Frequently, more often, most often ...
In order to define the most common animals, one has to look at species in terms of their distribution areas, on the one hand, and their stock numbers on the other. There are already some species that can be found either on all continents or because of their small body size occur in such large numbers that they can be confidently considered the most frequent representatives of their genera or orders.
Especially in terms of insects, this definition is easy, because among some species, the animals join together to huge colonies, which consist of up to several million copies. Especially the ants are significant in this context, which are represented on almost all continents of the earth. Scientists estimate the ants' worldwide stocks at several hundred billion specimens. Although ants are almost always considered a nuisance near human settlements, these insects are indispensable for a healthy environment. They are constantly on the lookout for food, exploiting dead organic material as part of their raids, contributing significantly to the proliferation of seeds, and creating nutrient-rich conditions essential to healthy plant growth by loosening up the soil.
Equally important for the intact ecological balance of their environment are the rudder crayfish or Copepoda, which comprise over 13,000 species and are detectable in all types of water of the earth. These tiny crustaceans are usually only a few millimeters long and utilize mostly dead plant material, algae and other organic remains. Some species also live parasitic. At the same time, they serve as an important food source for many fish and mammal species. Often Copepoda occur in such large numbers that divers can hear their movements as loud crackling noises.
Also among the fish is a group of small body size as the most common representative. The Brstenmäuler or Gonostomatidae comprise a total of 23 species and are usually only a few inches long. They mainly inhabit the deep sea and occur in the Pacific and Atlantic as well as in the Indian Ocean and the polar seas.
With seven billion individuals counted in 2011, humans are considered the most widespread mammal on Earth. He inhabits all continents and is considered by his intelligence as the most adaptable animal in the world. However, in terms of population numbers, man is certainly surpassed, at least in settlement areas of Rattus norvegicus, the mostly brown roan rat. Scientists suspect that the number of rats far exceeds that of humans, especially in cities, and that Rattus norvegicus is therefore also represented in billions worldwide.
The spread of the house mouse or Mus domesticus is closely related to that of humans, which is why it populates the earth in billions, like the brown rat. Both rodent species are extremely undemanding in terms of their living conditions and always find enough food near humans and their livestock. Rats live in human dwellings as well as in sewage systems, cellars, landfills and stables, and are often found in large packs defending their territory against conspecifics.
Among the reptiles, Zootoca vivpara, better known as the forest lizard, is the most common species in the world. It inhabits large parts of Europe and Asia and is found in arctic regions as well as in hot areas of the Mediterranean region. In North America, the multi-species garter snakes describe the most common reptiles. They inhabit almost all habitats in the United States, Central America, and Canada, and are found near humans as well as in the pristine wilderness.
In Africa, Quelea quelea, the songbird known as a blood-beak weaver, appears in huge swarms destroying entire crops within minutes. The population of these small birds is estimated at several billion specimens, which is why Quelea quelea is today considered the most common bird species in the world. Since many farms in Africa also use pesticides, many animals for which the widespread blood-beak weaver is an important source of food die from poisoning.