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Mystery of the Missing WHODUNNIT ? astodons, saber-toothed cats, wooly and enormous ground —these creatures and Mother giants roamed the earth for hundreds of thousands of . Then, during the most recent ice age, they abruptly disappeared. Between ten and fifty thousand years ago, two thirds of the genera of large alive on earth died. Now we may be in the midst of another great , and biologists fear that our most charismatic creatures, like the giant panda, the , and the African , are about to meet the same fate. At Berkeley, researchers are working to unravel the root causes of large , in hopes of learning how to protect currently threatened megafauna. One group is digging into the fossil record, investigating the origins of the late extinctions, while another is trying to understand the causes of present-day extinctions by looking in detail at a single species that roams the southwestern deserts, the California bighorn sheep. What these researchers find may help us mitigate the

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by Ruth Murray-Clay

30 berkeley science review Anthony Barnosky, a paleobiologist However, alone cannot . In , the first in Berkeley’s Integrative Biology account for their severity. During at to reach the continent, the department, studies the Pleistocene, least some of the major heating and Clovis-style hunters, arrived within 1500 which lasted from 10,000 to 1.8 million cooling events of the earlier Pleistocene, years of the majority of the extinctions years ago, in an effort to understand in the area. In Africa, where humans the mystery of the large mammal evolved with large animals rather than extinctions. This epoch of geologic invading as fully developed hunters, history was characterized by hugely extinction rates were lowest, affecting unstable climate, as the planet 18 percent of mammalian megafauna oscillated between frigid “ice ages” genera. lost 36 percent, and and warmer “interglacial” climates North and , where the more than twenty times. Many of first inhabitants were already the plants and animals living in the skilled hunters, lost 72 and 83 percent Pleistocene were similar to those respectively. In the , , inhabiting the planet today, but , and , most large, the epoch’s large mammal diversity slow-breeding animals that did survive was notably greater: mammoths occupied territories inconvenient for and , saber-toothed cats humans—they were nocturnal or they and dire wolves, giant beavers and lived in the deep forest, in trees, or on , and many others roamed LEFT: Male “Irish elk’” (Megaloceros giganteus), the globe. skeleton on display in the Geology Most of these behemoths persisted Department at Trinity College, Dublin. This dublin through the climate swings from Eurasian megafaunal species took three , one ice age to the next. But during thousand years to dwindle to extinction. the most recent ice age, between college

10,000 and 50,000 years ago, the large mammals went extinct in

rinity record numbers. At least 97 of the world’s approximately 150 genera RIGHT: The , or “saber-toothed ”, is the containing animals weighing more than California state fossil. These 44 kilograms disappeared, with the predators were not quite as most striking losses concentrated in the long as modern-day , but weighed about twice as Americas and Australia. What killed much. Fossils have been found them? The unusual severity of these all over North America and extinctions is the subject of a heated Europe; they went extinct about 10,000 years ago. debate between those who believe that climate change caused the losses and those who think that humans were the number of small, medium, primarily responsible. and large animals found in The extinctions began North America remained with the onset of the latest ice age and roughly constant, and none of lasted until the transition to the current the earlier ice ages caused the interglacial period. In northern high widespread extinctions characterizing tall mountains. Finally, the extinction and mid-latitudes (in Europe, Siberia, the most recent ice age. Global oxygen affected large, slow-breeding species and Alaska), two pulses of extinctions isotope records show that the rapid preferentially, rather than hitting small occurred. As the planet cooled between deglaciation at the end of the late animals first and indirectly affecting 20,000 and 45,000 years ago, warm- Pleistocene was neither faster nor more large animals through their food supply, adapted species were lost, and as the extreme than other similar events over as would be expected if climate change planet warmed between 9,000 and the past 700,000 years. were the primary culprit. For many 12,000 years ago, cold-adapted species Another major change was occurring scientists, these data suggests that disappeared. These correlations suggest on Earth during the humans were a significant factor in the

SMILODONa COURTESY causal UC MUSEUM OFrole PALEONTOLOGY/ for IRISH climate ELK COURTESY OF T change in the that differentiated it from prior ice late Pleistocene extinctions. extinctions. ages: humans were dispersing far from Barnosky and his collaborators

berkeley science review 31 A bighorn ewe in Robert Feranec and Alan Shabel the Old Woman from Berkeley, Paul Koch from mountains approaches a water UC Santa Cruz, and Scott Wing hole on a day when from the Smithsonian Museum of temperatures Natural History, summarize these reached 110 deg F. Bighorns depend on arguments in an October 2004 springs, tinajas, and review in Science. They suggest, water catchments however, that the conclusion to survive the long hot summers in the pegging humans as the cause of the Mojave. late Pleistocene is too simplistic. They argue that a closer look at perkins extinction events worldwide suggests big was

that while human impacts generated julia putting

the extinctions, climate change played a of

vital role in determining their regional together the differences, timing, and perhaps extent. two causes,” In their paper, the authors review the Barnosky courtesy evidence for causal factors producing says. extinction on a region-by-region basis. Barnosky and the members of his lab remained the same, with only For instance, they note that in Australia, have been contributing to the extinction minor adjustments during periods of extinctions of animals like the debate by studying the effect of climate climate change. In addition, they found occurred after humans arrived change on and evolution. that the “fine-tuning” of the ecosystem during a period of little climate change, In a paper published in the Proceedings generated by these minor adjustments while in Alaska, mammoths died of the National Academy of Science occurred by affecting species of smaller out during periods of climate change in June of 2004, Barnosky, Shabel, size and at lower trophic levels. These when very few humans were present. and collaborators including former findings differ markedly from the th In Eurasia, the evidence increasingly graduate student Christopher Bell use changes seen in the area during the 19 th suggests that Pleistocene climate change fossil evidence gathered at Porcupine and 20 centuries. Human has caused large mammals to shift their Cave in to study the effect targeted large animals and carnivores, ranges, contributing to their extinction. on mammal populations of glacial- changing the ecosystem from the top Eurasian extinctions also coincide with interglacial transitions from 600,000 down, causing the preferential loss of the spread and then an increase in to 1,000,000 years ago in the mid- large mammals, as was also observed in population sizes of modern humans. Pleistocene, long before humans arrived. the late Pleistocene. These populations may have impacted Mid-Pleistocene sites are less common What caused the double punch of megafauna as early as 30,000 years ago. than those representing the late humans and climate change to be so In North America, where dating is Pleistocene, and the differences between effective? Barnosky’s research suggests most robust, the case for an interaction them can help separate the effects of that humans directly affect large between both climate and human climate from the effects of humans. The animals through hunting or some other impacts is clear. Within 1500 years of group used the sediments in a sequence mechanism, while climate change affects the arrival of the Clovis hunters, who of mid-Pleistocene fossils to identify small animals first through their plant traveled from across the Bering glacial and interglacial conditions, and food supplies. To better understand how land bridge to North America roughly followed the area’s species composition these mechanisms work in our current 14,000 years ago, at least 15 species of and richness over transitions in the world, Clint Epps, a recent graduate megafauna went extinct in the region, Pleistocene, comparing their results of the department of Environmental including mammoths, mastodons, with species composition in historical Science, Policy and Management, has rhinos, saber-toothed , and giant and modern times. The group found focused on megafauna currently living ground sloths. The arrival of humans that the mammal community near in California: the desert bighorn sheep. and their practice of overkill coincided Porcupine Cave was structurally and Desert bighorn sheep (Ovis canadensis with a major climate change event, and functionally stable over the long periods nelsoni) live in small populations, Barnosky argues that if first contact of time preceding the historical period. usually fewer than 100 individuals, in had been significantly earlier or later, The species present in the region the arid mountain ranges of the Mojave, the extinction event would have been changed, but the species richness and Sonoran, and Great Basin deserts of the much less dramatic. “What made the functional relationships in the southwestern United States. Brown with

32 berkeley science review white highlights on the nose and rump, and human-related the sheep range in length from 150 to characteristics such 180 cm (5 to 6 feet) , and are armed as proximity to with spectacular thick, curved horns. towns and cities, Though they weigh up to 127 kg (280 , mining, lbs), the sheep jump and climb with and contact with enviable dexterity up steep mountain livestock that may slopes, protecting themselves from have carried disease. predators such as the mountain lion. The researchers The estimated 56 populations of desert were surprised to bighorn sheep in California occupy find that sheep

many small, isolated mountain ranges, extinctions were gwin separated by uninhabitable desert. most closely While each population on its correlated with local ren own has a low chance of long-term climate conditions. L A U of survival in this harsh environment, According to Clint Epps, recent graduate of the Department of the metapopulation, or system of climate records for the southwestern Environmental Science, Policy and Management, small populations, is more stable if desert region, the mean annual uses a spotting scope to pick bighorn sheep out from their surroundings. there is good connectivity between the temperature rose 1 degree Celsius courtesy individual groups. If one population (1.8 degrees Fahrenheit) from 1901 obstacle for sheep survival. In their quest dies out, individuals from neighboring to 1987, while, over the past century, to find new mountain range habitat, populations can recolonize the emptied annual precipitation dropped by about sheep must cross the broad and bleak habitat, ensuring the continuity of the 20%. Those places where the climate desert flatlands, which species in the region. was warmest and driest suffered the offer a suite of other threats. Epps, a lifelong outdoorsman and greatest number of sheep extinctions. In addition to increased exposure to self-described “hillbilly,” spends his Future changes are predicted to be more predators, dispersing bighorn sheep may summers camping in the sheep-occupied severe. encounter human-made barriers such mountain ranges of the Mojave Desert. Desert bighorn sheep numbers as interstate highways that are difficult Despite daytime temperatures that can decline in response to decreased rainfall or impossible to cross. In addition to reach 125° F (52° C), frequent vehicular because desert ecosystem productivity analyzing population extinction and the breakdowns, and extreme isolation, the is limited by available water. In general, effects of climate on diet quality, Epps desert can be rewarding during the hot the more it rains, the more desert collected pellets for analysis of fecal months. As Epps explains: “Summer plants will grow. Temperature increase, DNA. After an analysis of the genetic is the best time to find sheep in the however, can help or hurt sheep diversity of 27 populations of desert Mojave. The hotter the better—when survival, depending on timing. Warmer bighorn sheep, the group concluded they cluster near water, it’s much easier springs and summers lead to lowered that new highways and other human- to collect data on the sheep.” diet quality as plants wither in the heat, made barriers effectively isolated sheep In a paper published in the journal while warmer winters lead to earlier populations, halting gene flow across the Conservation Biology in February 2004, plant growth and higher diet quality in barriers and significantly reducing the Epps and his colleagues, including his the spring, a critical time for the survival genetic diversity of the groups in only advisor, professor Dale McCullough, of bighorn lambs. 40 years. examined current and historic sheep The sensitivity of sheep populations The California bighorn sheep case populations. Using data from the to climate change may result from their illustrates what may happen to other last 60 years, they found that thirty geographical distribution. While other large mammal species. As global of the eighty populations of sheep large mammals may gradually shift their temperatures change, a species’ habitat known to live in southern Californian territories north in response to global may shift from one place to another. mountain ranges over that period warming, bighorn sheep have a limited To survive, the animal must move with are now extinct. To determine the ability to move to new environments. its habitat. But human settlements primary causes of these extinctions, the Often, potential new ranges are and barriers can be as difficult to cross researchers looked at each mountain uninhabited for good reasons (like as the forbidding desert, and many range hosting a current or historic sheep higher temperatures and lower rainfall). animals will be deprived of a way to populations, and considered factors Complicating the situation, Epps and follow their preferred environments. As like annual precipitation, temperature, his colleagues found another significant a result, many animals face extinction berkeley science review 33 from climate change. This situation direct human impacts such as habitat extinction events were recolonized, but is particularly acute for desert species, fragmentation are occurring at rates these areas were not isolated by roads or which often live in naturally fragmented unprecedented in recorded history, human settlements. Whether or not the habitats: in addition to the desert sheep, leading Barnosky and his collaborators bighorn sheep in California ultimately the huge desert tortoises are also having to warn that climate change should survive, both studies clearly warn that difficulty. be considered a serious threat to the dual pressures of climate change and Barnosky emphasizes this point. large animal populations. If the late direct human impact may be too much “Animals respond to ‘normal’ climate Pleistocene may be taken as a guide, the for today’s megafauna to . change,” he explains, “by tracking their combination of direct human impacts climate across the landscape. In order and climate change is particularly Ruth Murray-Clay is a graduate for that to work, their life history has dangerous and could catalyze extinctions student in astronomy. to be such that they can move with massive enough to destabilize global their climate. There has to be habitat to ecosystems. Want to know more? Check out “Assessing the causes of Late pass through.” But In his studies of bighorn sheep, Pleistocene extinctions on the continents”: and fragmentation challenge nearly Barnosky says, Epps could easily be Barnosky et al, . Science 306:70-75 (2004). all of today’s large animals as human observing today processes similar to “Exceptional record of mid-Pleistocene development spreads into remote those that led to the late Pleistocene vertebrates helps differentiate climatic from corners of the globe. Large animals extinction. So how are California’s anthropogenic ecosystem perturbations”: Barnosky, A. D. Proceedings of The National are particularly sensitive to habitat bighorn sheep doing? They still have a Academy of Sciences USA 101:9297-9302 (2004). fragmentation because individuals often chance—“for the moment,” says Epps, require broad territories of continuous “they’re holding their own. Some “Effects of Climate Change on Population Persistence of Desert-Dwelling Mountain Sheep land. Their habitat is often the same populations are going up, and some in California”: Epps et al. Conservation Biology 18 land that people prefer, and for similar populations are going down. There’s (1): 102-113 (2004). reasons: like us, they need access to lots of variation in the system.” Over water and food. the years covered by the Berkeley group’s Today, both climate change and study, a couple of the areas hosting

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