Everyone wants precise numbers, but the truth is, estimating the number of animal species inhabiting our planet is an educated guess. There are many challenges.
Species counts are biased because we tend to study certain organisms more than others. Birds, as a group, have been extensively studied, so scientists believe that the current number of bird species (between 9,000 and 10,000) is a relatively good approximation of the actual number. Nematodes, also known as roundworms, are a less studied group of invertebrates, so we know very little about their diversity.
Habitat can make animal counting difficult. Animals living in the deep sea are not easily accessible, so naturalists know little about their diversity. Organisms that inhabit the soil or parasitize other animals are also difficult to locate and therefore difficult to quantify. Even terrestrial habitats like the Amazon rainforest can present insurmountable obstacles to species surveys.
Animal size often complicates species detection and counting. In many cases, smaller species are even more difficult to find and count.
The ambiguity of terminology and scientific classification affects species counts. How do you define a species? It's not always easy, especially when so-called "species" can hybridize. Furthermore, different classification methods also influence the number of species. For example, some models classify birds as reptiles, thus increasing the number of reptile species by as much as 10,000.
Despite these challenges, we still want to understand how many species inhabit our planet. This provides us with the perspective needed to balance research and conservation goals, ensuring that less popular animal groups are not overlooked and helping us better understand community structure and dynamics.
A rough estimate of the number of animal species
The number of animal species on our planet is estimated to be between thirty and thirty million. How did we arrive at this enormous estimate? Let's look at the major animal groups and see how many species belong to different categories.
If we divide all animals on Earth into two categories: invertebrates and vertebrates, it is estimated that 97% of all species are invertebrates. Invertebrates, or animals lacking a spine, include sponges, cnidarians, mollusks, flatworms, annelids, arthropods, and insects. Among all invertebrates, insects are by far the most numerous. There are at least 10 million species of insects, and scientists have not yet discovered, let alone named or counted them all. Vertebrates, including fish, amphibians, reptiles, birds, and mammals, comprise only 3% of all species.
The list below provides estimates of the number of species within different animal groups. Please remember that the sub-levels in this list reflect the taxonomic relationships between organisms. For example, this means that the number of invertebrate species includes all 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 its subgroups.
Animals: Estimated to be 30-30 million species
|
Invertebrates: comprising 97% of all known species
| |--Sponge: 10,000 types
| |--Cnidium monnieri: 8,000-9,000 species
| |--Mollusks: 100,000 species
Flatworms: 13,000 species
| |--Nematodes: More than 20,000 species
| |--Echinoderms: 6,000 species
| |-- Annelids: 12,000 species
| |--Arthropods
Crustaceans: 40,000 species
| |--Insects: 10-30 million+ species
Araneae: 75,500 species
|
Vertebrates: comprise 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 fish: 23,500 species
