7: Biodiversity

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.

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 7.1. 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 Canada). 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 26). The magnitude of this crisis is much smaller in Canada. Nevertheless, many of our native species have become extinct or otherwise at risk because of over-harvesting or habitat loss (Chapter 26).

A total of about 1.9 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 (Table 7.1). The scientist J.B.S. Haldane (1892–1964) was once asked by a theologian to succinctly tell, based on his deep knowledge of biology, what he could discern of God’s purpose. Haldane reputedly said that God has “an inordinate fondness of beetles.” This reflects the fact that, in any random sampling of all the known species on Earth, there is a strong likelihood that a beetle would be the chosen specimen.

*This is a conservative number. Some estimates suggest more than 30 million species of insects living in tropical forests alone (see text). Sources: Modified from Groombridge (1992), Heywood (1995), Environment Canada, (1997), Chapman (2009), and United Nations Environment Program (2001), and Bernhardt (n.d.).

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 Canada, 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 Canada and other relatively well-studies 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 United States, 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.

Richness of Communities

Biodiversity at the level of landscape (or seascape; collectively these are referred to as ecoscapes) is associated with the number of different communities 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 community would be judged as having little biodiversity at the level, compared with an ecoscape having a rich and dynamic mosaic of different communities.

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 Canada:

• Only about 0.2% of the original area of tall-grass prairie remains, the rest having been converted to agricultural use.
• Almost all of the Carolinian forest of southern Ontario has been destroyed, mostly by conversion to agricultural and urbanized landscapes.
• The survival of old-growth forest in coastal British Columbia is at risk, with the dry coastal Douglas-fir type being especially depleted. The loss of old-growth forest is mostly due to timber harvesting, which converts the ecosystem into a younger, second-growth forest (see Chapter 23).
• Throughout southern Canada, 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 salmonids (salmon and trout) in western Canada, and cod and redfish off the Atlantic Provinces.
• The habitats of various bottom types 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 Canadian 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.

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. 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 (Catharantus 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 tumours. 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 12 and 14). 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 Canadian 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 (see Chapters 14 and 26).

Canadian Focus 7.1. Medicinal Plants
Plants and products derived from them have always been vital to human survival, being used as sources of food, medicine, material, and energy. For instance, most foods eaten by people are the biomass of plants; the rest is animal or microbial products, but even these are produced indirectly from plants. Moreover, useful products are derived from a great richness of plant species—about 1,800 medicinal plants are commercially available in North America, and perhaps 20,000 worldwide. All of these bio-products are potentially renewable resources that can be harvested and managed on a sustainable basis (see Chapter 12).

Studies by anthropologists have repeatedly shown that Aboriginal peoples are intimately aware of useful medicinal plants that grow within their local ecosystems. This “traditional ecological knowledge” is helpful in identifying useful plants for further investigation by scientists. Nevertheless, only a small fraction of the enormous richness of biodiversity has been investigated by scientists for its potential to supply us with useful products. Because of the likelihood of discovering new bio-products, it is imperative that we continue to engage in “bio-prospecting” research. Work of this sort is ongoing in many countries, including Canada. Canada supports about 3,200 species of native plants, of which as many as 1,000 have been used for medicinal purposes, mostly by Aboriginal peoples. Of this relatively large number, several tens of species have become widely enough used that they are of significant commercial value. Some of them are being cultivated to supply the emerging herbal medicine markets, while others are still harvested from the wild. A few examples of Canadian species that are of interest as medicinal plants include the following:

• Yarrow (Achillea millefolium) is a widespread perennial herb of disturbed habitats and meadows that can be taken (often in capsule form) to treat the common cold, diarrhea, fever, and some other maladies, or used as a poultice to stanch the flow of blood from wounds. It is easily cultivated or may be gathered from the wild.
• Purple coneflower (Echinacea pallida var. angustifolia) is a perennial herb of prairie habitats that is widely drunk as a root extract. The root may also be chewed or taken in other forms to prevent or treat the common cold, sore throat, bacterial infections, and other ills. It is easily cultivated and is one of the most widely used herbal medicines in North America.
• Evening primrose (Oenothera biennis) is a widespread biennial herb of disturbed habitats and meadows that may be taken as a whole-plant infusion to treat asthma and gastrointestinal disorders, or as a pressed-oil product as a nutritional supplement. It is easily cultivated or can be gathered from the wild.
• Ginseng (Panax quinquefolius) is a perennial understorey plant of eastern hardwood forest that may be taken as a root infusion as a general tonic or to treat headache, cramps, fever, rheumatism, and other maladies. It is cultivated on a five- to seven-year rotation, and may be the most widely used herbal medicine in the world. It should not be gathered from the wild because past over-harvesting has rendered it endangered.
• Pacific yew (Taxus brevifolia) is a tree-sized plant of the humid of the west coast, and Canada yew (T. canadensis) a shrub of eastern forest. An extract of bark or leaves containing the chemical taxol has proven useful in the treatment of certain malignancies, particularly ovarian and breast cancers. Biomass for processing is gathered from wild plants, but local over-harvesting has been an issue in some areas. Plantations of Pacific yew and other yews are being established to relieve the pressure on slow-growing populations of wild plants.
• Cranberry (Vaccinium macrocarpon) is a widespread trailing shrub of bog wetlands that may be taken as a pressed juice as a source of vitamin C, to treat urinary tract infections and kidney ailments, and for other purposes for which its diuretic properties are useful. The species is extensively cultivated and is also gathered from wild habitats.

Reference and Additional Information
Small, E. and P.M. Catling. 1999. Canadian Medicinal Plants. Ottawa, ON: NRC Research Press.
Deur, D. and N. Turner (editors). 2005. Keeping It Living: Traditions of Plant Use and Cultivation on the Northwest Coast of North America. Seattle, WA: University of Washington Press.

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. 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.”

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. However, enlightened people or societies would not facilitate the endangerment or extinction of species or natural communities.

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 favour 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.

Monera

Monerans 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 7,643 species of bacteria have been named (Table 7.1), but there are many additional species that have not yet been described by microbiologists. 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, Legionnaire’s 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 moulds, 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 14 phyla and 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. It is likely that the protists will eventually be divided 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 moulds (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 three major divisions (phyla) of fungi, distinguished mainly by their means of sexual reproduction. Asexual reproduction is also common.

The zygomycetes (division Zygomycota) achieve sexual reproduction by the direct fusion of hyphae (the thread-like tissues of fungi), which form resting spores known as zygospores. There are about 600 named species, the most familiar of which are the bread moulds, such as Rhizopus, with their fluffy mycelium (a loosely organized mass of hyphae).

The ascomycetes (division Ascomycota) include about 30,000 named species, some of which are commonly known as a cup fungus or morel. During sexual reproduction, ascomycetes form numerous microscopic, cup-shaped bodies known as asci, which are located in specialized 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 Basiodiomycota) include about 16,000 named species. Sexual reproduction involves a relatively complex spore-producing structure known as a basidium, which depending on its shape may be called a mushroom, puffball, toadstool, or shelf fungus. In Canada, the largest of these structures is developed by the giant puffball (Calvatia spp.), which has a ball-like basidium 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.

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 meadow mushroom (Agaricus campestris), while the most prized wild mushroom is the extremely flavourful 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 Canada and elsewhere, and psilocybin (Psilocybe spp.) of more southern regions of North America and Central America. Some wild mushrooms are deadly poisonous even when eaten in tiny quantities. The most toxic species in Canada are the destroying angel (Amanita virosa) and deathcap (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, 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 Hepaticophyta), of which there are about 6,500 species
• mosses (Bryophyta), including about 10,000 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 100 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 of vascular plants:

• whisk ferns (division Psilophyta), containing several species
• club mosses and quillworts (Lycophyta), about 1,000 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 550 species of firs, hemlocks, pines, redwoods, spruces, yews, and others
• flowering plants (Anthophyta), containing a diverse assemblage of about 235,000 species

The flowering plants are also known as angiosperms, because their ovules are enclosed within a specialized membrane, and their seeds within a seedcoat. 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 aristata).

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 12,000 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 12,000 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 12,000 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 85,000 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 jewellery. 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 6,000 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 63,000 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 63 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 850 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 30,500 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 6,515 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 8,734 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 9.990 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 5,487 species, which first evolved about 220 million years ago (the earliest fossil mammals are difficult to distinguish from reptiles). 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 mammalsinclude 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.