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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.
• 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.
• Yet nematodes, also known as roundworms, are a scarcely-studied group of animals and consequently we have little grasp of how diverse this group of animals may be.
• Habitat makes counting animals difficult in certain cases. Counting animals is often complicated by the location of the animals.
• Deep sea organisms, for instance, are not easy to access so we have less understanding of their diversity.
• Organisms that inhabit the soil or parasitize other organisms are likewise challenging to locate and therefore difficult to quantify.
• Animal size often complicates the detection and counting of species. In many instances, the smaller the species, the more difficult it is to find them and count them.
• Ambiguities in terminology and scientific classification impact species counts. How do you define a species? It's not always easy—when species are capable of cross-breeding how do you declare them as separate groups? Additionally, classification approaches influence species counts. For example, some classification approaches place birds within the reptile group, thus boosting the number of reptiles by as much as 10,000 species.

Despite these challenges, it is valuable to have some idea of how many species inhabit our planet—it gives us the perspective necessary to balance research and conservation objectives to ensure some groups of animals are not overlooked and helps us to better understand community structure and dynamics.

Some Rough Estimates of Species Numbers

The estimated number of animals on our planet falls somewhere in the vast range of 3-30 million species (Erwin 1983, Wolosz 1988). How do we come up with that estimate? Let's take a look at some groups of animals to see how many species fall within the various categories.

If we were to divide all animals into two groups, invertebrates and vertebrates, an estimated 97% of all species would be invertebrates. Invertebrates include animals that lack backbones such as sponges, cnidarians, molluscs, platyhelminths, annelids, arthropods, and insects, to name just a few. Of all invertebrates, the insects are by far the most numerous. There are so many species of insects that scientists have yet to discover them all, let alone name or count them. Estimates of the total number of insect fall in the range of 1 to 30 million. The vertebrates represent the remaining 3% of all species and include species that are the most familiar to us: amphibians, reptiles, birds, fishes, mammals.

The list below provides estimates of the number of species within various groups of animals. The indentation levels in the list below reflect the taxonomic relationships between organisms. This means that the number of invertebrates includes all the groups that are 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
|        |--Molluscs: 100,000 species
|        |--Platyhelminths: 13,000 species
|        |--Nematodes: 20,000+ species
|        |--Annelida: 12,000 species
|        `--Arthropods
|            `--+--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

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