The diversity of the plant kingdom (1 of 10)

People are often astonished at how many plants there are in the world. Estimates of course vary, but here are the best facts and figures Plant Talk can find.

  1. Flowering plants (Angiospermae) dominate the contemporary flora of the Earth and constitute virtually all terrestrial vegetation. There are an estimated 250,000 species, with the greatest diversity in the moist tropics. Most Plant Facts will be on flowering plants, but all other types of plant will be covered also.
  2. The orchids (Orchidaceae) have more species than any other family of flowering plants with 25,000-35,000 species recognized, mostly in the tropics. Although some species do remain to be discovered, the difference between these estimates is mostly one of different taxonomic concepts.
  3. It is likely that 10 to 15% of the Earth's flowering plants have not yet been described. Most will be in the moist tropics, especially remote parts of Latin America.
  4. The Gymnosperms consist of the conifers (500 species), the cycads (100 species) and a few other small families. A conifer, California Redwood (Sequoia sempervirens), is the tallest tree on earth (110 m high). Another, the Bristlecone Pine (Pinus aristata) was thought to be the oldest, at about 4900 years, but a huge Huon Pine (Dacrydium franklinii) recently found in Tasmania may be thousands of years older still.
  5. The ferns and their allies (the latter mostly horsetails, club- mosses and quillworts) account for about 12,000 species, mostly in the moist tropics. They vary from moss-like delicate filmy ferns to tree-ferns up to 15 m or more tall.
  6. The bryophytes comprise about 8000 mosses and 6000 liverworts. Although distributed throughout the world, most species occur in cool or temperate, consistenty moist climates, such as temperate South America.
  7. Lichens consist of a fungus and a photosynthetic partner, either a green alga or a cyanobacterium ('blue-green alga'). Estimates of known species vary from 13,500 to 17,000, but since only an estimated 50-70% of them are known, there may be as many as 20,000 species in all. Temperate rainforests have the greatest diversity of lichens, but in arctic and alpine habitats lichens comprise most of the vegetation. Tropical lichens are much less well known, especially as they tend to grow in forest canopies.
  8. There are 4 main types of macro-algae:
    • Green Algae (Chlorophyta) live in fresh- or sea-water. There are some 1040 species. Their highest recorded diversity is in the N. and W. Atlantic Ocean, followed by the seas around Japan.
    • Brown Algae (Phaeophyta) are mostly marine and comprise over 1500 species, with the greatest diversity in the N. Atlantic and the N. Pacific. They include the kelps, with fronds up to a hundred metres long, and the wracks or 'seaweeds' of the rocky shore.
      Red Algae (Rhodophyta) comprise over 2500 species and are mostly marine. As with other algal groups, the richest floras are those of the seas around Japan, the tropical and subtropical W. Atlantic, and the N. Atlantic.
    • Stoneworts (Charophyta) are green algae mostly found in clean freshwater such as ponds and lakes. A recent revision recognises 440 taxa, the highest numbers from India and Asia. They are particularly susceptible to pollution: in Britain, 17 out of the 30 species present are in the Red Data Book.
  9. Microorganisms for the most part are best seen as a third Kingdom separate from both plants and animals (although fungi are traditionally the domain of botanists). A recent conference of experts concluded that less than 5% of the world's microorganisms have been described.
    Microscopic algae are very poorly known. The estimated figure of 350,000 species has a large margin of error! Even a well-botanized country like Britain has no accepted list of its microscopic algae.
    Bacteria. About 4000 species have been described, but there are vast numbers of bacteria in soils, deep-sea sediments and in the digestive tracts of animals. One estimate is of 3 million species in all.
    Fungi. About 70,000 species are described, mostly tiny or microscopic plants rather than large mushrooms and toadstools, but there may be over 1.5 million species in total. A later page will provide facts on fungi.
    Protozoa. Scientists know about 40,000 species of these unicellular organisms but estimate there are 100,000 species or more.
    Viruses. About 5,000 species are known, and 500,000 are estimated.
  10. As the plants get smaller, we know less about them. Note how we know 85-90% of the flowering plants but only 1% or less of some microscopic organisms. Of course species concepts differ in different groups, but this is still a staggering statistic. Why do we know so little of some groups?
    Sources: Most of the facts above are taken from the WCMC report Global Biodiversity (cited below). The material in that report on higher plants was prepared by John Akeroyd (and in part Hugh Synge), the material on lower plants by staff at the Department of Botany, Natural History Museum, London, and the material on microorganisms by Professor D.L. Hawksworth, International Mycological Institute, UK. The figures on British stoneworts (Fact 8) are from Stewart, N.F. and Church, J.M. (1992), Red Data Books of Britain & Ireland: Stoneworts, JNCC, Peterborough, UK. For further information, see Groombridge, B. (Ed.), Global Biodiversity: Status of the Earth's Living Resources, Chapman & Hall, London, 1992.

The above facts do not attempt to show the taxonomic relationships and phylogenies of the above groups.