2.2: Biomass is not directly related to the number of species

At present, about 1.7 to 2 million species have been described, but the number is estimated to be between 3 and 100 million species. A figure on the previous page represents the relationship between species richness and biomass of different taxa. Bacteria, archaea, and viruses are not included in this representation because species definition is problematic for these organisms.

Fungi, arthropods

Figure \(\PageIndex{1}\). Antarctic krill Euphausia superba. It lives in large groups, called “swarms”, sometimes reaching densities of 10,000 to 30,000 individuals per cubic meter. [Source: Krill666.jpg: Uwe Kils, CC BY-SA 3.0, via Wikimedia Commons]

Whereas groups like insects (a class of arthropods) dominate in terms of species richness (with about 1 million described species) [15], their relative biomass fraction is minuscule. Some species contribute much more than families or even entire classes. For example, the Antarctic krill species Euphausia superba (Figure 5) contributes ≈0.05 Gt C to global biomass [16], on par with other prominent species such as humans (≈0.06 Gt C) or cows. This value is comparable to the contribution of other members of the arthropod phylum, termites [17], which contain many species, and far exceeds the biomass of entire vertebrate classes such as birds (≈0.002 Gt C). Other groups, such as nematodes [18],surpass any other animal species in terms of number of individuals, but constitute only about 1% of the total animal biomass (Figure 6).

Figure \(\PageIndex{1}\). Relationship between abundance and biomass of different taxa. The total number of individuals in each taxon is plotted against the total biomass of the taxon. Error bars reflect the projected uncertainty in the biomass estimate. [Source: diagram after Bar-On et al. ref [4]; Open Access article distributed under CC BY-NC-ND 4.0 license]

In this way, the picture that arises from taking a biomass perspective of the biosphere complements the focus on species richness that is commonly held (see Figure 4). This survey puts into perspective claims of the dominance of groups such as termites [16], ants [19], nematodes [20] and prokaryotes [21]. In contrast, the biomass of {tooltip}amphibians{end-text}Formerly known as batrachians, amphibians are a class of tetrapod vertebrates with approximately 7000 species. The frog, the toad, the newt, the salamander are amphibians. They have the particularity of having an aquatic larval life and an aerial adult life (like the frog), whose population is in spectacular decline [22], remains poorly characterized.