Key Concepts

After completing this chapter, you will be able to

1. Outline the concept of biodiversity and explain its constituent elements.
2. Explain the reasons why biodiversity is important and should be preserved.
3. Define the classification of life in terms of species, genus, family, order, class, phylum, and kingdom.
4. Describe the six kingdoms of life.

Biodiversity

Biodiversity is the richness of biological variation. It is often considered to have three levels of organization (Figure 6.1):

1. Genetic variation within populations and species
2. Numbers of species (also known as species richness)
3. And the variety and dynamics of ecosystems on larger scales, such as landscapes and seascapes

Figure 6.1 Types of Biodiversity. Biodiversity in a given area can be characterized by genetic diversity within populations of species, the number of species (i.e., species richness), and by the diversity of ecosystems. Source: Biodiversity diagram by ABMI is licensed under CC BY 4.0.

Genetic Variation

In almost all species, individuals differ genetically – that is, in terms of information encoded in their DNA. This variation constitutes genetic biodiversity at the level of populations, and ultimately of the species (see Image 6.1).

However, there are exceptions to this generalization. Some plants, for example, have little or no genetic variability, usually because the species relies on asexual (vegetative) means of propagation. In such species, genetically uniform clones can develop, which consist of plants that, although discrete, nevertheless constitute the same genetic “individual.” For example, clones of trembling aspen (Populus tremuloides) can develop through vegetative propagation, in some cases covering more than 40 ha and consisting of thousands of trees. Such aspen clones may be the world’s largest “individual” organisms (in terms of total biomass). Similarly, the tiny plant known as duckweed (Lemna minor), which grows on the surface of fertile waterbodies, propagates by developing small vegetative buds on the edge of its single leaf. These break off to produce “new” plants, resulting in a genetically uniform population. These interesting cases are exceptions, however, and most populations and species contain a great deal of genetic variation.

Image 6.1. Species are a familiar element of biodiversity. The jaguar (Panthera onca) is a widespread large predator in South and Central America. This one was photographed in Tambopata National Park, Peru. Source: B. Freedman.

A high level of genetic diversity in a population is generally considered a desirable attribute. With greater genetic diversity, populations are more likely to have resistance to new diseases and to be more adaptable to changes in environmental conditions. In general, small populations with little genetic diversity are thought to be at risk because of inbreeding and low adaptability. Examples of such populations-at-risk include the several hundred beluga whales (Delphinapterus leucas) living in the estuary of the St. Lawrence River and the population of only about 150 panthers (Felis concolor coryi) in Florida.

Richness of Species

Species richness is the number of species in a particular ecological community or in another specified area, such as a park, province, country, or, ultimately, the biosphere. Species richness is the aspect of biodiversity that people can most easily relate to and understand.

It is well known that many tropical countries support a greater species richness than do temperate countries (such as the U.S.). In fact, tropical rainforest supports more species than any other kind of ecosystem. Unfortunately, species-rich rainforest in tropical countries is being rapidly destroyed, mostly by conversion into agricultural land-uses and other disturbances. These changes are causing the endangerment or extinction of many species and are the overwhelming cause of the modern-day biodiversity crisis (see Chapter 14). The magnitude of this crisis is much smaller in the U.S. Nevertheless, many of our native species have become extinct or otherwise at risk because of over-harvesting or habitat loss (Chapter 14).

A total of about 2 million species have been identified and given a scientific name. About 35% of these “known” species live in the tropics, 59% in the temperate zones, and 6% in boreal or polar latitudes. However, it is important to recognize that the identification of species is very incomplete. This is especially true of tropical ecosystems, which have not yet been thoroughly explored and characterized. According to some estimates, the global richness of species could range as high as 30–50 million, with 90% of them living in the tropics, particularly in rainforests.

Most of the species that biologists have named are invertebrates, with insects making up the bulk of that total, and beetles (order Coleoptera) comprising most of the insects (Figure 6.2).

Figure 6.2. Numbers of Species Described vs. Undescribed in Various Groups of Organisms. The distribution of numbers of known and undescribed (estimated) species on Earth, grouped by major taxonomic groups; according to Chapman 2009. Absolute number of species on the left (orange = estimated number of yet to be described species, blue = already described). Right: percentage of species already described (green) and estimated to be not yet known (yellow). Source: “Species Distribution Biodiversity after Chapman” by Gregor Hagedorn is licensed under CC BY-SA 4.0.

Furthermore, it is believed that many tropical insects have not yet been described by biologists – perhaps more than another 30 million species, with many of them being small beetles. This remarkable conclusion initially emerged from research by T.L. Erwin, an entomologist who was studying tropical rainforest in South America. Erwin treated small areas of forest canopy with a fog of insecticide, which resulted in a “rain” of dead arthropods that was collected in sampling trays laid on the ground. In the trays were large numbers of unknown species of insects, most of which had a highly localized distribution, being limited to only a single type of forest or even to a particular species of tree.

Clearly, biologists know remarkably little about the huge numbers of relatively small, unobtrusive species that occur in poorly explored habitats in the tropics and elsewhere, such as the deep ocean. However, even in a relatively well-prospected country like the U.S., many species of invertebrates, lichens, microbes, and other small organisms have not yet been discovered. Of course, larger plants and animals are relatively well known, partly because, for most people (including scientists), these have greater “charisma” than small beetles, microbes, and the like. Still, even in the U.S. and other relatively well-studied countries, new species of vascular plants and vertebrate animals are being discovered.

Compared with invertebrates and microbes, the species richness of other groups of tropical-forest organisms is better known. For example, a survey of rainforest in Sumatra, Indonesia, found 80 species of tree-sized plants (with a diameter greater than 20 cm) in an area of only 0.5 hectare. A study in Sarawak, Malaysia, found 742 woody species in a 3-ha plot of rainforest, with half of the species being represented by only a single individual. A similar study in Amazonian Peru found 283 tree species in a 1 ha plot, with 63% represented by only one individual and 15% by only two. In marked contrast, temperate forest in North America typically has fewer than 9-12 tree species in plots of this size. The richest temperate forest in the world, in the Great Smokies of the eastern U.S., contains 30–35 tree species, far fewer than occur in tropical forest. More northern boreal forest, which covers much of Canada, has only 1-4 species of trees present.

A few studies have been made of the richness of bird species in tropical rainforest. A study of Amazonian forest in Peru found 245 resident bird species, plus another 74 migrants, in a 97-ha plot. Another study found 239 species of birds in a rainforest in French Guiana. In contrast, temperate forest in North America typically supports 30-40 species of birds. Not many comprehensive studies have been made of other kinds of biota in tropical ecosystems. In one study, a 108 km² area of dry tropical forest in Costa Rica was found to contain about 700 species of plants, 400 vertebrate species, and 13,000 species of insects, including 3,140 kinds of moths and butterflies.

Another important component of biodiversity is functional diversity, which generally concerns the range of things that organisms do in communities and ecosystems. A widely adopted definition of functional diversity is “the value and the range of those species and organismal traits that influence ecosystem functioning” (Tilman, 2001). Many organismal traits are functionally important, both from an individual fitness perspective and a community function perspective (e.g., plant photosynthetic pathway, presence of secondary metabolites, flowering time, predator tooth morphology, pollinator size, nocturnal vs. diurnal foraging, etc.). Measuring functional diversity is important for understanding ecosystem structure and function, but is also important for maintaining ecosystem services for the purposes of conservation and land management. The roles that species play can be extremely beneficial to humanity by providing food, clean water, and myriad other services that support our everyday lives. If an area is species-rich, but all of those species are functionally similar, then some species can be lost without a subsequent loss of any ecosystem functionality. Conversely, if an area has few species, but all of them are doing something functionally different, then any loss of species will sacrifice some ecosystem functionality.

Richness of Ecosystems

Biodiversity at the level of landscape (or seascape; collectively these are referred to as ecoscapes) is associated with the number of different ecosystems that occur within a specified region, as well as their relative abundance, size, shape, connections, and spatial distribution. An area that is uniformly covered with a single kind of ecosystem would be judged as having little biodiversity at the level, compared with an ecoscape having a rich and dynamic mosaic of different ecosystems (Image 6.2).

Image 6.2. Ecosystem Biodiversity. This intertidal community sustains various algae, barnacles, mussels, starfish, and other species that vary in their tolerance of environmental stress associated with tidal cycles. Source: “tide pools” by Mookie Forcella is licensed under CC BY-NC 2.0.

Because natural ecoscapes contain many species and communities that have evolved together, it is as important to conserve this level of biodiversity as it is to protect genetic and species diversity. Natural communities, landscapes, and seascapes are being lost in all parts of the world, with the worst damages involving the destruction of tropical forest and coral reefs. However, dramatic losses of this level of biodiversity are also occurring in the United States:

• Only about 1-4% of the original area of tall-grass prairie remains, the rest having been converted to agricultural use.
• The survival of old-growth forest in coastal California is at risk, with the long-standing Redwoods and Sequoias feeling the effects of climate change and its influence on increasing frequency and intensity of wildfires. Throughout the U.S., wetlands of all kinds have been destroyed or degraded by pollution, in-filling, and other disturbances.
• Natural fish populations have been widely decimated, including mixed-species communities in the Great Lakes, populations of salmon near Alaska, and cod along the Atlantic coast.
• The habitats of various bottom dwelling organisms of aquatic ecosystems have been obliterated by the extensive practice of bottom-dragging in fisheries on the continental shelves, with great consequences for dependent types of ecological communities.

In all of these examples, only remnant patches of endangered natural communities and ecoscapes remain. These are at great risk because they are no longer components of robust, extensive, naturally organizing ecosystems.

The Value of Biodiversity

Biodiversity is important for many reasons. The value of biodiversity provides credence for its conservation. The reasons why biodiversity is important can be categorized into several groups.

Utilitarian Value

Humans are not isolated from the rest of the biosphere, in part because our survival depends upon having access to products of certain elements of biodiversity. Because of this requirement, humans must exploit species and ecosystems as sources of food, biomaterials, and energy—in other words, for their utilitarian value (also known as instrumental value).

For instance, all foods that we eat are ultimately derived from biodiversity. Moreover, about one-quarter of the prescription drugs dispensed in North America contain active ingredients extracted from plants. In addition, there is a wealth of additional, as yet undiscovered products of biodiversity that are potentially useful to people. Research on wild species of plants, animals, and microorganisms has discovered many new bio-products that are useful as food, medicines, materials, or other purposes (Image 6.3). Like many of the species already known to be useful, some of the newly discovered ones have a potentially large economic value.

To illustrate the importance of medicinal plants, consider the case of the rosy periwinkle (Catharanthus roseus), a small herbaceous plant that is native to Madagascar, a large island off northeastern Africa. One method used in the search for anti-cancer drugs involves screening large numbers of wild plants for the presence of chemicals that have an ability to slow the growth of tumors. During one study of that kind, an extract of rosy periwinkle was found to counteract the reproduction of cancer cells. Further research identified the active chemicals to be several alkaloids, which are probably synthesized by the rosy periwinkle to deter herbivores. These natural biochemicals are now used to prepare the drugs vincristine and vinblastine, which have proved to be extremely useful in chemotherapy to treat childhood leukemia, a cancer of the lymph system known as Hodgkin’s disease, and several other malignancies.

The exploitation of wild biodiversity can be conducted in ways that allow the renewal of harvestable stocks. Unfortunately, many potentially renewable biodiversity resources are overharvested, which means they are managed as if they were non-renewable resources (they are being “mined”; see Chapters 11 and 12). This results in biological resources becoming degraded in quantity and quality.

Sometimes, over-exploited species become locally extirpated or are even rendered globally extinct, and when this happens their unique values are no longer available for use by humans. The great auk and passenger pigeon are examples of North American species that were made extinct by over-harvesting. Local and regional extirpations have been more numerous and include the cougar, grizzly bear, timber wolf, and wild ginseng over most of their former ranges.

Image 6.3. Broadleaf Plantain. Many elements of biodiversity provide products useful to people as food, materials, and medicines. Indigenous people have relied on their traditional knowledge of the plants in their community for a wide variety of ailments that we now treat with modern medicine. Even the extremely common pioneer species, broadleaf plantain (Plantago major) that many consider a weed, is not only edible, but has medicinal uses for its anti-inflammatory and anti-bacterial properties. It is commonly used as treatment for stings, sores, or wounds. In addition to broadleaf plantain, many common plants and “weeds” actually have immense health and medicinal properties. Source: “Broadleaf plantain” by RD Stevenson is licensed under CC BY-NC 2.0.

Provision of Ecological Services

Biodiversity provides many ecological services that are critical to the stability and integrity of ecosystems as well as the welfare of humans (Figure 6.3). They include nutrient cycling, biological productivity, control of erosion, provision of oxygen, and removal of carbon dioxide and its storage as organic carbon. All of these services are critical to the welfare of people and other species, but they are not usually assigned economic value. In part, this is because we do not yet have sufficient understanding and appreciation of the “importance” of ecological services and of the particular species and communities that provide them. According to Peter Raven, a famous botanist and advocate of biodiversity, “In the aggregate, biodiversity keeps the planet habitable and ecosystems functional.”

Figure 6.3. Benefits of and Threats to Biodiversity. This eco-wheel image shows natural resources provided by biodiversity, the benefits and beneficiaries, and drivers of change. Source: EPA.

Intrinsic Value

Biodiversity has its own intrinsic value (or inherent value), regardless of any direct or indirect worth in terms of the needs or welfare of humans. This value is fundamental to all elements of biodiversity, and is irreplaceable. This intrinsic value raises certain ethical questions about actions that threaten biodiversity. Do humans have the “right” to impoverish or exterminate unique and irretrievable elements of biodiversity, even if our species is technologically able to do so? Is the human existence somehow impoverished by extinctions caused by our actions? These are philosophical issues, and they cannot be resolved by science alone.

Biodiversity Is Worthwhile

Many people firmly believe that wild biodiversity and natural ecosystems are worthwhile and important. They cite the utilitarian and intrinsic values of biodiversity, but may also mention less tangible opinions, such as the charisma of many species (such as wolves, pandas, and baby harp seals) and the spirituality of natural places (such as towering old-growth forest and other kinds of wilderness). Because this belief is becoming increasingly widespread and popularized, it is having a major influence on politicians, who are including biodiversity issues in their agendas for action—threats to biodiversity have become politically important.

Undoubtedly, there is an undiscovered wealth of products of biodiversity that are potentially useful to humans. Many of these bio-products will be found in tropical species that have not yet been “discovered” by biologists. Clearly, the most important argument in favor of preserving biodiversity is the need to maintain natural ecosystems so they can continue to provide their vast inventory of useful products and their valuable ecological services. In addition, biodiversity must also be preserved for its intrinsic value.

Classification of Organisms

Biologists classify species into higher-order groupings on the basis of their relatedness and similarities. Similarity is judged using information about anatomy, development, biochemistry, behavior, and habitat selection. These classifications are made by systematists (biologists who study the evolutionary relationships among groups of organisms) and taxonomists (who focus on naming groups of organisms).

The systematics of life is organized hierarchically, with levels ranging through subspecies, species, genus, family, order, class, phylum, kingdom, and domain. This system is illustrated in Figure 6.4.

Figure 6.4. Biological Classification. The hierarchical, systematic classification of organisms is illustrated by the taxonomic breakdown of domestic dogs. Source: Concepts of Biology is licensed under CC BY 4.0.

A species is described using two Latinized words, known as a binomial. If a subspecies is also recognized, the name has three Latin words (such as Pseudotsuga menziesii glauca, the interior form of the Douglas-fir). Many species also have a scientifically recognized “common name,” and they may also have informal common names. For example, the scientifically recognized common name of the widespread tree Populus tremuloides is trembling aspen, but this species is also known as aspen, golden aspen, mountain aspen, poplar, quaking asp, quaking aspen, trembling poplar, and that old-time favorite, “popple.” Some of the common names have only a local use and are unknown in other parts of the range of the species. Common names may also overlap among species—for instance, both the balsam poplar (Populus balsamifera) and large-toothed aspen (P. grandidentata) are often called “poplar.” To avoid the ambiguities associated with common names, species are assigned a globally recognized binomial and sometimes a “proper” common name. Because of this system, biologists working in Canada, the U.S., Germany, Turkey, Russia, China, and other countries where the animal Ursus arctos occurs all know it by its binomial. In English, this animal is known as the grizzly or brown bear, and in other languages by other common names. But no one is confused by its scientific binomial name.

The Organization of Life

Most biologists divide all of Earth’s species into six major groups, known as kingdoms (Figure 6.5). Although somewhat controversial and subject to ongoing refinement, this systematic organization is believed to reflect the evolutionary relationships among groups of organisms. The kingdoms and their major characteristics are briefly described below.

Figure 6.5. The Six-Kingdom System of Biological Classification. Source: Encyclopædia Britannica.

Archaea and Bacteria

These organisms are the simplest of single-celled microorganisms and include bacteria and blue-green bacteria, the latter being photosynthetic. They are prokaryotes, because their genetic material is not contained within a membrane-bounded organelle called a nucleus. Organisms in the other kingdoms have nuclei within their cells and are called eukaryotes. Prokaryotes also do not have other kinds of organelles, such as chloroplasts, flagella, or mitochondria. They were the first organisms to evolve, about 3.5 million years ago. It was not until 1.5 billion years ago that the first eukaryotes appeared.

At least 9,000 species of bacteria have been named (Figure 6.2), but there are many additional species that have not yet been described by microbiologists (could be as many as 10⁷-10⁹ species!). The diversity of bacteria includes species capable of exploiting a phenomenal range of ecological and metabolic opportunities. Many are decomposers, found in “rotting” biomass. Some species are photosynthetic, others are chemosynthetic, and still others can utilize virtually any organic substrate for their nutrition, either in the presence or absence of oxygen. Some bacteria can tolerate extreme environments, living in hot springs as torrid as 78°C, while others are active as deep as 400 m in glacial ice.

Many bacterial species live in mutually beneficial symbioses (mutualisms) with more-complex organisms. For example, some live as a community in the rumens of cows and sheep, and others live in the human gut, in both cases aiding in the digestion of food. Other bacteria, known as Rhizobium, live in the roots of leguminous plants (such as peas and clovers), where they fix atmospheric nitrogen gas into a form (ammonia) that plants can use as a nutrient (see Chapter 5).

Many bacteria are parasites of other species, causing various diseases. For example, Bacillus thuringiensis is a pathogen of moths, butterflies, and blackflies and has been used as a biological insecticide against certain pests in agriculture and forestry. Species of bacteria also cause important diseases of humans, including cholera, diphtheria, gonorrhea, Legionnaires’ disease, leprosy, pneumonia, scarlet fever, syphilis, tetanus, tooth decay, tuberculosis, whooping cough, most types of food poisoning, and the “flesh-eating disease” caused by a virulent strain of Streptococcus.

Protista

Protists include a wide range of simple, eukaryotic organisms, comprising both unicellular and multicellular species. Protists include foraminifera, protozoans, slime molds, and single-celled and multicellular algae. The latter group includes the large seaweeds known as kelps, some of which are over 10 m long. The kingdom Protista consists of about 60,000 named species, which vary enormously in their genetics, morphology, and function. Many biologists believe that the Protista is a catch-all group of not-so-closely related groups. Recently proposed taxonomy divides Protista into several kingdoms because of accumulating evidence of key differences among groups and recognition that the other, more-complex eukaryotic kingdoms (fungi, plants, and animals) evolved from different protistan ancestors.

Several phyla of protists, broadly known as algae, are photosynthetic. These groups include the diatoms (Bacillariophyta), green algae (Chlorophyta), dinoflagellates (Dinoflagellata), euglenoids (Euglenophyta), red algae (Rhodophyta), and brown algae such as kelps (Phaeophyta). Algae are important primary producers in marine and freshwater ecosystems. Some seaweeds are harvested to extract chemicals known as alginates, which are important additives to many foods and cosmetics. Uncommon marine phenomena known as “red tides” are blooms of certain dinoflagellates that produce extremely toxic metabolites.

Other phyla of protists are heterotrophic in their nutrition. These groups include the ciliates (Ciliophora), forams (Foraminifera), slime molds (formerly Myxomycota), amoebae (Rhizopoda), and unicellular flagellates (Zoomastigina). Forams are unicellular microorganisms that form an architecturally complex shell of calcium carbonate, the remains of which may accumulate over geological time to form a mineral known as chalk- the white cliffs of Dover in southern England are made of foram remains. Trypanosomes are unicellular flagellates that are responsible for sleeping sickness, a disease of humans and other vertebrate animals. Certain species of amoebae are parasites of animals, including amoebic dysentery in humans. The ciliate Giardia causes a water-borne disease known as hiker’s diarrhea (or beaver fever), the risk of which is a reason why even the cleanest-looking natural water should be boiled or otherwise disinfected before drinking.

Fungi

This kingdom consists of yeasts, which are single-celled microorganisms, and fungi, which are multicellular and filamentous. Fungi evolved at least 400 million years ago, but they may be much older than that because their remains do not fossilize well. Fungal cells excrete enzymes into their surroundings, which then externally digest complex organic materials. The fungus then ingests the resulting simple organic compounds. All fungi are heterotrophic—most are decomposers of dead organic matter, while others are parasitic on plants or animals. There are seven major divisions (phyla) of fungi, and both sexual reproduction and a sexual reproduction is also common. The two largest of these groups, the Basidiomycota and the Ascomycota, are discussed briefly below.

The ascomycetes (division Ascomycota) include about 33,000 named species, some of which are commonly known as cup fungus. During sexual reproduction, ascomycetes form numerous microscopic, spore-producing structures known as asci, which are often (but not always) located in cup-shaped fleshy structures called ascocarps. Familiar species include yeasts, morels, and truffles, as well as the pathogens that cause chestnut blight and Dutch elm disease (see below).

The basidiomycetes (division Basidiomycota) include about 31,000 named species. Sexual reproduction involves a relatively complex fruiting body known as a basidiocarp, which depending on its shape may be called a mushroom, puffball, toadstool, or shelf fungus (spores are produced within specialized structures called basidia on the surface of part of the basidiocarp). In the U.S., one of the largest of these structures is developed by the giant puffball (Calvatia spp.), which has a ball-like basidiocarp with a diameter up to 50 cm.

Lichens are mutualisms between a fungus and either an alga or a blue-green bacterium. Most of the lichen biomass is fungal tissue, which provides habitat and inorganic nutrients for the photosynthetic partner, which in turn provides organic nutrition to the fungus. Another type of mutualism, known as a mycorrhiza, involves a relationship between plant roots and certain fungi. This relationship is beneficial to the plant because it allows more efficient absorption of inorganic nutrients from the soil, especially phosphate. About 80% of plant species develop mycorrhizae.

Fungi are ecologically important because they are excellent decomposers, allowing nutrients to be recycled and reducing the accumulation of dead biomass (Image 6.4).

Image 6.4. Fungi as Decomposers. Fungi are critical for nutrient cycling by decomposing organic matter, including coarse woody debris, and releasing nutrients into the soil and atmosphere. Source: “Beautiful Bonnet” by Holger Krisp is licensed under CC BY 3.0.

Various kinds of fungi are economically important because they spoil stored grain and other foods, are parasites of agricultural or forestry plants, or cause diseases in humans and other animals. Ringworm is a disease of the skin, usually the scalp, which is caused by various fungi. The chestnut blight fungus (Endothia parasitica) was accidentally introduced to North America and wiped out the native chestnut (Castanea dentata), which used to be a prominent and valuable tree in eastern forests. The Dutch elm disease fungus (Ceratocystis ulmi) is another introduced pathogen that is killing elm trees (especially white elm, Ulmus americana).

Economically useful fungi include a few species of yeast that can ferment sugars under anaerobic (O₂-deficient) conditions, yielding gaseous CO₂ and ethanol. The CO₂ raises bread dough prior to baking, while brewers take advantage of the alcohol production to make beer and wine. Other fungi are used to manufacture cheese, soy sauce, tofu, food additives such as citric acid, and antibiotics such as penicillin.

Some mushroom-forming fungi are cultivated as a food, while other edible species are collected from natural habitats. The most commonly cultivated species is the button mushroom (Agaricus bisporus), while the most prized wild mushroom is the extremely flavorful truffle (Tuber melanosporum). Some wild mushrooms contain chemicals that induce hallucinations, feelings of well-being, or other pleasurable mental states, and are sought by people for religious or recreational use. These include the fly agaric (Amanita muscaria), a species widespread in North America and elsewhere, and Psilocybe spp. of more southern regions of North and Central America. Some wild mushrooms are deadly poisonous even when eaten in tiny quantities. Among the most toxic species in North America are the destroying angel (Amanita virosa) and death cap (A. phalloides).

Plantae

Plants are photosynthetic organisms that manufacture their food by using the energy of sunlight to synthesize organic molecules from inorganic ones. Plants evolved from multicellular green algae about 430 million years ago, and the first tree-sized ones appeared 300 million years ago. Plants are different from algae in that they are always multicellular, have cell walls rich in cellulose, synthesize a variety of photosynthetic pigments (including chlorophylls and carotenoids), and use starch as their principal means of storing energy. Plants are extremely important as photosynthetic fixers of CO₂ into organic carbon, and they are dominant in terrestrial ecosystems, where algae and blue-green bacteria are sparse. Plants can be separated into 12 divisions (phyla), which are aggregated into two functional groups.

Bryophytes are relatively simple plants that lack vascular tissue and do not have a waxy cuticle covering their foliage, a characteristic that restricts these plants to moist habitats. The bryophytes consist of the following:

• Liverworts (division Hepatophyta), of which there are about 9,000 species
• Mosses (Bryophyta), including about 12,000 species, which are prominent in some wetlands, especially in bogs, where the dead biomass of peat mosses (species of Sphagnum) accumulates as a partially decayed material known as peat, which is mined as a soil conditioner and a source of energy
• Hornworts (Anthocerophyta), with 200 species

Vascular plants are relatively complex and have specialized, tube-like, vascular tissues in their stems for conducting water and nutrients. There are nine divisions (phyla) of vascular plants:

• Whisk ferns (division Psilophyta), containing several species
• Club mosses and quillworts (Lycophyta), about 1,200 species
• Horsetails or scouring rushes (Sphenophyta), 15 species
• Ferns (Pterophyta), 12,000 species
• Cycads or sago palms (Cycadophyta), 100 species
• Gnetums (Gnetophyta), 70 species
• Ginkgo (Ginkgophyta), with one relict species (Ginkgo biloba)
• Conifers (Coniferophyta), including about 600 species of firs, hemlocks, pines, redwoods, spruces, yews, and others
• Flowering plants (Anthophyta), containing a diverse assemblage of about 391,000 species

The flowering plants are also known as angiosperms, because their ovules are enclosed within a specialized membrane, and their seeds within a seed coat. The conifers, ginkgo, and gnetums lack these structures and are referred to as gymnosperms. Together, the angiosperms and gymnosperms are known as seed plants. Their seeds develop from a fusion between specialized haploid cells known as pollen and ovules, in a process called pollination.

The seed plants are extremely diverse in their form and function. The tallest species are redwood trees (Sequoia sempervirens), which can exceed 100 m in height. The smallest is an aquatic plant known as watermeal (Wolffia spp.), only the size of a pinhead. Many seed plants live for less than one year (these are “annual” plants), while the age of others can exceed 4,500 years—for example, the oldest bristlecone pines (Pinus longaeva; Image 6.5).

Image 6.5. The World’s Oldest Trees. The bristlecone pine, Pinus longaeva, is the longest living plant, with known specimens nearly 5,000 years old. Source: “Gnarly Bristlecone Pine” by Rick Goldwaser is licensed under CC BY 2.0.

Many flowering plants grow as shrubs or trees. Rigid, woody tissues in their stems provide mechanical strength that allows these plants to grow tall against the forces of gravity and wind. Other angiosperms lack rigid stem tissues and grow as herbaceous plants that die back to the ground at the end of the growing season.

Species of angiosperms are important crops in agriculture, while both conifer and angiosperm trees are prominent in forestry. Plants are also economically important as sources of biochemicals in industry and medicine, and because they provide the food and habitat required by so many other organisms, including many animals that are used by people as food.

Animalia

Animals are multicellular organisms, and most are mobile during at least some stage of their life history, having the ability to move about to search for food, to disperse, or to reproduce. Animals are heterotrophs: they must ingest their food, ultimately consuming the photosynthetic products of plants or algae.

Most animals (except the sponges) have their cells organized into specialized tissues that are further organized into organs. Almost all animals reproduce sexually, a process that involves the joining of haploid gametes from a male and female to produce a fertilized egg. Animals comprise the bulk of identified species of organisms, with insects being the most diverse group. Apart from these broad generalizations, animals are extremely diverse in their form and function. They range in size from the largest blue whales (Balaenoptera musculus), which can reach 32 m in length and 136 tonnes of weight, to the smallest beetles and soil mites, which are less than 1 mm long and weigh a few milligrams.

The animal kingdom includes about 35 phyla. The majority occur in marine habitats, with a smaller number in freshwater and on land. All animals in all the phyla except one are considered to be invertebrates (with no backbone), while the phylum Chordata includes the vertebrates – animals with a backbone. The most prominent phyla are described below.

Sponges (phylum Porifera) include a marine group of about 5,000 species plus 150 freshwater ones. Sponges are simple animals, sessile (non-mobile) as adults, with no differentiation of tissues into organs. They filter-feed on organic matter suspended in their watery habitat. The slow flow of water through sponges is driven by surface cells that use flagella, tiny whip-like structures, to move water over their surface.

Cnidarians (phylum Cnidaria) include about 9,000 species, almost all of which are marine. Familiar groups include corals, hydroids, jellyfish, and sea anemones. Cnidarians have a simple, gelatinous body structure. They display radial symmetry, meaning a cross-section in any direction through their central axis yields two parts that are mirror images. Jellyfish are weakly swimming or floating animals, with a body form known as a medusa. Most other cnidarians are sessile as adults, being attached to a bottom substrate. Cnidarians are carnivores that use tentacles ringing their mouth opening to capture prey, often after subduing the victim by stinging it with specialized cells. Corals develop a protective casement of calcium carbonate and are important reef-building organisms.

Flatworms and tapeworms (phylum Platyhelminthes) include about 20,000 species of soft-bodied, ribbon-shaped animals. Many flatworms are free-living scavengers or predators of small animals, while tapeworms and flukes are internal parasites of larger animals, including humans.

Nematodes (phylum Nematoda) include 15,000 species of small, worm-like creatures. These animals are round in cross-section and are abundant in almost all habitats that contain other forms of life, ranging from aquatic habitats to desert. Many species are parasites, living in or on their hosts. Virtually all plants and animals are parasitized by one or more species of nematodes, which are often specialized to a particular host. Species of hookworms, pinworms, and roundworms are important parasites of humans. The Trichinella roundworm causes a painful disease known as trichinosis, while Filaria causes filariasis, a tropical disease.

True worms (phylum Annelida) include about 22,000 species of tubular, segmented, soft-bodied animals. Most worms are marine, but others occur in freshwater and moist terrestrial habitats. Worms are divided into three major groups: bristleworms or polychaetes, typical worms or oligochaetes (including earthworms), and leeches or hirudineans. Most feed on dead organic matter, but leeches are blood-sucking parasites of larger animals. Earthworms provide an important service by helping to recycle dead biomass in many terrestrial habitats.

Molluscs (phylum Mollusca) comprise about 100,000 species of clams, cuttlefish, octopuses, oysters, scallops, slugs, snails, and squids. Many have a hard shell of calcium carbonate that protects the soft body parts. Other molluscs, such as squid and octopus, lack this hard shell. Molluscs are most abundant in marine and freshwater habitats, with relatively few terrestrial species. Most are herbivores or scavengers, but some are predators. Various species are used by humans as food, and several produce pearls, used for making jewelry. Some slugs and snails are pests in agriculture, while others are alternative hosts for certain parasites, such as the tropical fluke that causes schistosomiasis in humans.

Arthropods (phylum Arthropoda) comprise the largest group of organisms. There are more than a million named species and likely millions of others that have not yet been described. Arthropods have an exterior skeleton (exoskeleton) made of a polysaccharide known as chitin, with their body parts segmented to allow movement. They have at least three pairs of legs. The most abundant groups are the spiders and mites (class Arachnida), crustaceans (Crustacea), centipedes (Chilopoda), millipedes (Diplopoda), and insects (Insecta). Insects alone make up more than half of all named species. Arthropods are of great economic importance, with some species being used by people as food (such as lobster), and others used to produce food (such as the honey of certain bees). Termites damage buildings by eating wood, while various insects are pests in agriculture. Species of mosquitoes, blackflies, fleas, and ticks spread diseases of humans and other animals, including malaria, yellow fever, encephalitis, and plague.

Echinoderms (phylum Echinodermata) include about 7,000 species of marine animals, such as brittle stars, sand dollars, sea stars, sea cucumbers, and sea urchins. Echinoderms have radial symmetry as adults. Most have an exoskeleton of calcium carbonate, some are covered with spiny projections, and some move about using large numbers of small, tube-feet. Sea urchins and sea cucumbers are harvested as a minor source of food, popular in some Asian countries.

Chordates (phylum Chordata) are the most familiar group of animals. Distinctive characters (in at least the embryonic phase) include a hollow nerve cord that runs along the dorsal (top) surface and a flexible, rod-like dorsal structure (the notochord), which is replaced by the vertebral column in adults. There are about 65,000 species of chordates, divided among three subphyla. The tunicates (Urochordata) are composed of about 1,000 species of marine animals, including sea grapes and sea peaches. Tunicates have a small notochord and adults are sessile filter-feeders. The lancets (Cephalochordata) consist of 23 species of filter-feeding marine animals, which have a long, laterally compressed body. The vertebrates (Vertebrata) comprise almost all species in the group, most of which have a vertebral column as adults. The major classes of living vertebrates are the following.

• The jawless fishes (class Agnatha) include 120 species of lampreys and hagfishes, which first evolved 470 million years ago. These marine or freshwater animals have a notochord and a skeleton of cartilage.
• The cartilaginous jawed fishes (class Chondrichthyes) consist of 1,000 species of dogfish, rays, sharks, and skates, all of which occur in marine habitats. Cartilaginous fishes evolved more than 410 million years ago.
• The bony fishes (class Osteichthyes) include about 30,000 species of typical fish, such as cod, salmon, tuna, and guppies. The first bony fishes evolved about 390 million years ago.
• The amphibians (class Amphibia) consist of 7,000 species of frogs, salamanders, toads, and legless caecilians. The first amphibians evolved about 330 million years ago. Early stages in the life history (egg and larva) are aquatic, but adult stages of many species can live in terrestrial habitats.
• The reptiles (class Reptilia) include 10,000 species of crocodilians, lizards, snakes, and turtles. Reptiles first evolved about 300 million years ago. Extinct groups include the dinosaurs, plesiosaurs, and pterosaurs, the last of which became extinct about 65-million years ago. Reptiles were the first fully terrestrial animals, capable of completing all stages of their life history on land (although some species, such as turtles, are highly aquatic as adults). Reptiles have a dry skin and lay eggs on land. Their young are miniature versions of the adults.
• The birds (class Aves) consist of 10,000 species, which first evolved about 225 million years ago from small, dinosaurian ancestors. Birds are homeothermic (warm-blooded), are covered in feathers, lay hard-shelled eggs, and have a horny covering of the jaws known as a beak. Most species can fly, the exceptions being the largest birds, penguins, and many species that evolved on islands lacking predators.
• The mammals (class Mammalia) consist of 6,500 species, which first evolved about 220 million years ago (the earliest fossil mammals are difficult to distinguish from reptiles) (Image 6.6). Mammals became prominent after the extinction of the last dinosaurs, about 65 million years ago. Mammals are homeotherms, have at least some hair on their body, feed their young with milk, and have a double circulation of the blood (i.e., a four-chambered heart and fully separate circulatory systems for oxygen-poor and oxygen-rich blood). There are three major groups of mammals: Monotremes are a few species of egg-laying mammals that live in Australia and New Guinea—the platypus and several species of echidnas. Marsupials bear live young that at birth are at an extremely early stage of development. After birth, the tiny young migrate to a special pouch (the marsupium) on the mother’s belly where they develop further while feeding on milk. Examples of marsupials include kangaroos, koala, and wallabies, which live only in Australia, New Guinea, and nearby islands, and the opossum of the Americas. Placental mammals include many familiar species of the Americas, Africa, and Eurasia. Placental mammals give birth to live young that are suckled by the mother. Humans are a species of placental mammal.

Image 6.6. Playing with Dogs on the Beach. Humans and dogs are species of mammals. Humans (Homo sapiens), along with other great apes, are in the family Hominidae. Dogs (Canis lupus familiaris) are a domesticated subspecies of the wolf and are in the family Canidae. Source: “Playing with Dogs on the Beach” by donjd2 is licensed under CC BY 2.0.

Conclusions

Biodiversity is the richness of biological variation—it exists at the levels of genetics, species richness, and community diversity on landscapes and seascapes. Biodiversity is important to the survival of humans and their economy, and also to all other species. Biodiversity also has inherent value. Human activities have resulted in the extinction of many elements of biodiversity, and the survival of many others is being placed at grave risk (Chapter 14). Damage to biodiversity is a principal aspect of the environmental crisis.

Questions for Review

1. What are the major components of biodiversity? Provide an example of each.
2. Pick any species in which you are interested. Illustrate the hierarchical classification of life by giving the scientific names of its species, genus, family, order, class, phylum, and kingdom.
3. What are the six kingdoms of life? Identify several groups within each of the kingdoms.

Questions for Discussion

1. Why is biodiversity important? Outline several reasons.
2. Discuss the notion that all species are similarly “advanced” in the evolutionary sense but may vary greatly in their complexity.
3. All elements of biodiversity are considered to have intrinsic value. What does this mean? Can it be fully justified in a strictly scientific context?
4. Choose an economically important “pest,” such as the house mouse (Mus musculus), a disease-carrying mosquito (such as an Anopheles species), or the groups A and B Streptococcus bacteria that cause deadly infections. Now suppose that a new method has been discovered to eradicate that pest, which would cause its global extinction. Based on ideas about intrinsic value and other considerations, could you mount a logical defense of the pest to argue against its extinction?

Exploring Issues

1. You are a biodiversity specialist, and a group of politicians has asked why it should spend public money to protect an endangered species occurring within their jurisdiction. You know that these people are skeptical, and that if you do not convince them to preserve the species and its habitat, it may become extinct. What information and arguments would you include in your presentation to the politicians?
2. Make a comprehensive list of products of biodiversity that you use in a typical day. The list can include raw and processed foods, medicines, materials, and sources of energy.

References Cited and Further Reading

Begon, M., R.W. Howorth, and C.R. Townsend. 2014. Essentials of Ecology. 4th ed. Wiley, Cambridge, UK.

Bernhardt, T. n.d. The Canadian Biodiversity Website. Heritage Canada and the Redpath Museum, McGill University, Montreal, PQ. http://canadianbiodiversity.mcgill.ca/english/

Bolandrin, M.F., J.A. Klocke, E.S. Wurtele, and W.H. Bollinger. 1985. Natural plant chemicals: Sources of industrial and medicinal materials. Science, 228: 1154-1160.

Boyd, R. 1988. General Microbiology. Mosby Year Book, St. Louis, MO.

Chapman, A.D. 2009. Numbers of Living Species in Australia and the World, 2nd ed. Australian Biological Resources Study, Department of the Environment, Canberra. http://www.environment.gov.au/node/13875

Ehrlich, P.R., and A. Ehrlich. 1981. Extinction: The Causes and Consequences of the Disappearance of Species. Ballantine, New York, NY.

Environment Canada. 1997. The State of Canada’s Environment. Ottawa: State of the Environment Reporting Organization, Environment Canada, Ottawa, ON.

Erwin, T.L. 1991. How many species are there? Revisited. Conservation Biology, 5: 330-333.

Freedman, B. 1995. Environmental Ecology. 2nd ed. Academic Press, San Diego, CA.

Freedman, B., J. Hutchings, D. Gwynne, J. Smol, R. Suffling, R. Turkington, R. Walker, and D. Bazeley. 2014. Ecology: A Canadian Context. 2nd ed. Nelson Canada, Toronto, ON.

Gaston, K.J. (ed.). 1996. Biodiversity: A Biology of Numbers and Difference. Blackwell Science, Cambridge, UK.

Gaston, K.J. and J.I. Spicer. 2004. Biodiversity: An Introduction. 2nd ed. Blackwell Science, Cambridge, UK.

Groombridge, G. 1992. Global Biodiversity. World Conservation Monitoring Center. Chapman & Hall, London, UK.

Groombridge, B. and M.D. Jenkins. 2002. World Atlas of Biodiversity: Earth’s Living Resources in the 21st Century. University of California Press, Berkeley, CA.

Heywood, V.H. (ed.). 1995. Global Biodiversity Assessment. Cambridge University Press, Cambridge, UK.

Janzen, D.H. 1987. Insect diversity in a Costa Rican dry forest: Why keep it, and how. Biological Journal of the Linnaean Society, 30: 343-56.

Miller, K. and L. Tangley. 1991. Trees of Life. Beacon, Boston, MA.

Myers, N. 1983. A Wealth of Wild Species. Westview, Boulder, CO.

Perlman, D.L. and G. Adelson. 1997. Biodiversity: Exploring Values and Priorities in Conservation. Blackwell Science Publishers, Cambridge, UK.

Pough, F.H., C.M. Jans, and J.B. Hirser. 2012. Vertebrate Life. 9th ed. Prentice Hall, Upper Saddle River, NJ.

Raven, P.H., G.B. Johnson, K.A. Mason, and J. Losos. 2013. Biology. 10th ed. McGraw-Hill, Columbus, OH.

Reaka-Kudla, M.L., D.E. Wilson, and E.O. Wilson (eds.). 1997. Biodiversity II: Understanding and Protecting Our Biological Resources. National Academy Press, Washington, DC.

Terborgh, J., S.K. Robinson, T.A. Parker, C.A. Muna, and N. Pierpont. 1990. Structure and organization of an Amazonian forest bird community. Ecological Monographs, 60: 213-238.

Tilman, D. Functional diversity. S.A. Levin (Ed.). 2001. Encyclopedia of Biodiversity, vol. 3, Academic Press, New York, pp. 109-120.

United Nations Environment Program. 2001. Global Biodiversity Outlook. Secretariat of the Convention on Biological Diversity, Montreal, PQ.

Wilson, E.O. (ed.). 1988. Biodiversity. National Academy Press, Washington, DC.