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Transcript
What Role Has Extinction Played in the History of Life? Extinctions: Evolutionary History Has Been Marked by Periodic Mass Extinctions Climate Change Contributed to Mass Extinctions Past and Present Catastrophic Events May Have Caused the Worst Mass Extinctions The extinction of the Dinosaurs was caused by an asteroid impact with the Earth about 65 million years ago Continental Drift from Plate Tectonics Evolution of the Earth with Time: Continental Drift 200 Million Years Ago 50 Million Years Ago The continents are passengers on plates moving on Earth's surface as a result of plate tectonics. (a) About 340 million years ago, much of what is now North America was positioned at the equator. (b) All the plates eventually fused together into one gigantic landmass, which geologists call Pangaea. 150 Million Years Ago (c) Gradually Pangaea broke up into Laurasia and Gondwanaland, which itself eventually broke up into West and East Gondwana. (d) Further plate motion eventually resulted in the modern positions of the continents. 100 Million Years Ago Continental Drift - 150 mya Present Continental Drift - 100 mya 1 Continental Drift - 75 mya Continental Drift - 50 mya Continental Drift - 0 mya Extinction events Paleontological archives reveal the existence of 5 large extinction events in the earth history = Time periods where extinction rates are greater than mean expected values Asteroid collision (65 106 yr): - global cooling - marine transgression - volcanic activity - nuclear winter Extinction of 50% of marine species Extinction of whole plant and animal families (including organisms like dinosaurs) Formation of Pangea (250 dominant 106yr): - habitat destruction - climatic modifications Extinction of 90% of living species 2 Greatest Hits: Mass Extinctions Roughly 2 BYA: Most of life on earth wiped out due to pollution (O2) Permian Mass extinction 250 MYA 90 - 95% of marine species became extinct K-T (Cretaceous) Event 65 MYA 85% of all species became extinct End-Ice-Age Mass Extinction (10 TYA) Current on-going mass extinction. Causes of these mass extinctions Massive environmental perturbation Extra-terrestrial impacts Volcanoes Climate change Biological agents Comet Shoemaker- Levy 9 July 1994 Impact size if S-L 9 had hit Earth (12+ such impacts!) Permian, K-T extinctions Show strong signals of impacts (iridium layer, shock quartz, other signals) Climate change following impact event 3 Toba extinction event. 75,000 YA Toba, Sumatra, Indonesia Toba Caldera energy released about one gigaton of TNT 3000 times greater than Mount St. Helens Led to a decrease in average global temperatures by 3 to 3.5 degrees Celsius for several years Believed to have created population bottlenecks in the various homo species that existed at the time Eventually leading to the extinction of all the other homo species except for the branch that became modern humans 15000 ybp to 1500 AD "North American Pleistocene Landscape," by Karen Carr Human colonization Humans invaded at end of the last ice age (15,00010,000 ybp) Spread across the continent and through South America in < 1000 years! Clovis hunters migrated south through an ice-free corridor east of the Canadian Rockies and moved south of the glaciers around 11,500 years ago 4 Pleistocene extinctions Shortly after glaciers retreated at the end of the Pleistocene, over thirty-five different genera of mammals became extinct Twenty-seven vanished from the world completely, and eight of them were only in North America. These mammals are known as “megafauna” for their massive size and their large populations. Mastodon (Mammut americanum) source: Illinois State Museum 5 The “Overkill” hypothesis Entered "the happiest hunting ground ever known." An abundance of animals wholly unadapted to human predation. Humans had easy hunting and exposure to few new illnesses The animals simply couldn't keep up with humans. Effects on megafauna in North America Large, slowly reproducing creatures targeted Precipitated an extinction cascade When the large herbivores disappeared, so did their natural predators Loss of large grazers also caused a major shift in the plant communities (e.g., from prairie to forest). Many smaller fauna that fed off the carcasses of the megafauna or would live off their "leftover" plant food also went extinct. NORTH AMERICAN VEGETATION AT 13,000 YBP, 11,000 YBP, AND AT PRESENT Criticisms Where are the kill sites? Very few in North America as they are in New Zealand Compare with Africa and Asia? Humans and large animals have co-evolved for long periods? Also 10 extinct families/genera of birds? All scavengers? Synergisms Changes at the end of the Pleistocene were by far the most rapid and amazing in the earth's history. Many large mammals confined to extremely limited ranges as the result of climatic change Presumably exceptionally vulnerable to human predation as a result. 6 Disease? (Ross McPhee) Other similar extinction events Humans could have brought with them microbes Infested with, but adapted to the pathogens perhaps carried by the lice in their hair, the fleas on their dogs, or within their own bodies Island extinctions Humans next conquered the islands. Madagascar, New Zealand, Cyprus, the Caribbean, and the Pacific. Large, tame, and flightless birds vanished. New Zealand and Madagascar, lost their large flightless birds almost instantaneously with the arrival of people 1,000 years ago. The moas of New Zealand Polynesia expansion Reached last inhabitable areas – Pacific Islands – within last 1,000-4,000 years Bird bones persist to, but not through, first indication of human presence Every one of ~800 Pacific Islands likely had its own species of rail A new species of flightless bird has been discovered on Calayan, a remote island in the Philippines 18/08/2004 With Stone Age technology, exterminated >2,000 bird species, or ~15% of world’s total Estimate may be 50% too low 7 Continental Extinctions European Expansions John James Audobon wrote of the passenger pigeon in 1844 in The Birds of America: "In the autumn of 1813, I left my house at Henderson (Kentucky), on the banks of the Ohio, on my way to Louisville. In passing over the barrens a few miles beyond Hardensburgh, I observed the pigeons flying from northeast to southwest, in greater numbers than I though I had ever seen them before…” The passenger pigeon became extinct in the wild. This is “Martha", the last passenger pigeon, died at 1:00 p.m. on September 1, 1914, at age 29, in the Cincinnati Zoological Garden. 8 Tom Nixon once shot 120 bison in 40 minutes. Also killed over 3200 bison in 35 days! Lions and wolves were virtually eliminated from eastern North America, and grizzly bears all but the remotest parts of the Rockies. Some 31 million bison were killed for hides between 1868 and 1881. 200,000 Native American Plains Indians were thought to harvest 2 million buffalo a year, a mere third of the bison's annual birth rate. 9 Current extinctions Comparison islands-continents Islands Molluscs Birds Mammals Others Total 151 104 90% 34 59% 74 62% 363 75% 11 10% 24 41% 46 38% 121 25% 79% Continents 40 21% 85 (2.1%) mammals, 113 (1.3%) birds have gone extinct since 1600; most in the last 150 years. Number of species extinct since 1600 Current extinctions 50% of fungi species in Europe may have become extinct in past 60 yrs. The Big Picture Background rate that is thought to have occurred during most of the Earth’s history. Based on the fossil record for marine animals, it has been estimated that the annual extinction rate was about 1 species in every million to ten million species. If there are 1 - 10 million species on Earth, then 1-10 species should go extinct each year. Current extinctions 20% of the world’s freshwater fishes extinct or in serious decline. Comparisons Based on recent extinctions among birds and mammals about 1% of species have gone extinct every 100 years. Extrapolated means that about 1 in every 10,000 species are currently going extinct each year. These estimates, suggest that the current extinction rate is about 100-1000 times greater than the background rate (other estimates 40 - 10,000 x background rate). 10 Extrapolations Extrapolate that ~ 34 species go extinct daily based on so many species occur in rain forest that we can ignore other habitats, z = 0.15, rainforest is lost at about 1% per year. Tremendous uncertainty but all conclude that 10,000s of species will go extinct in the next few decades. Oceanic islands - Z = 0.18 -0.37 . Habitat islands - Z = 0.17 - 0.72 . Ecological Theory of Extinction MacArthur-Wilson Theory of Island Biogeography Metapopulation dynamics Demographic Stochasticity MacArthur-Wilson Island Biogeography (1967) Interested in predicting species numbers on islands Numbers represent a balance between extinction and immigration Prediction: lower extinction rate on larger islands Prediction: higher immigration rates on islands closer to mainland 11 Size does matter: Species-area curves One of MacArthur & Wilson’s key observation was the species-area curve, predicting the number of species S simply from area A. S = CAz Log(S) = C + z*log(A) Key is species-area exponent z How can we apply this to conservation?? Complications Species-area slope (z): between islands vs. patches within an island Even if species-area curve exact, only tells us how many species will be lost, NOT which particular ones 12 Non-equilibrium Island Biogeography Application to conservation biology Isolated patches of habitats are essentially islands When isolation is increased, the island is no longer in equilibrium and the number of species is expected to decline Hence, loss of species on ever-isolated islands is expected Need to maximize patch size Need to maximize exchange between patches When should you NOT maximize exchange? Risk of disease/pathogens spreading Patches are sufficiently genetically different End 13