Diversity of organisms
The total number of species living on the planet is imprecisely known and published estimates vary from 12 million to over 100 million. This is because there are large numbers of species yet undiscovered and undescribed, and there is not always agreement on whether certain organisms should be regarded as separate species. The total number of known species including all animals, plants and micro-organisms is about 1.4 million, and over half of these are insects! Taxonomists have fairly complete records for the best known groups (e.g. birds with 9, 881 species world-wide). It is now also reasonably clear where the main gaps in our knowledge are, and intensive sampling of species-rich groups (e.g. insects) and species-rich areas (e.g. moist tropical forests) is now taking place to provide a more reliable picture of global and regional species richness, and a stronger basis for estimating the number of species.
In order to study the enormous diversity of organisms they need to be organised into manageable groups. This grouping of organisms is known as classification and the study of biological classification is called taxonomy. The usual method of classifying organisms, although not the only one in existence, follows the system originally proposed by the Swedish naturalist Carl Linnaeus (1707-1778) who gave each organism a two-part scientific name - a genus name and the species name (e.g. Homo sapiens). It is a hierarchical system of groupings based on evolutionary relationships. The sequence in the hierarchy is as follows: Kingdom-Phylum-Class-Order-Family-Genus-Species.
The five major groups, or kingdoms, are outlined below:
Bacteria Kingdom Only visible under the high power of the light microscope. Unlike other organisms (which have eukaryotic cells), they are single prokaryotic cells, i.e. without a proper nucleus (the DNA is not protected by a nuclear membrane), and live just about everywhere - air, water, soil, or inside animals and plants. They are often categorised according to their shape:
spherical (cocci) bacteria (e.g. Streptococcus, Staphylococcus) often mass together forming clumps or chains;
rod-shaped (bacilli) bacteria (e.g. Escherichia coli) also often form chains, some cells are curved or spiral and sometimes have flagella.
Many are beneficial, others cause disease.
Protoctist Kingdom A wide range of organisms divided into two main groups:
single-celled protoctists which are larger than bacteria and usually visible under the low power of the microscope. They may be plant-like, feeding by photosynthesis (e.g. Euglena in ponds; Pleurococcus on trees), or animal- like, taking in organic food (e.g. Amoeba and Paramecium in ponds). They live mainly in water or inside other organisms.
algae (e.g. seaweeds; Spirogyra blanket weed in ponds) are simple multicellular organisms without definite roots, stems or leaves. They are photosynthetic and live mainly in water.
Fungus Kingdom Single-celled (e.g. yeast), or multicellular (e.g. mushrooms, and moulds such as Penicillium and bread mould) which grow from a network (mycelium) of interwoven threads called hyphae. The hyphae have nuclei, and cell walls containing chitin, not cellulose. They have no chlorophyll and feed saprophytically absorbing organic substances.
[NB Viruses do not fit into any of the five kingdoms. They are smaller than bacteria and consist of genetic material (DNA or RNA) enclosed in a protein coat. They are able to replicate themselves inside other living cells, but are generally not considered living as they do not show many of the characteristics that define a living organism.]
Plant Kingdom The term plant, in everyday usage, generally refers to a complete, herbaceous specimen. Some children have difficulty assigning trees, or parts of plants (such as fruits, vegetables, flowers or seeds) to the plant kingdom. Plants are multicellular organisms able to photosynthesise. They have cells with cell walls, nuclei and chloroplasts. The largest, most highly evolved and most familiar group is the phylum of flowering plants (angiosperms). These have flowers for reproduction, produce seeds protected inside fruits, and range from small grasses to huge trees (e.g. oak, sycamore, fruit trees). The other phyla are non-flowering, often less conspicuous plants: the non-flowering trees (conifers) produce seeds in cones (e.g. pine, cypress); ferns (pteridophytes) can be large (e.g. bracken in woods) or small (e.g. water ferns in ponds), they have roots, stems and leaves (called fronds) and reproduce by spores; mosses and liverworts (bryophytes) are small plants with tiny leaves one cell thick and single- celled rootlets, reproduce by spores, common on trees, soil, walls, edges of ponds, etc.
Animal Kingdom There is a tendency for children to only regard large land mammals as animals. The animal kingdom includes organisms as diverse as humans (including babies and children!), fish, worms and limpets. Animals are multicellular, heterotrophic organisms. Their cells are without cell walls, but enclosed within a cell membrane. They have a nervous system and are usually able to move themselves around.
Animals without backbones (invertebrates) include the following phyla:
Cnidarians have soft bodies and a ring of tentacles around the mouth. Usually found in marine habitats (e.g. sea anemones, jellyfish), although Hydra live in ponds.
Echinoderms have bodies protected by calcareous plates and spines. Numerous tube feet used for locomotion. Restricted to marine habitats (e.g. starfish and sea urchins).
Molluscs have soft, non-segmented bodies, often with a calcareous shell. Live on land (e.g. snails and slugs), in ponds (e.g. pond snails), on the seashore (e.g. shellfish), or in the sea (e.g. squids and octopuses).
Segmented worms have long, segmented bodies, roundish in cross section, often with bristle-like chaetae which help with movement. Live in soil (e.g. earthworms), ponds (e.g. leeches), or on coastal mudflats
(e.g. lugworms and ragworms).
Flatworms have flattish, non-segmented bodies. Live in ponds and damp places (e.g. flatworms), most are internal parasites of other animals (e.g. tapeworms and flukes).
Roundworms (nematodes) have long, whitish, non-segmented bodies, round in cross section. Some live in soil, marine or freshwater habitats, others are internal parasites of animals and plants.
Arthropods have hard exoskeletons, segmented bodies and jointed legs. The phylum includes the following classes:
Crustaceans are a very diverse group, with two pairs of antennae, usually breathing with gills and thus requiring moist habitats. Some are marine (e.g. crabs, lobsters and shrimps), some freshwater (e.g. water fleas), some terrestrial (e.g. woodlice).
Myriapods have long, segmented bodies with numerous legs. Live in damp, shaded habitats. Some are fast-moving carnivores (e.g. centipedes), others are slower and herbivorous (e.g. millipedes).
Arachnids have eight jointed legs. Live in most terrestrial habitats, from hot tropical deserts (e.g. scorpions) to freshwater (e.g. rare water spiders); others are external parasites on vertebrates (e.g. ticks and mites).
Insects have six jointed legs, one pair of antennae, a pair of compound eyes, a body divided into a head, thorax and abdomen, usually two pairs of wings, and breathe through holes called spiracles. Found in most terrestrial habitats (e.g. flies, beetles, fleas, ants, bees and butterflies - and their larvae!).
Animals with backbones (vertebrates) belong to the phylum Chordata. Chordates have a skull which surrounds a well-developed brain and a skeleton of cartilage or bone. They are five vertebrate classes:
Reptiles have hard, tough scaly skin, four legs, and breathe with lungs. Their eggs have a leathery shell and are laid on land. Most species are found in warm climates, some are marine (e.g. turtles and crocodiles), others are terrestrial (e.g. lizards and geckos). Some terrestrial species live in Britain (e.g. grass snake, adder, slow worm, common lizard).
Amphibians have four legs, smooth, soft skins with no scales. All lay their eggs in water, but some, including all the British species (frogs, toads and newts) spend most of their time on land in damp places. Adults have lungs, but the aquatic larvae (tadpoles) breathe through gills.
Fish have scaly skins, have gills for breathing and fins for movement and stability. They are found in all aquatic habitats; some live in saltwater (e.g. sharks, cod), others in freshwater ponds and lakes
(e.g. sticklebacks, pike), and others migrate up rivers from the sea (e.g. salmon, eels). Despite their names jellyfish and starfish are not fish!
Birds have feathers, some scales, two legs, two wings and a beak. They breathe air and lay hard-shelled eggs on land and are warm-blooded. Live in many habitats (e.g. mallards and moorhens in ponds; robins and blackbirds in woods and gardens). Some migrate great distances each year (e.g. swallows, cuckoos, warblers).
Mammals have hair (or fur), four legs which may be modified (e.g. wings in bats, arms in humans, flippers in dolphins). They are warm-blooded and breathe air. Young usually develop inside the mother and feed through the placenta. Milk is produced from mammary glands (e.g. humans, cats, dogs, mice, rabbits, cows, lions, elephants, kangaroos, whales, seals, dolphins, porpoises).
Locally occurring organisms can be identified and assigned to their taxonomic groups by using keys. Simple photographs and drawings can be of assistance, but carefully constructed, keys based on unambiguous taxonomic characteristics are more accurate and systematic.
Dichotomous keys are commonly used; at each stage in the process there is a choice between two characteristics, only one of which applies to the organism you are identifying.