Everyone wants hard figures, but the fact is that estimating the number of animal species that inhabit our planet is an exercise in educated guesswork. The challenges are numerous.
Species counts are biased by our tendency to study certain organisms more than others. Birds, as a group, have been extensively studied, so scientists believe that the estimated number of bird species alive today (between 9,000 to 10,000) is a relatively good approximation of the actual number. On the other hand, nematodes, also known as roundworms, are a little-studied group of invertebrates and consequently, we have little grasp of how diverse they may be.
Habitat can make counting animals difficult. Animals that live in the deep sea are not easy to access, so naturalists have less understanding of their diversity. Organisms that inhabit the soil or parasitize other animals are likewise challenging to locate and therefore difficult to quantify. Even terrestrial habitats, like the Amazon rainforest, can present insurmountable obstacles to a species census.
Animal size often complicates the detection and counting of species. In many instances, the smaller species are more difficult to find and count.
Ambiguities in terminology and scientific classification affect species counts. How do you define a species? It's not always easy, especially when supposed "species" are capable of cross-breeding. Additionally, different approaches to classification influence species counts. For example, some models classify birds as reptiles, thus boosting the species count of reptiles by as much as 10,000.
Despite these challenges, it's desirable to have some idea of how many species inhabit our planet. This gives us the perspective necessary to balance research and conservation objectives, to ensure that less popular groups of animals are not overlooked, and to help us better understand community structure and dynamics.
Rough Estimates of Animal Species Numbers
The estimated number of animal species on our planet falls somewhere in the vast range of three to 30 million. How do we come up with that whopping estimate? Let's take a look at the major groups of animals to see how many species fall within the various categories.
If we were to divide all the animals on earth into two groups, invertebrates and vertebrates, an estimated 97% of all species would be invertebrates. Invertebrates, animals that lack backbones, include sponges, cnidarians, mollusks, platyhelminths, annelids, arthropods, and insects, among other animals. Of all invertebrates, the insects are by far the most numerous. There are so many insect species, at least 10 million, that scientists have yet to discover them all, let alone name or count them. Vertebrate animals, including fish, amphibians, reptiles, birds, and mammals, represent a puny 3% of all living species.
The list below provides estimates of the number of species within the various animal groups. Keep in mind that the sub-levels in this list reflect the taxonomic relationships between organisms. This means, for example, that the number of invertebrates species includes all the groups below it in the hierarchy (sponges, cnidarians, etc). Since not all groups are listed below, the number of a parent group is not necessarily the sum of child groups.
Animals: estimated 3-30 million species
|--Invertebrates: 97% of all known species
| |--Sponges: 10,000 species
| |--Cnidarians: 8,000-9,000 species
| |--Mollusks: 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
|--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
Edited by Bob Strauss