Land Bridges

Ancient Environments, Plant Migrations, and New World Connections

Alan Graham

• Editore: University of Chicago Press
• Anno: 2018
• ISBN:
  9780226544328
• Argomento: Scienze biologiche
• Categoria: Scienza generale

Land bridges are the causeways of biodiversity. When they form, organisms are introduced into a new patchwork of species and habitats, forever altering the ecosystems into which they flow; and when land bridges disappear or fracture, organisms are separated into reproductively isolated populations that can evolve independently. More than this, land bridges play a role in determining global climates through changes to moisture and heat transport and are also essential factors in the development of biogeographic patterns across geographically remote regions.In this book, paleobotanist Alan Graham traces the formation and disruption of key New World land bridges and describes the biotic, climatic, and biogeographic ramifications of these land masses' changing formations over time. Looking at five land bridges, he explores their present geographic setting and climate, modern vegetation, indigenous peoples (with special attention to their impact on past and present vegetation), and geologic history. From the great Panamanian isthmus to the boreal connections across the North Atlantic and North Pacific Oceans that allowed exchange of organisms between North America, Europe, and Asia, Graham's sweeping, one-hundred-million-year history offers new insight into the forces that shaped the life and land of the New World.

PART ONE

Boreal Land Bridges

Bering Land Bridge

Beringia

In a quest for the fabled Northwest Passage between Asia and Europe in 1816,

Kotzebue and his Russian crew initially enjoyed . . . a clear passage through the Bering Strait north of Alaska. On August 16th a sailor from the masthead saw only open sea to the east. Kotzebue clearly thought himself on the brink of a major geographical achievement, one that would place him in the ranks of Cortez and Cook: he “cherished the hope of discovering a passage into the Frozen Ocean, more particularly as the strait appeared to run without impediment to the horizon” [Kotzebue, 1821]. He was foiled, in the end, only by the shallowness of the local waters ahead that made further northeastward navigation impossible. (Wood, 2014, 128)

The shallowness of the waters through the Bering Strait has long been a factor in determining continuity between Asia and North America, resulting from a combination of changes in sea level (isostacy) and elevation of the land (orogeny). As defined here, Beringia extends from approximately 75°N to 58°N latitude and from 120°W to 100°E longitude (Figs. P1.1, P1.2). To the west are Siberia and the Kamchatka Peninsula of Russia, and to the east are Alaska and the adjacent Yukon Territory, Northwest Territories, and the province of British Columbia, Canada. South of the BLB proper is the 1200-mile-long chain of 69 mostly volcanic peaks of the Aleutian Islands, separating the Pacific Ocean to the south from the Bering Sea and the Arctic Ocean to the north. The westernmost United States as defined by longitude is at Amatignak Island, or, as miles from the mainland, it is the uninhabited island of Attu in the Near Island Group of the Aleutian chain (173°E, 1100 mi/1770 km). Farther to the west are the Komandorsklye Ostrova (Commander) Islands belonging to Russia. The International Date Line runs between Attu and the Commander Islands.

P1.1. Index map of place names and physiographic features for Siberia, Kamchatka, and vicinity, western Beringia.

P1.2. Index map of place names and physiographic features for Alaska and vicinity, eastern Beringia.

Vast areas of Siberia and Alaska are sparsely populated, and locations are commonly cited by longitude and latitude using GPS instead of by distance from remote population centers. Yakutsk is said to be at 62/129 (62°02´N 129°44´E) rather than 1600 km northeast of Irkutsk. At sea in the early days of exploration, determining latitude was relatively easy because it could be read off reference charts by noting the altitude of the sun at noon. Longitude was more difficult to estimate, and estimation of longitude became possible only after instruments were developed that could keep accurate time for years in the corrosive saline environment on often heavily rolling ships. Having recorded the time at the starting point, and knowing the time at the ship’s current position, the hours under sail could be calculated and from the ship’s average speed a guess could be made of its position. Another problem is that the lines of longitude are 60 mi apart at the equator and converge toward the poles, which created a further difficulty with the “dead reckoning” method. In 1714 the British government established the Longitude Act, offering a prize of 20,000 pounds on the grounds that “nothing is so much wanted and desired at sea, as the discovery of the longitude, for the safety and quickness of voyages, the preservation of ships, and the lives of men” (Sobel and Andrewes, 1995). The prize was awarded to Yorkshire carpenter John Harrison for his marine chronometer, but that was not until 1773 and long after Vitus Bering first sailed the treacherous waters of Beringia in 1728 guided by stars, rumor, and notoriously faulty maps. Field studies in Beringia are still difficult because of extreme cold, intense winter storms, and prolonged darkness. Today, when visiting a site with the enticing name of Bering Land Bridge National Preserve on the Seward Peninsula, visitors are advised that it is the nation’s most remote and least visited national park, providing “solitude not often available in other parts of the country.” Park headquarters are in Nome, which itself is unconnected by paved roads to any major city, like the state capital Juneau. Access to the preserve is by chartered floatplane or by conventional aircraft utilizing a dirt landing strip. There are no accommodations within the preserve, and travel is on foot or by snowmobile. Visitors must bring their own food, tents, fuel, emergency supplies, and communication devices. They are cautioned to be prepared for sudden and extreme changes in weather, to provide an itinerary of their whereabouts, and to be especially clear about arrangements for departure. In spite of these difficulties, those who study the geology and natural history of Beringia mostly agree that the scenic beauty of the surroundings and the often novel and important research results make confronting the isolation and physical challenges worth the effort.

Background

Longstanding interest in the biology and geology of Beringia and the surrounding land is evident by the number of books, international symposia, and edited volumes devoted to the region over the years. Among the earliest are Outline of the History of Arctic and Boreal Biota during the Quaternary Period by Eric Hultén (1937; Fig. P1.3) and The Bering Land Bridge by David M. Hopkins (1967), based on a meeting of the VII Congress of the International Association for Quaternary Research in Boulder, Colorado, in 1965. These were followed by Arctic Geology (Second International Symposium on Arctic Geology, San Francisco, 1971; Pitcher, 1973), Paleoecology of Beringia (Hopkins et al., 1982), and Beringia in the Cenozoic Era (All-Union Symposium, “The Bering Land Bridge and Its Role in the History of Holarctic Floras and Faunas in the Late Cenozoic”; Kontrimavichus, 1984). There are summaries of the fossil plants (Boulter and Fisher, 1994), archaeology (West, 1981), mammoths (Haynes, 1991), and the “Mammoth Epoch” in Siberia (Ukraintseva, 1993). Interest is due to the long fascination with this remote land that formerly connected North America and Asia; early recognition of the biological similarities between eastern North America and central Asia, particularly the Hubei (formerly Hupeh) and Sichuan (Szechwan) provinces of China (e.g., Halenius, 1750; Gray, 1840, 1846; see Graham, 1966, 1971a); and more recently the discovery of vast oil and natural gas reserves on the North Slope of Alaska and in Siberia.

P1.3. Eric Hultén, prominent Swedish botanist, biogeographer, and early student of the Arctic vegetation. http://people.wku.edu/charles.smith/chronob/HULT1894.htm.

Interest in the biogeography and vegetation history lulled after Asa Gray’s work in the mid-1800s and was revived when, in July 1971, the Geological Institute of the Academy of Sciences of the USSR hosted the Third International Palynological Conference in the academic city of Akademgorodok just south of Novosibirsk. The conference included presentations on the stratigraphy and correlation of economically important strata and, hence, was of interest to economic geologists and to those in the petroleum industry, as well as the academic world. However, it was at the height of the Cold War, epitomized by the frequently cited and variously translated phrase, “We will bury you,” uttered by Soviet Premier Nikita Khrushchev at the Polish Embassy in Moscow in 1956. This pronouncement was followed by the launch of the world’s first satellite (Sputnik) by the Soviet Union on 4 October 1957; Khrushchev’s alleged shoe-banging incident at the United Nations in 1960 (the widely circulated photograph was faked); and the Cuban missile crisis in October 1962. The tensions between East and West had an effect on the selection of attendees even at non–military-related scientific conferences. When invitations were sent out in 1969 for the Russian meeting, I was one of the few North Americans issued a visa and allowed to attend. Since I was a relative newcomer to the field, this was surprising considering the number of well-known botanists and geologists anxious to interact with Russian colleagues long isolated behind the Iron Curtain. It was clear that careful attention had been given to the selection of foreign delegates. Members from the Western bloc nations were mostly limited to two per country so they could not dominate the conference or sway its adopted policies, recommendations, and resolutions.

My attendance was likely facilitated by the fact that as part of the XI International Botanical Congress in Seattle in 1969, I had organized the symposium “Floristics and Paleofloristics of Asia and Eastern North America” in collaboration with the Japan–United States Cooperative Science Program (Graham, 1971b). I invited the Academy of Sciences of the USSR to provide a speaker on the vegetation of the USSR, and they sent B. A. Yurtsev of the Komarov Botanical Institute in Leningrad. His paper was one of the few up-to-date, English-language summaries available at the time on the vegetation of Russia (Yurtsev, 1972). Once the conference in Akademgorodok was under way, isolated in the Hotel Gold Valley (Fig. P1.4), overt supervision of Westerners eased, but they were kept apart from their Soviet counterparts who at precisely 11:00 p.m. were shuttled back to their separate dormitories. This was symbolic of the difficulties in exchange of scientific information between East and West in the 1970s. Translations were provided for the papers but rather than translating simultaneously, the translator came to the podium, stood beside the speaker, and after each sentence repeated the sentence in the alternate language. Every 20-minute paper took 40 minutes, and the day sessions scheduled to end at 6:00 p.m. continued past midnight.

P1.4. Gold Valley Hotel, Akademgorodok, July 1971.

Monitoring the interaction between foreigners and the townspeople was less severe in the evening at the Hotel Gold Valley. One night a huge, bearded Russian workman came into the bar, took a bottle of vodka and two glasses from the shelf, and sat down. He poured two drinks, shouted, “Prost!” and repeated the ritual several times, then got up and left. The bartender said it was the locals’ way of welcoming strangers.

At dusk, as the sun set over the River Ob, there was a magnificent view of the densely forested landscape from the upper floor of the hotel. The taiga or boreal forest extended from horizon to horizon with only a single, narrow road running in a nearly straight line for 2000 miles between Novosibirsk and Moscow. It was easy to envision plants and animals of the past moving virtually unimpeded as climates changed. Then, in the Quaternary, they began moving preferentially eastward, especially the huge herds of bison and woolly mammoth, in response to cold conditions that developed slightly earlier and more intensely on the vast interior continental Siberian plain than on the more restricted and topographically diverse landscape of Alaska. Finally, sometime after about 18,500 years ago, the megafauna was followed by human hunters and gatherers. An important additional legacy of these movements were the plant propagules the human migrants inadvertently or deliberately introduced into the new lands over the millennia (van Kleunen et al., 2015; Rejmánek, 2015) and those attached to or within the fauna they were following. To judge from recent invasives like Frangula alnus (De Kort et al., 2016), Lythrum salicaria, and Pueraria lobata, the effect was probably local rapid simplification and decrease in biodiversity at places in the new land. Such introductions are not easily recognizable where time has been insufficient for morphological differences to develop between the original and introduced populations (see chapter 4). However, if microsatellite analyses could detect divergences as recent as 18,000 years ago, this might help in corroborating the time humans and some plants crossed into the New World. A similar approach has been used to detect recent expansion of Rhizophora in Florida and the Caribbean islands (Kennedy, 2014), ...

Indice dei contenuti

• Cover
• Title Page
• Copyright Page
• Contents
• Abbreviations, Time Scale, and Conversions
• Preface
• Introduction
• Part I: Boreal Land Bridges
• Part II: Equatorial Land Bridges
• Part III: Austral Land Bridge
• Acknowledgments
• Index