Whether in the forest, in gardens or in the city: ants are everywhere. However, until now it has been unclear how many of these social insects there are worldwide and how they are distributed across the globe. A research team has now collected field data from all over the world and re-estimated the number of ants on this basis. According to this, there are at least 20 trillion ants in the world. Their dry biomass of 12 megatons of carbon is equivalent to that of all birds and mammals combined and about 20 percent of human biomass, the team reports. The ants reach the greatest density in the forests and savannas of tropical regions.
Around 15,700 ant species are known and described worldwide, but their true diversity could be far greater: biologists estimate that just as many ant species have remained undiscovered. The mostly social insects are important actors in various ecosystems: they influence the soil composition and texture by consuming organic residues and digging when building their nests, they act as predators, but also as prey for many birds, mammals or other arthropods. As mushroom breeders and aphid partners, they also play an important role in the thriving of other organisms. "While ant diversity is clearly crucial for the functioning and maintenance of many ecosystems, the extent of its influence is also shaped by the sheer quantity and activity of ants," explain Patrick Schultheiss from the University of Hong Kong and the University of Würzburg and his colleagues.
20 quadrillion ants worldwide – at least
How many ants there are worldwide, however, could only be roughly estimated. Biologists usually started from the number of ants in a sample area and extrapolated the total number from this. Others estimated the number of ants based on the fact that they make up around one percent of all insects - but this is also highly uncertain given the high number of unreported known insect species. Schultheiss and his team therefore chose a different approach: they collated data from 465 studies worldwide in which ants were either counted using ground litter samples or caught using ground traps. However, since the latter reflects the activity of ants in an area rather than their absolute number, the researchers evaluated the data from both methods separately. In addition, they included data from 24 studies of ant numbers on trees using whole canopy sprays of insecticide.
From their evaluation of this data, Schultheiss and his team conclude that there must be around three quadrillion ants in the ground litter alone. "But the ants in the litter represent only a fraction of the global ant fauna," emphasize the biologists. If one also includes the ants living in other habitats, the total number of ants worldwide is around 19.8 trillion. They put the dry biomass of this global ant population at 12.3 megatons of carbon. "This corresponds to the combined dry biomass of all wild birds and mammals together, or around 20 percent of the dry biomass of all humans," the researchers explain. However, as they point out, this is a conservative estimate as there are many habitats and biomes for which data are scarce, including subterranean habitats, mangroves or boreal forests. "The true number of ants worldwide is therefore probably much higher," the team says.
Mostly in the tropics
According to the evaluations, the ants are not evenly distributed across the biomes and climate zones: "It is remarkable that 61 percent and thus almost two-thirds of the global number of epigeic ants live in only two biomes: the tropical rainforests and the tropical savannas," report Schultheiss and his colleagues. Animal species that live on or just above the ground surface are called epigeal. According to the litter samples, ant densities in subtropical and tropical forests and grasslands are on average twice as high as in their temperate counterparts. The data from the floor traps confirmed this. Accordingly, the activity of the ants in the tropical and subtropical areas is even three to four times higher than in temperate latitudes. "This underscores the importance of tropical regions for global ant abundance and biomass," the team said. Ant activity is particularly high in dry habitats.
According to the scientists, this could also explain why there are so many mammal species that specialize in ants as food, especially in the tropics. Species such as the anteater of the Americas, the pangolin of Southeast Asia, or the aardvark of Africa have independently evolved adaptations to digging up ant nests and scavenging ants with insensitive, long tongues. "The number of arthropods that mimic ants morphologically or in behavior to avoid being eaten by them or to be able to capture ants themselves increases in the direction of the tropics because the ants are more common there and this mimicry is therefore effective." , report Schultheiss and his team. However, they also emphasize that this distribution observed in terrestrial ant species probably also applies to the ant species below the ground or in the higher layers of trees and shrubs, which have hardly been sampled to date. "It is of paramount importance that we fill these gaps in order to obtain a more comprehensive picture of insect diversity and global biodiversity patterns, their drivers and impacts," the scientists write.
Source: Patrick Schultheiss (University of Hong Kong) et al, Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2201550119