Estimating Species’ Numbers

First thing's first, if we take that 8.7 million and divide it into two simple groups, invertebrates and vertebrates, then we'll find that an estimated 97% are in fact invertebrates. Invertebrates basically means without a backbone, and therefore include sall insects (which is an astronomical amount, believe you me). The vertebrates represent the remaining 3% of all species and include those animals that are perhaps more familiar to most of us: amphibians, birds, fish, reptiles and mammals. So, with this in mind, let's look at the figures in a little more details - starting with vertebrates (the children would love this)!


ANIMALS: estimated 3-30 million species (I'll go with Erwin)
VERTEBRATES: 3% of all known species

  • Reptiles: 7,984 species
  • Amphibians: 5,400 species
  • Birds: 9,000-10,000 species
  • Mammals: 4,475-5,000 species
  • Ray-Finned Fishes: 23,500 species

INVERTEBRATES: 97% of all known species

  • Sponges: 10,000 species
  • Cnidarians: 8,000-9,000 species
  • Molluscs: 100,000 species
  • Platyhelminths: 13,000 species
  • Nematodes: 20,000+ species
  • Echinoderms: 6,000 species
  • Annelida: 12,000 species


  • Crustaceans: 40,000 species
  • Insects: 1-30 million+ species
  • Arachnids: 75,500 species


The Science of Counting Species

The science of counting species revolves around the equally scientific term - biodiversity. Biodiversity (if I can put it simply) is the variety of organisms at all levels of an organisation. In that sense; biodiversity can refer to the genetic diversity within a population, the species diversity within a community, or the habitat diversity within an ecosystem. Put simply, so you and I can understand; biodiversity isn't just about counting species (despite my feeling compelled to know the exact number of each group). The truth is that scientists have to ask a million and one questions when they try to classify new animals; such as, why are some groups of animals so diverse and why are some species rare while others are widespread. Interesting work!