CK-12 Biology I - Honors

by CK-12 Foundation

  Quantify Earth’s species diversity, according to scientists’ current understanding.

  Describe patterns of biodiversity in space.

  Trace changes in biodiversity throughout Earth’s history.

  Examine the evidence for the Sixth Extinction.

  Compare the Sixth Extinction to major extinctions before humans.

  Discuss the direct economic benefits of biodiversity.

  Evaluate ecosystem services provided by biodiversity.

  List the intangible (cultural, spiritual, religious) benefits of biodiversity.

  Relate biodiversity to social and political stability.

  Consider that biodiversity has intrinsic value apart from benefits to humans.

  Assess the potential for early human activities to contribute to Ice Age extinctions of large animals.

  Identify habitat loss as the primary cause of the Sixth Extinction.

  Relate the introduction of exotic species to loss of biodiversity.

  Explain the extent to which over exploitation has affected all levels of biodiversity.

  Connect energy use to extinction.

  Describe the effects of population growth and unequal distribution of resources on biodiversity.

  Recognize that pollution of water, land, and air contributes to the loss of species.

  Acknowledge that your daily activities and decisions can significantly help to protect biodiversity.

  Evaluate your consumption – of food, clothing, furniture, and cleaning products.

  Appreciate the importance of water resources and know how to use them wisely.

  Evaluate your choice and use of energy sources.

  Assess the importance of minimizing waste, and of using best practices for waste disposal.

  Know how to avoid transporting and releasing exotic species.

  Realize that you can practice sustainable management of your own land, from small yards to local, state, and federal lands which also belong to you.

  Describe sustainability and its role in decision-making.

  Explain how learning and active citizenship can contribute to protecting biodiversity.

  Introduction

  Humans, like all species, depend on certain natural resources for survival. We depend on land and soils to grow crops, which transform solar energy into food. We use the Earth’s freshwater lakes, rivers, and groundwater for drinking. We rely on the atmosphere to provide us with oxygen and to shield us from radiation. We rely on Earth’s biodiversity for food, clothing, and medicines. We utilize all of the “basic four” (biodiversity, land, water, air) for recycling of nutrients and disposal of waste. Natural ecosystems, as Odum suggests, provide services for all species: they maintain soil, renew the atmosphere, replenish freshwater supplies, dispose of wastes, and recycle nutrients. In our dependence on these services, we are like all other species.

  Yet in many ways, we do not behave like other species. We supplement food and animal energy with fossil fuel energy. We harvest natural resources to exhaustion, and produce waste beyond levels that the Earth can process. We alter biodiversity, land, water, air and fossil fuels beyond nature’s ability to repair. As you learned in your study of population biology, our population has grown beyond Earth’s carrying capacity, compounding problems of resource use and waste disposal. Only recently have we learned to appreciate the full value of these resources – and the potential for harm from our own activities. Our economics have not caught up to our relatively new understanding: we do not yet pay the costs of maintaining all of “nature’s services.”

  This lesson will explore biodiversity – the “millions of organisms and hundreds of processes - operating to maintain a livable environment.” The topic is timely, critical, and colorful: you will encounter warnings of a Biodiversity Crisis and the Sixth Extinction, and species identified as “an Elvis taxon” or “a Lazarus taxon.” More importantly, by the end of your study, you will have some tools you can use in your daily life to help protect the great diversity of Earth’s life.

  What is Biodiversity?

  “The first rule of intelligent tinkering is to save all the pieces.” --attributed to Aldo Leopold, but probably a shortened version of: "To save every cog and wheel is the first precaution of intelligent tinkering." - Aldo Leopold, Round River: from the Journals of Aldo Leopold, 1953

  What are the “cogs” and “wheels” of life?

  Although the concept of biodiversity did not become a vital component of biology and political science until nearly 40 years after Aldo Leopold’s death in 1948, Leopold – often considered the father of modern ecology - would have likely found the term an appropriate description of his “cogs and wheels.” Literally, biodiversity is the many different kinds (diversity) of life (bio-). Biologists, however, always alert to levels of organization, have identified three measures of life’s variation. Species diversity best fits the literal translation: the number of different species (see the chapter on Evolution of Populations ) in a particular ecosystem or on Earth (Figure below). A second measure recognizes variation within a species: differences among individuals or populations make up genetic diversity. Finally, as Leopold clearly understood, the “cogs and wheels” include not only life but also the land (and sea and air) which supports life. Ecosystem diversity describes the many types of functional units formed by living communities interacting with their environments.

  Although all three levels of diversity are important, the term biodiversity usually refers to species diversity. How many species do you think exist on Earth? What groups of species do you think are most abundant? Consider your own experience, and your study of biology up to this point. Think carefully, and write down your answer or exchange ideas with a classmate before you read further.

  Figure 18.1

  The most accessible definition of biodiversity is species diversity. How many species exist on Earth?

  What is the Species Diversity of Earth?

  There are three good answers to this question. As a member of one of Earth’s most intriguing species, you should know them all!

  1) Scientists have identified about 1.8 million species. (Figure below)

  Figure 18.2

  Among 1.8 million identified species (A), 1,315,378 are Animals (B), 287,655 are Plants (C), and only 259 are Archaebacteria. The Animal Kingdom is dominated by the Class Insecta, and the Plant Kingdom is dominated by flowering plants.

  The relative numbers of species in each of the six kingdoms of life is shown in Figure A above. The Animal Kingdom (dominated by the Insects, as shown in Figure B above) includes the great majority of known species, and Archaebacteria, by far the fewest. Most scientists agree that Eubacteria and Archaebacteria are seriously underrepresented, due to their small size and chemistry-based diversity. This leads to a second, and perhaps better answer to our question:

  2) No One Knows How Many Species Currently Live on Earth!

  Does this lack of knowledge surprise you? Scientists are still discovering new species - not only microorganisms but also plants, animals, and fungi. At least 5 new species of marsupials, 25 primates, 3 rabbits, 22 rodents, 30 bats, 4 whales or dolphins, a leopard, and a sloth were identified between 2000 and 2007 – and these include only mammals! The vast majority of Eubacteria, Archaebacteria, Protist, and even Insect species may be yet unknown because their small size, remote habitats, and the chemical distinctions between species make them so difficult to detect. These challenges, however, have not prevented scientists from estimating Earth’s biodiversity – bringing us to the third answer to our question:

  3) Scientists Estimate that Between 5 and 30 Million Species Inhabit the Earth.

  Estimates vary widely – from 2 million to 117.7 million, underlining our lack of knowledge. Most estimates fall between 5 and 30 million. Much remains to be learned about the diversity of microorganisms. For example, scientists have recently discovered that Archaebacteria – originally considered limited to extreme environments - may constitute as much
as 40% of the ocean’s microbial biomass. Few species have been identified. Estimates of global diversity of the better-studied Eubacteria vary from millions to billions, with orders of magnitude of error. As for multicellular organisms, the most “species-dense” terrestrial ecosystems, such as coral reefs and tropical rain forests, harbor most of the undiscovered species (Figure below). Ironically, these ecosystems are also disappearing quickly. In summary, our estimates of biodiversity remain crude. However, the following conclusion is clear: given the current rapid loss of species, we will never know many of the species we are losing.

  Figure 18.3

  Coral reefs (above) and tropical rain forests (below) have the greatest biodiversity of the many ecosystems on earth. They are also among the most threatened habitats. Because our knowledge of their species is incomplete, we are clearly losing species we do not (and never will) know.

  Biodiversity Patterns in Space

  Are Earth’s 1.8 million known species evenly distributed across its surface? You may already be aware that the answer is a resounding “No!” We will compare two regions with relatively high diversity to begin our analysis.

  Minnesota has relatively high ecosystem diversity, because three of the Earth’s six major terrestrial biomes converge in this state (Prairie, Deciduous Forest, and Coniferous Forest). By contrast, Costa Rica comprises almost entirely of Tropical Rain Forest, and has only one quarter of the land area of Minnesota (Figure below).

  Figure 18.4

  The state of Minnesota () includes three major biomes and four times the land area of the country of Costa Rica (), which is predominately a tropical rainforest. compares the biodiversity of Minnesota to that of Costa Rica.

  You might expect, then, that Minnesota would have a higher species diversity. Several groups of organisms are compared in the Figure below. Note that a column is included for you to research your own state or region!

  Figure 18.5

  A comparison of species diversity within categories supports the increase in diversity from the poles to the equators. Costa Ricas increased diversity is due in part to greatly increased niches: diversity begets diversity! For example, poison dart frogs mature in tiny epiphyte pools, and strangler figs climb existing trees and starve their hosts of sunlight. Does your state or region support this overall spatial pattern of biodiversity?

  Clearly, biodiversity is much higher in Costa Rica than in Minnesota. Collecting leaves for your biology class in Costa Rica, you would need to study 2,500 different trees in order to identify the species! And you’d need to look carefully to distinguish tree leaves from those of the many epiphytes (plants which grow on top of others), vines, and strangler figs which climb the trunks and branches, “cheating” their way to the sunlight at the top of the canopy. In Minnesota, keys to native trees include just 42 species of conifers and deciduous broadleaved species. There, vines are relatively rare, and epiphytes are limited to colorful lichens.

  The differences in biodiversity between Minnesota and Costa Rica are part of a general worldwide pattern: biodiversity is richest at the equators, but decreases toward the poles. Temperature is undoubtedly a major factor, with warmer, equatorial regions allowing year-round growth in contrast to seasonal limitations nearer the poles.

  Generally, the more species, the more niches – so diversity begets diversity.

  Does your country, state or region fit the general pattern of decreasing biodiversity from equator to poles?

  Biodiversity Patterns in Time

  How has Earth’s biodiversity changed across time? The fossil record is our window into this pattern, although the window has limitations. Microorganisms are poorly preserved and distinguished only with difficulty; gene sequence studies of living bacteria have begun to fill in some missing data. For all organisms, recent rock layers are more accessible and better preserved than ancient ones.

  Despite these drawbacks, fossils and gene studies show a distinct pattern of increasing biodiversity through time. As discussed in the chapter on the History of Life, the origin of life is not clearly understood; evidence suggests that life did not appear on Earth until perhaps 4 billion years ago. For several billion years, unicellular organisms were the only form of life. During that time, biodiversity clearly increased, as Eubacteria and Archaebacteria emerged from a common ancestor some 3 billion years ago, and Eukaryotes emerged by endosymbiosis about 2 billion years ago. However, we have not accurately measured the diversity of even today’s microorganisms, so we have little understanding of changes in the diversity of microorganisms beyond these major events.

  The emergence of multicellular life about 1 billion years ago certainly increased biodiversity, although we have little way of knowing whether it might have negatively affected the diversity of microorganisms. Fossils remain relatively rare until the famed Cambrian explosion 542 million years ago. Since then, a much more detailed fossil record (Figure below) shows a pattern of increasing biodiversity marked by major extinctions.

  Figure 18.6

  The fossil record for marine species over the past 542 million years shows a gradual increase in biodiversity interrupted by five major extinctions. Some scientists view the recent rapid rise in diversity as a result of better preservation of more recent rock layers and fossils.

  The dramatic increase indicated for the last 200 million years is somewhat disputed. Some scientists believe it is a real increase in diversity due to expanding numbers of niches – diversity begets diversity, again. Others believe it is a product of sampling bias, due to better preservation of more recent fossils and rock layers. Most scientists accept the general pattern of increasing diversity through time, interpreting the magnificent biodiversity of life on Earth today as the result of billions of years of evolution.

  Most scientists also accept at least the five major mass extinctions shown in Figure above, and some hold that regular cycles govern extinction. Causes for these extinctions (more completely discussed in the History of Life chapter) remain incompletely understood; hypotheses include global climate change, major volcanic and continental drift events, dramatic oceanic change, and/or extraterrestrial impact or supernova events.

  Increasingly accepted is a current Sixth or Holocene Extinction event. According to a 1998 survey by the American Museum of Natural History, more than 70% of biologists consider the present era to be a sixth mass extinction event - perhaps one of the fastest ever. We will explore the Sixth, or Holocene, Extinction in the next section of this lesson.

  The Current Loss of Biodiversity

  “For one species to mourn the death of another is a new thing under the sun.” -Aldo Leopold A Sand County Almanac, 1949

  Over 99% of all species that have ever lived on Earth are extinct. During the 5 major extinctions recorded in the Phanerozoic fossil record (Figure above), more than 50% of animals disappeared. Evidently, extinction is natural. However, current extinctions may differ significantly in rate and cause. The IUCN (International Union of Concerned Scientists) has documented 758 extinctions since 1500 CE; for example, 6 species of giant, flightless Moa (Figure A below) disappeared from New Zealand shortly after the arrival of Polynesians. Estimates of extinctions for the last century range from 20,000 to 2,000,000 species; as for diversity, we simply do not know the true figure.

  Figure 18.7

  A gallery of species which have succumbed to the Sixth Extinction: A: one of six species of birds which disappeared after Polynesians first arrived and began to hunt and clear forests in New Zealand about 1500 CE. B: reconstruction of a woolly mammoth, one of many large mammals which became extinct at the end of the last Ice Age, due to human hunting, disease, and/or climate change. C: a reconstruction of the meter-tall flightless Dodo, which disappeared within a hundred years of its discovery, probably due to forest destruction and introduced predators. D: the Golden Toad recently discovered in 1966, has been officially extinct since 1989. Amphibians as a group have declined sharply throughout the world during the past three decades.

/>   Many scientists begin the Sixth Extinction with the Ice Age loss of large mammals and birds - part of a continuum of extinctions between 13,000 years ago and now. During that time, 33 of 45 genera of large mammals became extinct in North America, 46 of 58 in South America, and 15 of 16 in Australia. Climate change and/or human “overkill” are hypothetical causes. Supporting the significance of the “sudden” arrival of humans are the low numbers in Europe and South Africa, where humans had coevolved with large animals. The woolly mammoth (Figure B above) is one of the many examples of large mammal extinctions from this period.

  The first species to become extinct during recorded human history was the Dodo (Figure C above), a flightless bird which had evolved without predators on an island in the Indian Ocean. Described in 1581, the fearless Dodo experienced hunting, forest habitat destruction, and introduced predators, and became extinct before 1700 – a story repeated for many more species over the following three centuries. Unfortunately, the story extends back in time, as well; over the past 1100 years, human activity has led to the extinction of as many as 20% of all bird species… a tragic loss of biodiversity.

  Harvard Biologist E.O. Wilson estimated in 1993 that the planet was losing 30,000 species per year - around three species per hour. In 2002, he predicted that if current rates continue, 50% of today’s plant and animal species will be extinct within the current century – compared to hundreds of thousands or even millions of years for pre-human mass extinctions. A dramatic global decline in amphibian populations in less than 30 years headlines the recent rise in extinction. Herpetologists report that as many as 170 species have become extinct within that time, and at least one-third of remaining species are threatened. Costa Rica’s Golden Toad (Figure D above), first described in 1966, was last seen in 1989 and has become a poster species for amphibian declines.