|Biodiversity and Conservation|
source ref: biobook.html
(Archaeopteryx from Casiano Zoo)
|BIODIVERSITY AND CONSERVATION
HISTORY OF LIFE ON EARTH
THE DAWN OF LIFE (PALEOZOIC)
THE AGE OF REPTILES (MESOZOIC)
|Registered UCI students: view the slide show for this chapter or download it: http://darwin.bio.uci.edu:80/~sustain/protected/chap1slides.ppt|
"If today is a typical day on planet Earth, we will lose 116 square miles of rainforest, or about an acre a second. We will lose another 72 square miles to encroaching deserts, as a result of human mismanagement and overpopulation. We will lose 40 to 100 species, and no one knows whether the number is 40 or 100. Today the human population will increase by 250,000. And today we will add 2,700 tons of chlorofluorocarbons to the atmosphere and 15 million tons of carbon. Tonight the Earth will be a little hotter, its waters more acidic, and the fabric of life more threadbare." ...... David Orr (1991), " What is Education For?".
This hypertext book deals with the problems of trying to preserve biological diversity on the earth. We will discuss the history of life on earth, the reasons for depletion and extinction of animals and plants at various times including the present, the reasons for being concerned about these losses, and what can be done to preserve some of what is left.
Read the Living Planet 2002 Report, by the World Wildlife Fund, to see graphic illustrations of humanity's impact on the natural resources of the planet, and read about a new comprehensive plan to save the planet, drafted by an unprecedented combination of scientists, environmentalists and business leaders in August 2000.
We live at a critical time for the conservation of biological resources on earth. The Living Planet Index, a measure reflecting the state of the world's forests, freshwater and marine ecosystems, fell by 37% between 1970 and 2000 according to the Living Planet 2002 Report.
In the recent past, the level of biological diversity was the highest the world has ever seen. However, the number of species is not known, even to the nearest order of magnitude. The number of described species is about 1.4 million; but the total is estimated at about 5 million. It has taken 3.5 billion years for this biodiversity to evolve, and we are rapidly destroying it. Whereas the natural rate of extinction is estimated at about one species per year, the present rate is estimated at 10,000 times that - about one per hour - and almost all of these losses are caused by human activities. We probably have already lost 1 million species, and several more million will be lost in the first few decades of the 21st century.
Table 1 gives the number of recorded extinctions between 1600 and 1983. Notice that:
- the total number of recorded extinctions is over 700;
- over half of these are vascular plants;
- a very large fraction are island forms, especially with reptiles and birds.
The data are probably most accurate for birds and mammals since these are the most conspicuous of the types of organisms listed. 113 species of birds and 83 species of mammals were definitely lost during this period.
Table 1. Recorded Extinctions, 1600 to 1983
|Mainland a||Island b||Ocean||Total||Approximate
Source: Reid and Miller 1989 Keeping Options Alive-The Scientific Basis for Conserving Biodiversity.
Table 2 gives the number of extinct animal species, and also includes counts of the number of animal species considered to be under different degrees of threat.
2. Number of Endangered and Extinct Animal Species
|class||critically endangered||endangered||vulnerable||total threatened||extinct|
The number of inconspicuous forms, like insects, that have gone extinct is probably much higher than the number shown on the list. Many of these species are becoming extinct before they are even studied or named by scientists.
In the U.S., where research is probably more intensive than in many other countries, 631 species are known to have been lost since 1642, and the total is probably well over a thousand. Hawaii, Alabama, and California lead the nation in number of extinctions.
The reasons for extinction are changing. In prehistoric times, natural disasters and competition with other species were the main causes. In historic times, overexploitation and exotic species introductions have caused many extinctions. But today, the main problems facing wildlife are destruction of habitat and pollution.
Tropical forest is being destroyed at the rate of 40,000 square miles = an area the size of Ohio, per year. This is mainly due to slash-and-burn agriculture in areas of high population growth, in which small areas are cleared and used for a few years until they become infertile, and then more acreage is cleared. About 44% of the original tropical moist forest on the earth is now gone. It has been estimated that 15-20% of all species will become extinct by the year 2000 because of the destruction of tropical forests. This rate is about 10,000 times as high as the rate prior to the existence of human beings.
Other habitats are also being destroyed - temperate forests, deserts, wetlands, and coral reefs are all being destroyed at alarming rates, either for profit or to make room for housing, agriculture, ports and other human activities. Damming of rivers has depleted salmon populations in the Pacific Northwest to such an extent that many of the runs are extinct and others have been listed as endangered.
A 1996 study by the World Conservation Union found that 25 percent of mammal and amphibian species, 11 percent of birds, 20 percent of reptiles and 34 percent of fish species are threatened with extinction. About 10 percent of the world's tree species are in danger of extinction.
The fundamental reason for the degradation and loss of habitat is the explosive growth of the human population. Since 1900 the world's population has more than tripled. Since 1950 it has more than doubled, to 6 billion. Every year 90 million more people (= 3x the population of California) are added to the planet. All of these people need places to live, work and play, and they all contribute to habitat loss and global pollution.
Our generation is the first one that really became aware of the fact that the human population is causing irreparable damage to the planet - to the air, water and soil of the planet and to its biological resources. Ours is not the first generation to do damage to the planet, but we are the first to realize the extent of the problem.
Ours is the only generation that can prevent a massive loss of biological diversity. Huge losses have already taken place and we will have to make major changes in the way we treat the planet if we are to save it in anything like an intact state.
If we are lucky, ours will be the first generation in which the main principles guiding our governments will not be economic and political systems, but will be learning to live on the planet without destroying it: learning to live, work, grow food, trade, and develop a sustainable way of life that serves the continuing needs of our descendants, and the other species present on earth, as well as ourselves.
Take this course and sign up for the Interdisciplinary Minor in Global Sustainability
(UC students) Join CALPIRG (California Public Interest Research Group) and get experience with activism
Read about Issues and Write Letters about them (here are some tips)
Read the ideas of one of the main authors on biodiversity: Speciation and Biodiversity, an Interview with Edward O. Wilson
Subscribe to some of the following information sources:
Biodiversity Associates | EnviroLink | Environmental News
Network | Gaia Forest Conservation Archives | Breach Marine Protection | GREEN's Home Page
| Indonesian Nature Conservation Database | League of Conservation Voters | MARMAM: Marine
Mammal Research and Conservation | National Audubon Society Desktop Activist Guide |
Natural Resources Defense Council | Sierra Club Home Page | SC Action | Sierra Club: The
Planet | Center for Biological Diversity | Trout Unlimited | Environmental Letters,
Articles, and Book Reviews by Mike Vandeman | Whale and Dolphin Conservation Society |
Sustainable Development Institute | Global Stewards - Sustainable Living and Environmental
Activism | World Rainforest Report | Global Response
The purpose of Conservation Biology is to find ways of maintaining the high levels of biological diversity that are seen in today's world. But first, in order to appreciate that diversity, we need to understand how it has evolved, and what a complicated series of geological, climatic and biological events have led up to the present day situation. Therefore, we will review briefly the history of life on earth. Visit the virtual Museum of Paleontology at the University of California at Berkeley to find more information and illustrations regarding any of the topics or geological periods mentioned.
The chart shows the life forms that have been present on the earth since Cambrian times, and whose fossils are therefore found in different layers of rocks. Each time a new layer of rock is deposited (usually by sedimentation) it is on top of the older layers, so the chart reflects this order. Four eras, some subdivided into periods and epochs, are recognized, and are characterized by typical life forms.
The history of global biological diversity is best seen in the marine animals since the ocean is where life started, and marine animals are the best represented in the fossil record. The chart in the slide show shows the number of families of marine organisms on the earth at different times in biological history.
Past Biodiversity, by Jeremy B. C. Jackson and Kenneth G. Johnson. Science
2001 September 28; 293: 2401-2404.
Biological diversity was dramatically depleted by five mass extinction episodes at the ends of the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods. At each of these times a large fraction of existing species was wiped out, leaving the survivors to repopulate the biological world. The most famous of these was the extinction at the end of the Cretaceous because this ended the age of dinosaurs and made possible the evolution and dominance of mammals. But it was not the most devastating of the mass extinctions.
Multicellular animals first appeared about 600 million years ago in the early Paleozoic and there was a rapid rise in number of families during the Cambrian and Ordovician. Diversity remained relatively constant (perhaps even declining) up until about 200 million years ago and then it rose again to its current all-time high of close to 800 families.
1. Precambrian: The origin of life.
2. Paleozoic ("Ancient Life"): The origin of plants, most invertebrate types, the first vertebrates (back-boned animals, including fishes, amphibians, and reptiles).
3. Mesozoic (the "age of reptiles"): The origin of flowering plants, dinosaurs, birds, and mammals.
4. Cenozoic (the "age of mammals"): The diversification of flowering plants, insects, birds and mammals, and the appearance of humans.
The origin of life: the Earth was formed about 5 billion years ago. A series of giant meteorites ("the late heavy bombardment") essentially sterilized the planet about 3.8 billion years ago. Rocks 3.5 billion years old contain microfossils of primitive one-celled organisms without a nucleus ("prokaryotes") resembling bacteria and blue-green algae, and carbon isotope ratios characteristic of biological materials, representing the earliest clear signs of life. The first living cells with a nucleus ("eukaryotes") appeared 2 billion years ago, and the first organisms made up of multiple cells (multicellular algae) about 1.8 billion years ago.
A third major group of organisms, called Archaea, consisting of about 500 species but making up about 30% of the biomass on Earth, was not discovered until 1977. They live in the most extreme environments on Earth - the hottest, coldest, and highest-pressure environments, so they are sometimes called "extremophiles". Most of their known biomass is in the Antarctic.
After 3 billion years of very little change, all of a sudden during the Cambrian period, there was a frenzy of evolutionary innovation that generated just about all of the major types (phyla and classes) of marine invertebrates plus many others that no longer exist. Accurate dating methods used in 1993 (involving measurements of uranium and lead isotopes) indicate that the explosion happened during a very short period - from 533 to 525 million years ago. This is sometimes called the Cambrian explosion. What caused it is not known - one of the most plausible suggestions is that it followed shortly after marine animals evolved protective shells and cuticles that allowed them to exploit new ecological niches. Another possibility is that more complex body organization became possible after the atmospheric oxygen reached a certain threshold.
Many of the important fossils telling the story of the
Cambrian explosion were found in a fossil bed in the Canadian Rockies called Burgess
Shale, where both soft- and hard-bodied animals were buried in an underwater mudslide and
preserved in water so deep and oxygen-free that they did not decompose. They are described
in a book by Stephen J. Gould called Wonderful Life. View pictures of
the actual fossils, and an on-line exhibit at the Tyrrell museum in Alberta, Canada.
|Two different views on the role of chance and selection in evolution:|
|Wonderful Life: The Burgess Shale and the Nature of History by Stephen Jay Gould (1990). W.W. Norton & Company.||The Crucible of Creation: The Burgess Shale and the Rise of Animals by Simon Conway Morris (1998). Oxford University Press, 272 pp.|
The Cambrian explosion probably generated over 100 major animal groups with fundamentally different body plans, but only about 30 survived to this day. Examples of those that survived are arthropods (insects, spiders, crustaceans), echinoderms (sea urchins and starfish), mollusks (snails, clams), and chordates, the latter represented in the Cambrian by worm-like animals with a rod-like backbone but eventually giving rise to the vertebrates. Although large numbers of new families evolved in the later periods of earth history, these later innovations did not include major reorganizations of the body plan, but rather variations on themes that were established in the Cambrian.
We will look at the kinds of animals and plants that were present on the earth at different periods, starting with a jump to the Carboniferous because at that time, both plants and animals had moved out on to the land and were beginning to diversify.
During the Carboniferous the climate was hot and humid, and there were extensive swampy forests dominated by giant tree ferns and conifers, club mosses, and horsetails. The decomposed remains of these plants gave rise to the major coal and oil reserves of today. There were no flowering plants and no grasses.
Some of the Carboniferous fish breathed air through two lungs, and they had developed lobe-like fins. The fascinating search for a surviving relative, the Coelacanth, has been recounted in Samantha' Weinberg's "A Fish Caught in Time": click on the picture. The Carboniferous fish gave rise to modern bony fishes. In these the fins have lost their lobes, and one lung has been lost and the other converted to a swim bladder. They also gave rise to the amphibians, in which both lungs have been retained and the limbs strengthened and specialized for walking. These amphibians, like modern frogs and salamanders, hatched from eggs and spent their larval period in the water as tadpoles, then moved out on to the land as adults. They were the first land-dwelling vertebrates, and some of them were massive animals two or three feet long. In addition to the primitive amphibians there were some very primitive reptiles that were totally terrestrial, like today's lizards. In these animals the egg gave rise directly to a miniature version of the adult, which could survive on the land.
The arthropods of the Carboniferous had also moved onto the land and had in fact taken to the air. Many of the ancient insects were gigantic compared to present day ones, among them mayflies with a 14-inch wing span, and giant six-winged insects, in which the first pair of wings had been already reduced to nubs but not eliminated as they are in all present-day insects. These insects had long sucking mouthparts with which they sucked the juices out of large primitive pinecones. Feeding on these insects were dragonflies with a 30-inch wingspan!
The land was increasing in altitude in the Permian and the climate was cooler and dryer than it was in the Carboniferous. The landscape at this time was one of low hills with small streams and lakes covered in primitive vegetation such as ferns. Reptiles such as the large sail-backed carnivore Dimetrodon, were common at this time. A number of these early reptiles had elaborate sails on the back. The purpose of these structures is not known although they are thought to have been involved in temperature regulation; that is, to radiate heat when the animal was too hot and to absorb it when the animal was too cold.
The most successful land animals of the middle Permian were the many kinds of mammal-like reptiles, which were quite varied and included both herbivores and carnivores. Most of them were large, heavy and slow moving. We can't tell if they were warm-blooded or if they had mammary glands, like true mammals, but they did have a jaw articulation like that found in mammals and they had several types of teeth, which is a mammalian feature.
At the end of the Permian, 245 million years ago, huge numbers of animals became extinct. About 96% of all marine animal species and 52% of the families disappeared in the greatest mass extinction the world has ever known. Recent data shows that 8 of 27 orders of insects, 21 of 27 families of reptiles, 6 of 9 families of amphibians and most of the terrestrial plant life disappeared at this time. This happened over a remarkably short period - about one million years. The most popular theory is that it was caused by flood volcanism in Siberia - huge continent-sized floods of scorching hot magma, about a mile deep, poured out from fissures in the Earth's crust and spread over the land. The eruption lasted for about a million years, generating about 2 million times as much lava as was involved in the eruption of Mount St. Helens in 1980. This might have generated enough debris to block out sunlight and trigger an ice age, enough sulfuric acid to acidify the oceans, and/or enough carbon dioxide to cause global warming by a greenhouse effect. However, studies reported in 2001 suggest an alternative explanation - that the extinction might have been caused, like the later one that finished off the dinosaurs, by a giant meteorite hitting the Earth. Whatever caused the Permian extinctions; it took about 100 million years for the species diversity to recover from this event.
Reptiles and mammal-like reptiles dominated the landscape in the Triassic. Late Triassic was the time when the first true mammals, descendants of the mammal-like reptiles, appeared. The first ones were small, like Megazostrodon, resembling a modern shrew.
At the end of the Triassic many of the amphibians, reptiles and most of the mammal-like reptiles disappeared and were replaced by the dinosaurs and crocodiles in addition to some early turtles, lizards and frogs.
The Jurassic and Cretaceous together were the age of giant reptiles. For 150 million years, the dominant vertebrates on the land were the dinosaurs. (By comparison, humans have been on the earth for only about 3 million years).
The dinosaurs are composed of two distinct orders:
The Ornithischians (bird-hip dinosaurs) included the duck-billed dinosaurs or Hadrosaurs; Stegosaurs with their plate-like armor along the back; Ankylosaurs which were heavily armored and flattened, and the rhinoceros-like horned dinosaurs or Ceratopsians (example - Triceratops).
The Saurischians (lizard-hips) included the Sauropods - gigantic herbivorous dinosaurs with extremely long necks and tails, such as Brontosaurus and Brachiosaurus - the largest terrestrial animals that ever lived; and Theropods - carnivorous dinosaurs with enormous skulls, powerful teeth and ridiculously small front legs, Tyrannosaurus being the best known example but Giganotosaurus was bigger. A new species of theropod, about the size of a dog, was recently discovered on Madagascar.
The dominant animals of the oceans were the ichthyosaurs, some of which were as large as medium sized whales, the long-necked plesiosaurs and some marine crocodiles. The dinosaurs also took to the air and evolved some spectacular and huge flying reptiles called pterosaurs. One of them, Quetzalcoatlus, was the largest flying vertebrate the world has ever known, with a 40-foot wingspan!
A different, much smaller type of dinosaur is thought to have evolved into the first bird, Archaeopteryx, some time in the Jurassic.
The predominant land plants in the Jurassic were the Cycads, primitive palm-like seed plants. A few species of cycads still exist in tropical and sub-tropical regions today, and although they are used as ornamental plants some of them are facing possible extinction. Early Cretaceous saw the evolution of flowering plants (angiosperms). The flowers provided a new food source for pollinators, mainly insects, with profound effects on the evolution of those forms.
The end of the Cretaceous saw the culmination of dinosaur evolution. Dinosaurs were more varied and adapted for more different modes of life than any other group, before or since. There were at least 100 species of dinosaurs. Some of them even lived in the Arctic, which was probably a lot warmer than it is now. They had achieved a level of evolutionary success that guaranteed them a permanent place in history, and yet they were soon to disappear entirely from the face of the earth. Several different groups of bird-like dinosaurs had evolved, and all except one - the ancestor of all modern birds - died out at about the same time as the rest of the dinosaurs.
Species, Speciation, and the Environment by Niles Eldredge
The Sixth Extinction by Niles Eldredge