The Next Species: The Future of Evolution in the Aftermath of Man

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The Next Species: The Future of Evolution in the Aftermath of Man Page 5

by Michael Tennesen


  This showed that before Talcher had become one of India’s largest coal deposits, formed by steaming tropical swamps, it had been part of an enormous ice field. The Blanfords returned to Calcutta and reported to their boss that ice sheets had once covered India. But this raised important questions in the geological community. How could glaciers form in the tropics? Had India once been much closer to the poles? Did continents move?

  Further evidence for the shift of landmasses was uncovered in 1912 when Britain’s Captain Robert Scott led a harrowing expedition to the South Pole, having to cope with blizzards and temperatures as low as minus 23 degrees. Though he and his men made it, they did so thirty-three days after a Norwegian team. Captain Roald Amundsen, its leader, left a Norwegian black marker flag and a note to the British at the pole. Losing the race for his country was enormously disconcerting for Scott, who wrote in his diary: “The POLE. Yes, but under very different circumstances from those expected. Great God! This is an awful place and terrible enough for us to have labored to it without the reward of priority.” Scott and most of his men froze to death trying to get back, though they’d carried most of their finds almost the distance.

  Scott’s second in command, Edward Evans, survived, but upon returning to New Zealand wrote a letter criticizing their leader for not jettisoning all records and specimens of weight that the party had collected on their treacherous adventure. Scott and the team members Edward Wilson and Henry Bowers had died in a tent that was but 12.7 miles (20 kilometers) south of One Ton Depot, a spot on the Ross Ice Shelf where the party had cached food and supplies. Scott’s body was found beside thirty-five pounds of coal and fossil rocks that the captain apparently considered more sacred than his own life. The samples included the first find this far south of Glossopteris, a seed fern that had become extinct over 200 million years ago. For such a tree to survive, a much warmer climate than the icy world Scott had found at the South Pole would have had to exist, scientists speculated. Or maybe the land that the South Pole stood upon had once been in the tropics?

  Alfred Wegener, a German geophysicist, who first described the lateral movement of the earth’s great landmasses in his 1915 book The Origin of Continents and Oceans, gathered evidence for this argument. Wegener noted that the continents of Africa and South America fit together quite nicely, and he found reports that fossils on the adjacent coastlines of both continents were similar. Scientists had previously suggested that land bridges once joined them, but Wegener countered this belief, saying that they had moved. He noted that India, Antarctica, and Australia looked like they could fit together, and proposed that they had all once been joined in a supercontinent that he called Pangaea. His book was the first place that name appeared. Wegener proposed that the world of today was but the dispersing remnants of this supercontinent, which 250 million years ago began to break apart.

  Continental drift, or the slightly more evolved concept of plate tectonics, has been for scientists the driving engine behind evolution and the creation of new species for over a hundred years. In the days of the great Pangaea, all major landmasses had gathered together, and this merging of lands coalesced life—the outcome being fewer species. But as Pangaea began to separate, the isolation that followed proved the best breeder of species, creating a greater number of plants and animals.

  ISLAND BIOGEOGRAPHY

  There are, however, other ways to make new species. Alfred Russel Wallace, often credited with cofounding the theory of evolution, traveled through the Amazon and Southeast Asia in the mid-1800s. He studied hundreds of animals and tried to determine why they were found where they were. He thought it was significant that rivers and mountain ranges frequently marked the boundaries of species ranges. Many scientists believed that climate determined a species’ range, but Wallace found similar climate regions with very different species and declared that geography had a lot more to do with it.

  This theory of island biogeography, as promoted by Wallace and others, began as a way of explaining the species richness of actual ocean or lake islands, but grew to incorporate the species richness of landlocked islands as well. Scientists modified their definition of islands in the late twentieth century to include other isolated habitats such as mountains surrounded by deserts, lakes surrounded by dry land, and natural habitats surrounded by landscapes altered by man. Today, scientists have modified this concept further, using it to explain any ecosystem surrounded by divergent ecosystems. It could be an island surrounded by water, or a spring surrounded by desert, a mountain peak surrounded by lowlands, or grassland surrounded by human housing.

  It is not a simple concept. What is considered an island for one organism may not be an island for another: some organisms located on mountaintops may also be found in valleys, being adaptable to both elevations. But others may be ecologically adapted only to the peaks and thus view the valleys as chasms that cannot be crossed. It may depend on whether the animal is a generalist, suitable to a wide range of environments, or a specialist, adapted to a much more specific niche. Isolated environments created in a mountain range can increase the variety of species in the range overall.

  AN ISLAND IN THE ANDES

  A typical example of a landlocked island is the mountainous region of Vilcabamba, the range in the Andes that I visited with Conservation International biologists. Deep river valleys surround the mountaintops, isolating them just like an ocean. The cloud forests here house many unique species, including some that have yet to be identified by scientists. Vilcabamba is a monument to natural diversity, as it showcases the broad range of possibilities to life.

  On the day after Peruvian army helicopters had deposited our team into the dripping-wet cloud forests of Vilcabamba, I got up before dawn to survey the area’s birds with Tom Schulenberg, an ornithologist with the Cornell Lab of Ornithology. We skirted the bog in the middle of the forest near our campsite looking for the feathered creatures, careful to avoid wet sinkholes in the moss that could swallow one’s leg up to the thigh. Schulenberg aimed binoculars as well as microphones at the edge of the forest, claiming he could hear four times as many birds as he could see. Though the elevation here was too high for parrots and toucans, the ornithologist’s Peruvian assistant, Lawrence López, captured a plush-capped finch, an Azara’s spinetail, and a yellow-scarfed tanager that he pulled from his jacket pocket and proclaimed, “Look at that beauty,” before he released it.

  In the evening, I followed Mónica Romo, a biologist with the office of Conservation International in Lima, Peru, who set nets by the forest edge to capture bats that she estimates spread almost 50 percent of the seeds in the forest through their feces. The next day I followed Romo down trails freshly cut with a machete to lay mammal traps. Romo was knowledgeable about all these animals, but also bore a self-professed sweet tooth, and was envious when the camp’s peanut butter was added to the bait used to capture these small creatures. “I hope they appreciate it,” she said. Mindful of the presence of fer-de-lance, one of South America’s most potent and aggressive vipers, I stowed my traps in the open, while Romo hid hers in every dark corner. They collected forty species of mammals, including a very large rodent that had never been described before.

  A few days later, I accompanied Brad Boyle, a Canadian botanist who specializes in tropical plants. With his Peruvian counterparts, he laid a 165-foot (50-meter) line in the forest and started taking plant specimens on either side. He directed my attention to the orchids, bromeliads, mosses, and ferns perched on the limbs of the trees above, and declared that there were more species in that cluster of treetops than in most northern forests.

  He told me how difficult it was to discover a new species. It was not like “Hallelujah! We’ve just found a new species!”; it was more like “We’ve just been through every similar-looking plant in the entire herbarium and can’t find anything that looks like this.” To declare something a new species requires a lot of work. Nevertheless he held up a tiny orchid a short while later and declared, “I’ll bet a c
ase of beer that’s never been described before.”

  The rains carved the valleys that surround Vilcabamba, making it a de facto island, but not all lands are so easily separated. It took the massive forces of earth’s molten core to break up Pangaea and to spread the continents wide. But now, through mass transportation, man has destroyed much of this hard-won separation. Similarly, as boats, trains, cars, and planes have spread across the land, so have the mammals, insects, reptiles, and crustaceans that have hidden in the bilges, trunks, and storage facilities of these vehicles. This has destroyed much of the isolation that created these many species, taking them to places where their presence can be ruinous to the resident animals.

  In the United States the resultant spreading of some of these displaced species has been purposeful if not downright ridiculous. In 1890 an eccentric Shakespeare fanatic, Eugene Schieffelin, decided to introduce all the birds mentioned in Shakespeare’s plays to the United States. Schieffelin released sixty starlings one fateful day in New York’s Central Park, and from that introduction, the US now has 200 million.

  These starlings, as well as sparrows and pigeons, make up the majority of the birds Americans see most days in urban environments. Yet none of these birds is native to the United States. “Invasives,” as we call these exotic plants and animals, compete for food with true natives like eastern bluebirds and purple martins. Since local birds tend to migrate south for the winter, while invasives stay home, there is little nesting space available when the native birds return. And this is all because some dutiful citizen felt the New World would be more civilized if it were populated with the Bard’s birds.

  Invasive plants often flourish outside their native habitats because the insects, diseases, and animals that naturally check their growth at home are not present in their new digs.

  The spread of an invasive species can also be caused by the inability of local animals to deal with new immigrants. The brown tree snake had such a free rein when it was introduced to Guam from the Solomon Islands after World War II. Scientists speculate that the snake probably snuck onto Guam inside the wheel well of an airplane, since Guam has an active air base. The brown tree snake spread across the island’s jungles over the last sixty years and is responsible for the extinction or severe reduction of a number of native species that had no defense against the snake. Biologists recently attempted to control the snakes by air-dropping into the jungle dead mice laced with about 80 milligrams of acetaminophen—equal to a child’s dose of Tylenol, all that’s needed to kill an adult brown tree snake. The results are not yet conclusive.

  Similarly, licensed and unlicensed animal traders over the last couple of decades brought Burmese pythons into Florida. When pet owners found that the snakes either took up too much space or tried to swallow the family dog, they let them go in local and national parks. Since 2002, more than 1,800 pythons have been removed from Everglades National Park and the surrounding areas in Florida. Now the US Fish and Wildlife Service reports that northern and southern African pythons, reticulated pythons, boa constrictors, and four species of anaconda have joined the Everglades pythons. Biologists believe that tens of thousands of these snakes now live in the park.

  Invasive species can come from afar or they may grow up locally and penetrate spaces formally occupied by other species when the right conditions arise. Native woody shrubs and trees are invading semiarid grasslands in the US, South America, Africa, and Australia as a result of overgrazing, fire suppression, and climate change.

  When animal grazing is controlled, grasses grow up naturally and provide kindling for natural or man-made fires, which stimulate more grasses but suppress the growth of woody shrubs. The control of woody shrubs, which inhibit grasses, is critical to pastoral communities on arid and semiarid lands, which make up 35 percent of the earth’s people. They must balance the need for grasses to feed their cattle, sheep, and goats, with the need for fires to control woody vegetation.

  To get a look at how shrubs outcompete grasses, I followed Rob Jackson, a professor of environmental earth system science at Stanford University, on a warm afternoon, down a tall ladder into the underground caverns of Powell’s Cave, about 150 miles (240 kilometers) west of Austin, Texas. We entered a world of stalactites and stalagmites in the porous limestone bedrock of the Edwards Plateau in west-central Texas. I scurried after Jackson through a maze of caverns and tight crawl spaces, over slippery pathways, and into rooms filled with glistening multicolored limestone structures, all carved by nature. We arrived at a point about sixty feet (eighteen meters) below the ground, where an underground stream gushes from the rock.

  Like the British geologists who went underground to look for evidence of glaciers, Jackson went deep to try to explain how native juniper trees on the Edwards Plateau have invaded grasslands and are taking over. He showed me several thick tree roots that appeared to burst from the limestone walls, reach down into the stream, and suck water out of it. He explained to me: “A single taproot can provide a third or more of the tree’s water during a drought.”

  Junipers put out roots along the full depths of their root systems so that they can get water from deeper roots in times of drought, and from shallower roots in times of rain. This gives them an advantage over grasses, which can pull only from their shallow roots no matter what the weather.

  Woody shrubs and trees like junipers have invaded arid and semiarid grasslands and savannas in the US. Their presence limits the grass available for land managers, ranchers, and wildlife. Studies show that increased shrub and tree growth can rob from one-third to two-thirds of the stream water.

  Juniper, mesquite, creosote, and Chinese tallow are problem plants in different parts of the US, particularly in the southern regions—the Great Plains, the Southwest, and the Gulf Coast. These plants existed here before, but overgrazing, fire suppression, and climate change have allowed plant populations to explode. Their increased presence has led to thickets that don’t allow enough light or room for other plants or grasses. “Thicketization” was what Steve Archer, a professor at the School of Natural Resources at the University of Arizona, Tucson, called it when I met him at an Ecological Society of America conference in Austin, Texas. Archer studies the ecology, management, and restoration of rangelands, which are any extensive area of land occupied by native vegetation that is grazed by domestic or wild animals that eat plants.

  When huge herds of cattle were introduced to the western United States in the late 1800s, they devastated the grasses. This reduced the fuel for grass fires, and in their absence woody plants got better established. In earlier times, Indians regularly burned the grassy meadows here to clear brush and trees and open them up for hunting. Today fire suppression is one of the problems promoting the juniper invasion. Without grass fires, woody plants spread unabated.

  But it’s more than a recent problem. It goes all the way back fifteen thousand years ago to Ice Age hunters, who wiped out the large animals that once ate the woody plants in the grasslands of North America. In East Africa, they still have elephants that control woody plants, one of their natural foods. But the US no longer has wildlife populations that can do the job, so woody vegetation grows uncontrolled.

  Woody plants in the US can be bad for ranchers, farmers, and wildlife. Black-capped vireos and golden-cheeked warblers, both considered endangered in Texas, are two species that need a mixed landscape of forest and open grassland to thrive. Woody vegetation suppresses the grasses and the open space that comes with them.

  An example of the woody plant problem can be found at the Tallgrass Prairie Preserve near Pawhuska, Oklahoma, the largest protected area of tallgrass prairie in North America. Tallgrass prairies once spread throughout the Midwest, supporting enormous buffalo herds, and though there are still buffalo here, their numbers are small. Private ranches surround the preserve, and land managers at these ranches find that if they don’t burn grassland areas every year, woody vegetation forms canopies, which makes them immune to future fires.
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  Woody vegetation is also invading normally bald mountaintop environments in New Mexico as well. Here, bighorn sheep typically gather because they can see mountain lions approaching and escape. But as woody vegetation moves into these formerly bald areas, it allows mountain lions some cover from which to attack, wreaking havoc on bighorn populations.

  “Climate change, increasing atmospheric carbon dioxide, and on-the-ground changes like fire suppression and cattle grazing should speed the global transition to woody species,” says Jackson. “It’s not just a problem in Texas, but in South America, Africa, and Asia as well.”

  Climate change, its causes and effects, is another issue Jackson is going underground to work out. He is currently looking at problems with natural gas, which was once thought to be an ideal solution for some of our greenhouse problems, since it burns cleaner. It’s certainly a cleaner fuel than coal or oil, but Jackson is concerned with leaks that occur in transport. Belowground fracking can result in leakage into groundwater on the extraction side and old pipes can sprout leaks into urban soils on the delivery side.

  Jackson and the Boston University professor Nathan Phillips found natural gas (methane) escaping from more than 3,300 leaks in Boston’s underground pipelines, where there is a record of natural gas blowing up homes, regularly sending manhole covers into the sky, and killing trees.

  Still, despite all the importance given to greenhouse gases, Jackson thinks the spread of invasive species across the globe is more permanent and perhaps a more serious threat to our environment. We can reverse climate change in one thousand to ten thousand years, but the plague of invasives and the mixing of species worldwide is not one we are likely to recover from.

 

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