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Historical Patterns of Global Change Geological time scale is hierarchical • • • • Eon Era Period Epoch Index fossils • Wide-ranging species • Not restricted to microhabitats • Often marine organisms – – – – – planktonic, calcareous foraminiferans siliceous radiolarians (Protozoa) of the Cenozoic era chitinous colonial graptolites (Hemichordata) floating animals (Ordovician and Silurian) swimming ammonoid cephalopods (Mollusca) of Mesozoic – vascular plants of Carboniferous period Theory of Continental Drift • has had the largest impact on biogeography • evidence is conclusive Development of Plate Tectonics Theory • Lyell (mid 1800’s) • Antonio Snider-Pelligrini (1858) • F. B. Taylor (1908 - 1910) – American geologist • Alfred L. Wegener – meteoroligist who developed ideas of continental drift independently of Taylor while noting congruence of opposite coastlines on Atlantic Ocean; – working hypothesis presented in 1912; published in 1915 Main points of Wegener’s theory • continental rocks fundamentally different than those of ocean – floor, and are lighter & thus float on the mantel • major landmasses once united into a supercontinent, Pangaea – Pangaea broke into small continental plates, which moved apart Main points of Wegener’s theory • breakup of Pangaea began as a rift valley, which widened into an ocean – midoceanic ridges mark where continents were joined – distribution of major earthquake and volcanic regions, and mountain building regions are related to continental movement Main points of Wegener’s theory • continental blocks have retained their initial outlines, except in mountain orogeny regions – joining the margins of continents – lines up stratigraphic zones, fossils, and reconstructed paleoclimates • rates of movement range between 0.3 and 36 m per year • radioactive heating in the mantle may be the primary cause of continental movement – forces of movement are gradual Opposition to the theory • Involved shifting paradigm – New ideas challenged in science • Contained many factual errors • Testing model difficult – would have required much more geological and biogeographical evidence than presented • Lacked a plausible mechanism Acceptance of theory took time • Five decades after Wegener’s proposals, sufficient evidence was gathered by geologists, paleontologists and biogeographers • S. W. Carey (1955, 1958); Bullard et al (1965) demonstrated the fit of continents into Pangaea using submarine contours of continental shelves. Evidence for plate tectonics • Stratigraphic – Topographic features align along Wegener’s proposed connections of fragmented Gondwanaland • Paleoclimatic – Southern Hemisphere glacial scars aligned only if Gondwanaland reconstructed • Paleontological – The late Paleozoic glacial deposits of southern continents are covered with Permian rocks bearing the Glossopteris flora (arborescent gymnosperms of temperate climates) - circumscribes Gondwonaland – vertebrate fossil assemblages with similar distribution Evidence from Marine geology • WWII stimulated charting of ocean floor topography – discovery that submarine mountain ranges bisecting the oceans are segments of a continuous global system 65,000 km long • interpretation that midoceanic ridges are zones of seafloor spreading – trenches located far from ridges • up to 10 km deep • earth’s crust is very thin at the trenches • hypothesis that the crust is pulled downward into trenches and material is reincorporated into the mantle Evidence from paleomagnetism • Paleomagnetism – orientation of magnetized crystals, titanium and iron oxides, at the time of mineral formation when molten rock solidifies – by measuring the direction and declination of remnant magnetism in cooled lavas, it’s possible to determine the relationship of any landmass to the magnetic poles at time of formation – reversals of magnetic field occurs every 10,000 to million years Significance of magnetic stripes for continental drift theory • Basaltic rocks at the mid-oceanic ridges have normal field (present-day) magnetic properties • The widths of alternating magnetic stripes on the opposite sides of a ridge are often roughly symmetrical, and the stripes are generally parallel to the long axis of the ridge. • The banding pattern of any one ocean closely matches that of the others, and the ocean patterns correspond approximately to reversal timetables from terrestrial lava flows. Model of seafloor spreading • Proposed by Herman Hess (1960, 1962); R.S. Dietz (1961) • oceans formed by addition of material and spreading at mid-oceanic ridges – moving away from the ridges, basalt along the ocean floor increases in age and is marked by magnetic stripes • records polarity of prevailing magnetic field • differences in widths of stripes suggest rate of spreading varies over time • not uniform across oceans or parts of the same ocean Finally: a plausible mechanism! • Current model suggests that three forces may be responsible for crustal movements – ridge push – mantle drag – slab pull • 16 major plates are currently recognized – strong correlation with Wallace’s map of biogeographic regions So, what are biogeographic regions? • Biogeographic regions are defined by biotas and portions of the earth’s crust that share both evolutionary and tectonic histories – representative assemblages on each plate evolved in isolation from those on other plates. Plate boundaries classified into three types of zones • Spreading – midoceanic ridges and rift zones on continents • Collision – if plates are of roughly equal density, collision of plates will cause uplift and formation of mountains along plate boundary – if ocean plate is denser than the other, will sink below lighter continental plate to form subduction trench • Transform – plates of roughly equal density may slide and grind against each other, without subduction or mountain uplift Tectonic phenomena (earthquakes, volcanism, mountain formation, island arcs) closely associated with interactions among plates Changing face of the planet • Pangea – probably a temporary supercontinent during late Paleozoic and early Mesozoic – North: Laurasia – South: Gondwanaland Changing face of the planet • Gondwanaland – includes foundations of present-day South America, Africa, Madagascar, Arabia, India, Australia, Tasmania, New Guinea, New Zealand, New Caledonia, and Antarctica – most ancient of the Pangean land masses – great antiquity accounts for similarity of isolated biotas of southern hemisphere Changing face of the planet • Laurasia – Pre-Laurasian landmasses were isolated until early Devonian (400 mybp), when precursors of Northern Europe drifted northward from subantarctic latitudes to collide with precursors of North American and northern part of Siberia to form the “Sandstone Continent” Breakup of Pangaea • Initiated in early Jurassic (180 mybp). • Evolutionary radiations of most terrestrial families and many orders occurred after breakup of Pangaea and Panthalassa – resulted in rafting biotas, reduced gene flow, altered selective pressure and rapid speciation Selected Histories of Different Biogeographic Regions - Central America and the Antilles • Precursors of North and South America remained connected until about 160 mybp – connection severed by late Jurassic (ca 150 mybp) • chain of volcanic islands along the eastern edge of the Caribbean Plate – drifted eastward to form Proto Antilles archipelago • by Eocene – core of Greater Antilles achieved present position Selected Histories of Different Biogeographic Regions - Central America and the Antilles • Central American landbridge – first emerged during late Cretaceous (80 to 65 mybp) as chain of islands far east of current position – By late Miocene (10 to 5 mybp), Central American archipelago provided stepping stone route – 3.5 mypb, archipelago fused to form current landbridge. Selected Histories of Different Biogeographic Regions - Marine basins and island chains • Epeiric or epicontinental seas – form when sea levels rise and oceans flood continental plates • Mediterranean and Red Seas – Mediterranean started out as the Tethyan Seaway – Red Sea formed by rifting which began ca 35 mypb Selected Histories of Different Biogeographic Regions - Marine basins and island chains • Pacific Ocean – No central midoceanic ridge as seen in Atlantic ocean – Island chains form over hot spot • Triple junctions – where three plates rest against each other to form a complex of trenches • Pacific basin had six ancient plates – Phoenix; Kula; Pacific; Farallon; Cocos; Caribbean Freshwater systems • River system drainages have also shifted over time – – – – Amazon Mississippi St. Lawrence Great Basin of North America Climatic and Biogeographic Consequences of Plate Tectonics • Mountain ranges form when plates collide – subduction & volcanic activity – accretion – plates with similar densities • Biotic exchange, extinction, adaptive radiation, rapid speciation – follows history of contact and breakup of continental plates – diversity increases with area; diversity and similarity among biotas decrease with increasing isolation Climatic and Biogeographic Consequences of Plate Tectonics • Climatic shifts correlated with continental movements – Affects precipitation patterns, paleocurrents, wind, temperature – Position of land masses relative to poles and equator affect local climate – Landmasses over poles trigger episodes of glaciation and changes in sea level • Mass extinctions associated with tectonic events Glaciation and Biogeographic Dynamics of Pleistocene Glaciation and Biogeographic Dynamics of Pleistocene • Pleistocene and Holocene (past 2 million years) – events during this period had big impact on current distributions of taxa – but have noterased the patterns resulting from older periods – Recency of this period of time allows for sources of paleoecological data not available for older time periods Glaciation • Global glacial events associated with position of large landmasses relative to the poles • Glaciation during the Pleistocene was not result of land mass position relative to poles. – Climatic changes resulted from changes in the interceptions and absorption of solar radiation by earth’s surface due to changes in its orbit Milankovitch cycles • eccentricity of orbit (shape of ellipse) varies on 100,000 yr cycle • obliquity (tilt) varies from 22.1˚to 24.5˚ on a 41,000 yr cycle • precession (axis orientation) wanders on a 22,000 yr cycle Milankovitch cycles • combined effects cause fluctuations in the amount of solar energy striking the earth • transitions between glacial and interglacial periods influenced by feedback effects • temperatures were much warmer during most of last 600 my with less of a gradient from equator to poles – During the Miocene, global climates gradually became cooler and drier Pleistocene glaciation • several glacial-interglacial cycles with advance and retreat of continental glaciers – often 2 to 3 km thick – Massive enough to deform underlying crust – at maximum covered up to 1/3 of earth’s surface – unglaciated regions had very different climates Estimation of Paleotemperatures • Can measure oxygen isotope ratios in marine fossils and ice cores • Two common isotopes of oxygen in water are O16 and O18 – The lighter isotope (O16) evaporates faster than the heavier (O18), especially in warmer periods • marine exoskeletons: calcite oxygen ratios • ice cores: direct isotope measurements • Reconstruction of temperatures suggest that 10 major glacial periods occurred over past 2 million years The effect on global climate • Declines in air temperatures ranging 4° to 8°C • Climatic zones shifted toward the equator during glaciation and shifted northward in the interglacials – position of land masses, glaciers and major bodies of water complicated the patterns • Glacial winters less severe and glacial summers cooler The effect on global climate • Tropical regions tended to be cold and dry • SW North America was wetter • There was a strong association between glacial periods and monsoonal circulation and precipitation Take home message: What we see in current climatic regimes is not what was typical over the past 2 mybp Sea Level Changes during Pleistocene • Global changes resulting from freezing or melting of sea ice (Eustatic changes) • Isostatic changes reflect sea level changes when portions of the earth’s crust rise or sink into the athenosphere • During Wisconsin glacial maximum, nearly 1/3 of land in northern hemisphere covered in ice and there were huge fields of sea ice. Sea Level Changes during Pleistocene • Sea level dropped by 100 m – Other glacial periods saw drop in sea level up to 160 m – Sea levels have been up to 70 m higher than at present – Total changes of 230 m through global history • Land bridges form between biogeographic regions when sea level drops by exposing the continental shelves – e.g., Wallace’s line coincides with the division between landbridges of the Sunda Shelf (north) and the Australia-New Guinea-Tasmania land bridges. Glacial melt: • takes centuries for crust to rebound • with rising sea levels, flooding of interior regions caused shallow seas – forms corridors for dispersal of shoreline flora and marine and estuary species Biogeographic responses to glaciation • Environmental changes: – Changes in the location, extent and configuration of habitats – Changes in the nature of climatic and environmental zones – Formation and dissolution of dispersal routes • Responses by species: – Migration - moving with optimal habitat – Adaptation to local changes – Range reduction and extinction Biogeographic responses to glaciation • Biomes shifted 10° to 20° in latitude (toward equator) – Zonal vegetation belts compressed during glacial maxima, but relative positions were unchanged – North-South rivers and mountain systems in North America served as corridors for migration – E-W mountains and other barriers in Europe blocked migration • Upward migration of vegetation zones in mountainous regions since glacial retreat Biogeographic responses to glaciation • Shifts of vegetation zone occurred in Southern Hemisphere as well as in north • Biomes and communities often disintegrated rather than migrating as units – Responses to climatic change varied among species – Animals migrated more quickly than plants, but were influenced by vegetative Aquatic systems • Glacial activity of Pleistocene had major impact on lake formation • Many glacial lakes formed by ice damming – drainage of lakes spectacular and devastating when ice dams broke • e.g., Columbia River Gorge. • Other types of lakes from glacial activity – kettle lakes and glacial plunge pool lake – basins formed by scouring action of glaciers – dams formed by glacial moraines Pluvial lakes • Southwestern US has basin-and-range topography • low, flat basins filled with rainwater – slow rate of evaporation from cooler temperatures and high amounts of rainfall • Dried pluvial lakes – saline lakes and dry lake beds • Biogeographic consequences of disappearance of pluvial lakes – wholesale extinction of many plants and animals that lived in or around the lakes – dissection of large lakes resulted in vicariant speciation of survivors (e.g., pup fishes of SW NA) Biotic Exchange • Mountains: – shift to lower elevation during glacial maxima allowed for corridors to form between mountain ranges • remixing of previously isolated taxa – shift to more arid climates isolates peaks from one another during interglacials • formation of “sky islands” Biotic Exchange • Marine organisms experienced similar types of phenomena • Landbridges exposed during lowered sea levels functioned as corridors between continents, between continents and islands, or among islands • Landbridges fragmented marine biotas Biotic Exchange • Dispersal in terrestrial and marine biomes was assymetrical – More spp migrating from larger to smaller areas • Great American Interchange – a dominating transfer of northern species to the southern hemisphere Pleistocene Refugia • Neotropics: – lowland tropical rainforests may have contracted in size within past 40,000 years – Amazonia has ca. 6 centers of endemism and high species diversity – Insular nature of these areas of species richness and endemism led to the idea that they were Pleistocene refugia • islands of lowland rain forest that persisted during glacial maxima. Pleistocene Refugia • Cyclical vicariance or speciation pump model: – Populations became isolated as their prime habitat became fragmented – As glacier receded, refugia expanded to eventually reconnect to each other – Range expansion of taxa brought relatives back into contact • Objections to the model: – forests may not have contracted nearly as much as model suggests – molecular and phenotypic data indicate that many endemics are much older than the hypothesized refugia – there is little overlap in centers of endemism among different taxa – Haffer’s hypothesis not the most parsimonious explanation Pleistocene Refugia • Nunataks – Glacial refugia that persisted within or adjacent to the ice sheets (continental margins, pockets in mountain ranges, mountain tops, steep bluffs of coastline, between ice sheets) • Land south of ice also served as refugium. • Driftless area in southern Wisconsin and adjacent Illinois and Iowa – elliptical area bypassed by Laurentide glacial front Pleistocene Refugia • Three large coastal refugia in North America during Wisconsin glacier – Beringia – coastal regions of Pacific Northwest – Nova Scotia • Refugia notable for high amount of divergence in animals and plants, and high degree of endemism. Beringia • First proposed by E. Hultén in 1937 • Evidence to support hypothesis: – fossil pollen records show that mesic tundra was widespread in northern and central portions of the landbridge – strong evidence for the occurrence of deciduous trees and larch in Yukon at this time – mammalian fossils from 40,000 ybp show high diversity of large ungulates (woolly mammoth, woolly rhino, muskox, moutain sheep, steppe antelope, reindeer, horse, camel). Pleistocene extinctions • plants unable to disperse in advance of glaciers went extinct • mass extinctions of marine invertebrates during late Neogene associated with glacial cycles • terrestrial vertebrate extinction pattern very different from plants and marine invertebrates • extinctions did not occur until glaciers retreated Overkill Hypothesis • humans responsible for the extinctions • evidence to support hypothesis: – fossil evidence shows that prehistoric humans and large mammals coexisted and that humans hunted them (arrow points in carcasses) – extinctions were nonrandom • large animals became extinct at a higher rate than small mammals – Animals adapted to human predation (e.g., caribou, moose, deer, bighorn sheep) fared much better in terms of extinction – extinctions began in north and proceeded southward – Definite correlation of extinction patterns and human migrationpatterns