity of wing color patterns in tropical butterflies evolved. Heliconius species are highly distasteful. Their vivid wing patterns warn predators not to eat them. How have different butterfly species evolved similar wing patterns?

Based on the new sequence, scientists found that different species copy each other’s wing patterns by exchanging genes, a process thought to be very rare, especially in animals. Although many different species interbreed in the wild, their hybrid offspring often cannot reproduce successfully. But sometimes hybrids gain useful genes that help them adapt to changing conditions. Heliconius hybrids gain wing patterns that help them survive.

Kanchon Dasmahapatra, the a lead author of the study and a former Smithsonian fellow who worked with Jim Mallet at University College London notes: “What we discovered is that one butterfly species can gain its protective colour pattern genes ready-made from a different species by hybridizing with it–a much faster process than having to evolve one’s colour patterns from scratch.”

Some of the other genes in the sequence also surprised researchers. These butterflies, typically regarded as primarily visual insects, apparently have a rich array of genes for smelling and sensing chemicals in their environment, raising new questions about the links between perception and the origins of new species. Indeed, analysis carried out at the University of California by co-author Adriana Briscoe showed that butterflies have an even greater array of genes involved in chemical communication than moths, which depend on chemical signals for finding mates and host plants.

The study heralds a new era in genome biology and an important step in the Smithsonian’s goal to understand and sustain a biodiverse planet. Low-cost genetic sequencing opens doors to small research groups and individuals to sequence entire genomes, a technique formerly accessible only to labs with major government funding.

“Assembling a genome from scratch is still hard work: think Humpy-Dumpty,” said Owen McMillan, geneticist and Academic Dean at the Smithsonian Tropical Research Institute, “but it is getting easy, inexpensive, and is transforming how we do science. At the core, having a reference genome opens up new research possibilities and reveals previously unimagined connections.

Heavyweight trees are forest champs at sequestering carbon

John Barrat — Thu, 03 May 2012 16:36:41 +0000

Big trees three or more feet in diameter accounted for nearly half the biomass measured at a Yosemite National Park site, yet represented only one percent of the trees growing there.

This means just a few towering white fir, sugar pine and incense cedars per acre at the Yosemite site are disproportionately responsible for photosynthesis, converting carbon dioxide into plant tissue and sequestering that carbon in the forest, sometimes for centuries, according to James Lutz, a University of Washington research scientist in environmental and forest sciences. Lutz is lead author of a paper on the largest quantitative study yet of the importance of big trees in temperate forests being published online May 2 on PLoS ONE. The research was funded by the Smithsonian Center for Tropical Forest Science.

Image right: A handful of large-diameter trees per acre, such as these incense cedars, together with remains of big trees like the three-foot-wide white fir snag and downed debris account for half the forest biomass at a Yosemite National park study site. (Image by James Lutz)

“In a forest comprised of younger trees that are generally the same age, if you lose one percent of the trees, you lose one percent of the biomass,” he says. “In a forest with large trees like the one we studied, if you lose one percent of the trees, you could lose half the biomass.”

In 2009, scientists including Lutz reported that the density of large-diameter trees declined nearly 25 percent between the 1930s and 1990s in Yosemite National Park, even though the area was never logged. Scientists have found notable numbers of large trees dying in similar areas across the West.

The new 63-acre study site is one of the largest, fully-mapped plots in the world and the largest old-growth plot in North America. The tally of what’s there, including the counting and tagging of 34,500 live trees, was done by citizen scientists. The site is part of the network of the Smithsonian Center for Tropical Forest Science, a global network of 42 tropical and temperate forest plots including the one in Yosemite.

Image left: Washington State University’s Mark Swanson pulls a tape tight around a 4-foot-wide sugar pine, one of the 34,500 live trees counted and tagged for long-term study in a Yosemite National Park study plot. (Washington State University)

One implication of the research is that land managers may want to pay more attention to existing big trees, the co-authors said. In some younger forests that lack big trees, citizens and land managers might want to consider fostering the growth of a few big-trunked trees, Lutz added.–Source: University of Washington.

Not on a plane, but how did blind snakes ever get to the Pacific’s Caroline Islands?

John Barrat — Wed, 25 Apr 2012 14:01:52 +0000

Two new species of blind snakes found living on small, low-lying atolls in the Caroline Islands are an unexpected discovery that is quite difficult to explain, say scientists studying the reptiles. The new snakes were announced recently in a paper in the journal Zootaxa co-authored by Addison Wynn, a herpetologist at the Smithsonian’s National Museum of Natural History, and colleagues from the U.S. Geological Survey, the College of Micronesia and the Yap Institute of Natural Sciences. The Caroline Islands are located in Micronesia, an area north of the equator and far west of Hawaii.

Both snakes (newly named Ramphotyphlops hatmaliyeb and Ramphotyphlops adocetus) are believed to be indigenous to islands that only formed in the last 2,000-years. These islands are surrounded by water and have never had any connection to land.

“The question is,” Wynn says, “how did they get there?” The only other terrestrial snake found in the Caroline Islands east of Palau is a third blind snake species known to have been introduced by humans years ago.

“These new species extend the known range of blind snakes some 2,000 kilometers out into the Pacific Ocean, into areas where we didn’t know they occurred or could ever occur. We just didn’t expect to find blind snakes out there in the middle of the ocean,” Wynn says.

Image right: The new blind snake species “Ramphotyphlops hatmaliyeb.” (Photo courtesy Marjorie Falanruw)

Blind snakes live underground, are typically four or five inches long and easily mistaken for large worms. “They eat termites and small ants, and there are about 240 or so known species in the world,” Wynn says. “They spend their lives burrowing so their head is blunt and pointed to push their way through the soil. Their rudimentary eyes can only differentiate between light and dark and exist as pigment spots underneath scales on their head.”

When we first received a single specimen from the island of Ant Atoll, in the Carolines, Wynn explains, “we initially thought it must be a waif, brought by humans aboard a ship, because it was so unexpected.” As the scientists received additional specimens they were faced with the fact that the snakes are a species indigenous to the Caroline Islands. One hypothesis to their presence is that they are a relic population that somehow survived on temporary cays, reefs, platforms and other oceanic structures for thousands of years until the islands on which they now live were formed.

Image right: Top and side photographs of the head of the newly named blind snake species “R. adocetus” with accompanying illustration showing the unique scale pattern that scientists use to identify them as to species.

Because different species of blind snakes look so similar in appearance, an expert is normally required to tell species apart. Physical characteristics used to distinguish them are counts of the number of scales found along the back of the animal or scale rows around the animal’s body, shape and position of scales on the head, and color pattern.

Article link: “The unexpected discovery of blind snakes (Serpentes: Typhlopidae) in Micronesia: two new species of Ramphotyphlops from the Caroline Islands,” by Addison Wynn, Robert Reynolds, Donald Buden, Marjorie Falanruw and Brian Lynch appeared in the journal Zootaxa.

New ‘Bumblebee’ gecko discovered in Papua New Guinea

John Barrat — Wed, 18 Apr 2012 14:58:35 +0000

Biologists from the Papua New Guinea National Museum and the U.S. Geological Survey have discovered a new species of gecko, adorned like a bumblebee with black-and-gold bands and rows of skin nodules that enhance its camouflage on the tropical forest floor.

Specimens of the lizard, which measures about 5 inches from head to tail, were collected in May 2010 in Sohoniliu Village on Manus Island in Papua New Guinea. Herpetologists George Zug of the Smithsonian’s National Museum of Natural History and Rober Fisher of the USGS Western Ecological Research Center described the new species in a report published in “Zootaxa” this month.

“We’ve officially named it Nactus kunan for its striking color pattern — kunan means ‘bumblebee’ in the local Nali language,” Fisher says. “It belongs to a genus of slender-toed geckos, which means these guys don’t have the padded, wall-climbing toes like the common house gecko, or the day gecko in the car insurance commercials.”

Fisher found two individuals of the bumblebee gecko on Manus Island in 2010 and analyzed their genetics to show that the lizards were new and distinctive. Two additional species were found that trip, and the specimens await further analysis.

“This species was a striking surprise, as I’ve been working on the genus since the 1970s, and would not have predicted this discovery,” Zug says.

“Exploration of Manus Province is in its infancy, with many new species possible, and this joint expedition was our first to this region,” says Bulisa Iova, the reptile curator at the Papua New Guinea National Museum.

This research on Pacific lizard biodiversity was supported by the Smithsonian, U.S. Department of Defense and USGS. USGS regularly collaborates on biological surveys with partner nations, as part of its mission to provide scientific information that help government managers address critical natural resource issues.–Source: USGS

Development will reduce carbon stored in forests, Smithsonian & Harvard scientists predict

John Barrat — Mon, 16 Apr 2012 13:07:11 +0000

When most people look at a forest, they see walking trails, deer yards, or firewood for next winter. But scientists at the Harvard Forest and the Smithsonian take note of changes imperceptible to the naked eye — the uptake and storage of carbon. What they’ve learned in a recent study is that an immense amount of carbon is stored in growing trees, but if current trends in Massachusetts continue, development would reduce that storage by 18 percent over the next half century. Forest harvesting would have a much smaller impact.

Jonathan Thompson is Research Ecologist at the Smithsonian Conservation Biology Institute, Research Associate at the Harvard Forest, and lead author on the paper which appeared in the journal Ecological Applications in late 2011. “The rebounding forests of New England provide a tremendous public benefit by storing carbon that would otherwise contribute to climate change,” said Thompson. To put these findings into context he adds, “In Massachusetts, forests capture approximately 2.3 million metric tons of carbon each year. That’s equal to the amount of carbon dioxide emitted from the energy used by one million American homes annually.” He and his coauthors were able to estimate the extent to which development may chip away at that carbon sink, using an uncommon collection of long-term data and a distinct form of research known as scenario science.

Image right: From this 71-foot eddy-flux tower in a 200-year-old hemlock forest, Harvard Forest scientists have measured carbon dynamics and other ecosystem processes for more than 20 years as part of the Long-Term Ecological Research program. Located in a 35-hectare Smithsonian Global Earth Observatory plot and part of the core measurements for the National Ecological Observatory Network, this tower is a focal point for studies of the eastern hemlock tree and its impending demise from the invasive hemlock wolly adelgid, as well as phenology studies of succeeding hardwoods.
(Photos by David Foster)

For more than 30 years, scientists at the Harvard Forest have scaled towers into the forest canopy and measured the trunks of trees to track how much carbon is stored or lost from the woods each year. This treasure trove of data is part of the national Long-Term Ecological Research (LTER) Network, which is celebrating more than three decades of research this month. This important milestone is marked by six new papers released today in a special issue of the journal BioScience. The forest carbon research is one example of participatory scenario science — a growing trend in ecology featured in a paper by Thompson, David Foster, Director of the Harvard Forest, and their colleagues in the BioScience issue.

Image left: Summer Research Program students monitor soil respiration of decaying wood in a large study comparing carbon, water, and energy fluxes between harvested and unharvested sites.

Harvard Forest is one of four LTER sites in the northeastern U.S. and was awarded a grant by the National Science Foundation to join the Network in 1988. David Foster coauthored the Ecological Applications paper of 2011 and co-edited the new BioScience special issue. He notes, “With three decades of data meticulously collected as part of the LTER Network, we have reached a crucial transition where we are now able to tackle major environmental challenges, such as the fate of forest carbon, across large landscapes.”

Foster adds, “Over the last two centuries, forests have stored more carbon with each passing year in many parts of New England, but the turning point may be in sight for Massachusetts and other urbanizing landscapes if recent development trends continue.” But that’s not the end of the story for Foster: “The good news is that forests are resilient and history is not necessarily destiny. Our research makes a compelling case for expanding support for forestland protection and for the efforts of private landowners to keep their land forested. It reminds us that forests provide important infrastructure that we should invest in, just as we do major civil works projects.”

Preventing home invasions means fighting side-by-side for coral-dwelling crabs and shrimp

John Barrat — Fri, 30 Mar 2012 16:38:30 +0000

As any comic book lover knows, when superheroes band together the bad guys fall harder. The strength that comes in numbers is greater than the sum of its parts.

The same holds true, researchers have recently learned, when different species of crabs (genus Trapezia) and snapping shrimp (Alepheus lottini) in the central Pacific band together to defend their coral homes from hungry seastars. In these frequent conflicts “one-plus-one doesn’t always equal two, sometime it is more,” explains Seabird McKeon, a marine biologist at the National Museum of Natural History’s Smithsonian Marine Station in Fort Pierce, Fla. The crustaceans are much more effective when they fight together than when they fight alone, a process McKeon calls the Multiple Defender Effect. “It is a clear example of synergy, and one that underscores the importance of biodiversity in the ocean.”

Image right: A cushion seastar, bottom foreground, approaches a coral head of the genus Pocillopora looking for a meal. (Photos courtesy Seabird McKeon)

Even in a comic book one would be hard pressed to find an enemy more bizarre than the “cushion” seastar (Culcita novaeguineae), an animal used by McKeon in recent laboratory experiments with living corals (genus Pocillopora) and their defenders. To consume a coral, the seastar pushes its stomach outside its body and lays it over the coral like a cushion. It then hugs the coral close and “eats,” letting stomach acids and digestive juices do their work.

Image: Trapezia serenei, a tiny coral-dwelling crab.

The stationary coral is defenseless, yet the tiny crustaceans that live among its branches come to its aid, snipping and prodding an intruding seastar with their claws. “The coral itself is like a cauliflower head, a main central stem and lots of little branches,” McKeon explains. “Crabs gain protection from fish by living inside the coral structure.”

Once a mating pair of crabs takes-up residence on a coral head they do not tolerate the presence of other crabs of their same species. Crabs of other species however are ignored, as are snapping shrimp. As a result, some coral heads may have as many as five different species of defensive crustaceans living on them, all pairs of different species.

Image right: Alpheus lottini, a snapping shrimp.

In repeated experiments McKeon and colleagues measured the effectiveness of a single crab pair in preventing a seastar from eating their home coral. He found that one pair of crabs reduced the volume of coral eaten by about 19 percent, compared to a coral with no defenders. Two pairs of crustaceans working together, however, were able to reduce the volume of coral eaten by as much as 65 percent—the multiple defender effect.

The take-home lesson here, McKeon says, “is these crabs don’t allow others of their same species on their coral, yet the synergy of different pairs fighting together is critical to the defense of the coral. The multiple defender effect is an important new angle on why we must conserve biodiversity in the ocean.”

“Multiple defender effects: synergistic coral defense by mutualist crustaceans,” by C. Seabird McKeon; Adrian C. Stier of the University of Florida; Shelby McIlroy of the University at Buffalo and Banjamin Bolker of McMaster University; appeared recently in the scientific journal Oecologia.

New species of deep-sea catshark described from the Galapagos

John Barrat — Wed, 07 Mar 2012 20:08:01 +0000

Scientists conducting deep-sea research in the Galapagos have described a new species of catshark, Bythaelurus giddingsi, in the March 5 issue of the journal Zootaxa. The new shark is approximately a foot long and has a chocolate-brown coloration with pale, irregularly distributed spots on its body. The spotted patterns appear to be unique to each individual.

The findings were published in the article “Description of a new species of deepwater catshark, Bythaelurus giddingsi sp. nov., from the Galapagos Islands (Chondrichthyes: Carcharhiniformes: Scyliorhinidae),” co-authored by John McCosker and Douglas Long of the California Academy of Sciences, and Carole Baldwin of the Smithsonian’s National Museum of Natural History, It appeared int Zootaxa 3221: 48-59.

Image right: Bythaelurus giddingsi is a new species of deep-sea catshark from the Galapagos.

McCosker collected the first specimens of this new catshark while diving to depths of 1,400-1,900 feet aboard the Johnson Sea-Link submersible. “The discovery of a new shark species is always interesting, particularly at this time when sharks are facing such incredible human pressure,” said McCosker, lead author on the paper. “Many species have become locally rare and others verge on extinction due to their capture for shark-fin soup. The damage to food webs is dramatic, since sharks provide valuable ecological services as top-level predators—when they disappear, their niche is often filled by other species that further imbalance ecosystems. Most deepwater shark species are not very susceptible to overfishing; however, since this catshark’s range is restricted to the Galapagos, its population is likely limited in size, making it more susceptible than more widely distributed species.”

Image left: This photo shows John McCosker climbing into the Johnson Sea-Link submersible in the 1990s during a Galapagos expedition. (Photos California Academy of Sciences)

In the 1990s, McCosker and Baldwin made a series of dives inside the submersible Johnson Sea-Link to explore the marine life on the islands’ steep volcanic slopes and sandy bottoms. Submersibles allow scientists to explore a vast part of the Galapagos that was not accessible to Charles Darwin or earlier Academy scientists. It was during two such dives in 1995 and 1998 that the seven specimens used to describe B. giddingsi were collected.–Source: California Academy of Sciences

Fancy footwork and non-stick leg coating helps spiders not stick to their own webs

John Barrat — Thu, 01 Mar 2012 13:21:19 +0000

Researchers at the Smithsonian Tropical Research Institute and University of Costa Rica studying why spiders do not stick to their own sticky webs have discovered that a spider’s legs are protected by a covering of branching hairs and by a non-stick chemical coating. Their results are published online in the journal, Naturwissenschaften.

Image right: This mature female golden silk spider had just contacted the sticky line with her right leg IV and was about to extend this leg, thereby pulling additional line from her spinnerets. (Photo by C. Frank Starmer)

They also observed that spiders carefully move their legs in ways that minimize adhesive forces as they push against their sticky silk lines hundreds to thousands of times during the construction of each orb.

The web-weaving behavior of two tropical species, Nephila clavipes and Gasteracantha cancriformis, was recorded with a video camera equipped with close-up lenses. Another video camera coupled with a dissecting microscope helped to determine that individual droplets of sticky glue slide along the leg’s bristly hair, and to estimate the forces of adhesion to the web. By washing spider legs with hexane and water, they showed that spiders’ legs adhered more tenaciously when the non-stick coating was removed.

( “Spiders avoid sticking to their webs: clever leg movements, branched drip-tip setae, and anti-adhesive surfaces” by R.D. Briceño and W.G. Eberhard. 2012. Naturwissenshaften. DOI 10.1007/s00114-012-0901-9. Published online: 1 March 2012.)

Earthworms to blame for decline of Ovenbirds in northern Midwest forests, study reveals

John Barrat — Wed, 29 Feb 2012 13:33:55 +0000

A recent decline in Ovenbirds (Seiurus aurocapilla), a ground-nesting migratory songbird, in forests in the northern Midwest United States is being linked by scientists to a seemingly unlikely culprit: earthworms.

A new survey conducted in Minnesota’s Chippewa National Forest and Wisconsin’s Chequamegon-Nicolet National Forest by a research team led by Scott Loss of the University of Minnesota and the Smithsonian Migratory Bird Center has revealed a direct link between the presence of invasive European earthworms (Lumbricus spp.) and reduced numbers of Ovenbirds in mixed sugar maple and basswood forests. The results are detailed in a paper published on-line in the scientific journal “Landscape Ecology.”

Image right: An ovenbird. (Photo by Simon Pierre Barrette)

European earthworms are invading previously earthworm-free hardwood forests in North America the scientists say, and consuming the rich layer of leaf litter on the forest floor. In turn, herbaceous plants that thrive in thick leaf litter and provide cover for ground-nesting birds are thinning out, replaced by grasses and sedges. As a result, Ovenbird nests are more visible and vulnerable to predators and Ovenbirds searching for nesting sites reject these low-cover areas outright. Areas of reduced leaf litter also contain fewer bugs for the Ovenbirds to eat, requiring them to establish larger territories, resulting in fewer birds over a given area.

Image left: A forest without earthworms has a rich understory of herbaceous plants, tree seedlings, and shrubs, and a thick, spongy leaf litter layer. (Photo by Scott Loss)

The worms invading northern Midwestern forests (as well as forests in the northeastern U.S. and Canada) have been in the United States since soon after the first European settlers arrived, Scott Loss explains, brought over inadvertently in the ballast of ships, in the root balls of agricultural plants or on purpose for use in gardening. Only now is the leading edge of their continued invasion, caused mainly by logging activities and fishermen dumping their bait, reaching interior wilderness areas such as parts of the study site in the remote forests of Wisconsin and Minnesota. “Night crawlers [Lumbricus terrestris] and the slightly smaller red worms [also called leaf worms or beaver tails, Lumbricus rubellus], have the most damaging impacts to the soil, litter layer, and plants in forests that were historically earthworm-free,” Loss says.

Image right: A forest experiencing heavy earthworm invasion often has few remaining herbaceous plants and seedlings, no intact litter layer, and extensive patches of bare soil. (Photo by Scott Loss)

“Everyone has probably heard at one time or another that earthworms have really positive effects in breaking down soil and making it more porous,” Loss explains. “This is true in agricultural and garden settings but not in forests in the Midwest which have developed decomposition systems without earth worms.” Because the forested areas of the Midwest United States were once covered in glaciers, there are no native earthworm species present in the soil, Loss explains. “These earthworm-free forests developed a slow fungus-based decomposition process characterized by a deep organic litter layer on the forest floor.”

Image left: An Ovenbird incubating eggs on a nest. (Photo by Scott Loss)

Earthworms feed on this layer of leaf litter and make it decompose much faster, Loss says. “As a result, we see the loss of sensitive forest-floor species such as trillium, Solomons seal, sarsaparilla and sugar maple seedlings and a shift in dominance to disturbance-adapted species like Pennsylvania sedge.

One result is reduced nest concealment for the Ovenbird and increased predation by squirrel and bird predators.

The researchers found no decline in three other species of ground-nesting birds included in their survey—the Hermit Thrush (Catharus guttatus), Black-and-White Warbler (Mniotilta varia) and Veery (Catharus fuscescens)—nor did they find a correlation between Ovenbird decline and invasive worms in other forest types, such as red oak, paper birch and aspen.

Image right: Scott Loss uses a liquid-mustard mixture to sample earthworms. The mustard contains a skin irritant that causes earthworms to come to the surface. (Photo by Sara Schmelzer Loss)

“Our results suggest that Ovenbird density may decline by as much as 25 percent in maple-basswood forests heavily invaded by invasive earthworms,” the researchers conclude. “Maple-basswood forests are among the preferred ovenbird habitats in the region, comprise a considerable portion of the region’s woodlands…and are experiencing Lumbricus invasions across most of the northern Midwest.” Previous studies have demonstrated that invasive earthworms also are harmful to other native North American species, such as salamanders.

Image left: Ovenbird nests in earthworm-free forests (left, arrow pointing to nest opening) are well-concealed. In areas with invasive earthworms (right), nests are less concealed and therefore more vulnerable to predators. (Photos by Scott Loss)

There is reason for concern that the overall population of Ovenbirds could decline, Loss points out. “Ovenbirds migrate to Central America and the Caribbean and back every year, a trip during which they can fly into buildings and towers or get nabbed by a cat as they rest on the ground, and they also face loss of habitat on their breeding and wintering grounds. Now, here is yet another potential threat to their survival.”

The Smithsonian Migratory Bird Center is dedicated to fostering greater understanding, appreciation, and protection of the grand phenomenon of bird migration. Founded in 1991, we are located at the Smithsonian National Zoological Park in Washington, D.C.--John Barrat

“Invasions of non-native earthworms related to population declines of ground-nesting songbirds across a regional extent in northern hardwood forests of North America,” was co-authored by Scott R. Loss, Gerald J. Niemi and Robert B. Blair of the University of Minnesota.

Boom and bust cycle of marine biodiversity every 60 million years linked to uplifting of continents

John Barrat — Thu, 23 Feb 2012 18:53:49 +0000

A mysterious cycle of booms and busts in marine biodiversity over the past 500 million years could be tied to a periodic uplifting of the world’s continents, scientists report in the March issue of The Journal of Geology.

The researchers discovered periodic increases in the amount of the isotope strontium-87 found in marine fossils. The timing of these increases corresponds to previously discovered low points in marine biodiversity that occur in the fossil record roughly every 60 million years. Authors of the study are Adrian Melott, professor of physics and astronomy at the University of Kansas, paleobiologist Richard Bambach of the Smithsonian’s National Museum of Natural History, Kenni Petersen of Aarhus University, Denmark, and John McArthur of University College London.

Image right: Yosemite Valley and Half Dome from Glacier Point. (Photo by Jon Sullivan)

Melott, lead author, thinks the periodic extinctions and the increased amounts Sr-87 are linked. “Strontium-87 is produced by radioactive decay of another element, rubidium, which is common in igneous rocks in continental crust,” Melott says. “So, when a lot of this type of rock erodes, a lot more Sr-87 is dumped into the ocean, and its fraction rises compared with another strontium isotope, Sr-86.”

An uplifting of the continents, Melott explains, is the most likely explanation for this type of massive erosion event.

“Continental uplift increases erosion in several ways,” he said. “First, it pushes the continental basement rocks containing rubidium up to where they are exposed to erosive forces. Uplift also creates highlands and mountains where glaciers and freeze-thaw cycles erode rock. The steep slopes cause faster water flow in streams and sheet-wash from rains, which strips off the soil and exposes bedrock. Uplift also elevates the deeper-seated igneous rocks where the Sr-87 is sequestered, permitting it to be exposed, eroded, and put into the ocean.”

The massive continental uplift suggested by the strontium data would also reduce sea depth along the continental shelf where most sea animals live. That loss of habitat due to shallow water, Melott and collaborators say, could be the reason for the periodic mass extinctions and periodic decline in diversity found in the marine fossil record.–Source: University of Chicago Press Journals