In the past five decades, the human population has doubled to nearly 8 billion, yet we occupy only 0.03% of the Earth's land mass. As of 2023, scientists estimate that around 8.7 million species of plants and animals exist on our planet. Out of this vast number of species, which of them is considered the most abundant?
Determining Species Abundance
Species abundance refers to the total of individuals from a particular species within a given area. A species is considered abundant when it has a high population relative to the size of the area it inhabits. It can also include other performance measures for animals, plants, or other forms of life in an area, such as population density, number of breeding pairs, and even biomass.
It must be noted that absolute species abundance differs from relative species abundance. The former refers to the raw number of individuals of a species in a given area, while the latter counts the proportion of individuals of one species within the broader biological community. Relative species abundance also measures the evenness of the community's total population numbers as they are distributed across species.
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World's Most Abundant Species
Oceans make up about 70% of the Earth's surface. However, large portions are infertile and have less biodiversity than the land. Still, the biotically richest region of the land surface is a very thin crust compared with the oceans.
Because of this, it is no surprise that the most abundant organism on Earth is an ultra-small marine bacterium named Candidatus Pelagibacter ubique. It was only discovered in 2002 and has not been named yet.
In bacteriology, the word Candidatus is a term used to mean 'candidate taxa.' These refer to species and other taxa that have been discovered and given an informal name but have not yet passed the requirements for formal naming under the International Code of Nomenclature of Bacteria.
An estimated 2 x 10^28 individuals of Pelagibacter ubique are in the surface waters of temperate oceans. For comparison, there are only around 10^21 stars in all the galaxies in the observable universe. This means that there are around 10 million times more of this species of bacterium in the Earth's ocean than stars in the universe.
Pelagibacter ubique is not only ubiquitous, but it is also fascinating in terms of its role in understanding the limitations of life. Pelagibacter and its relatives live in environments where nutrients are rare, with concentrations that almost reach the lower limits suitable for life.
Scientists believe Pelagibacter ubique has adapted to this condition by streamlining its genomes and cell structures to execute only life's most important functions. In other words, these organisms live, allowing them to be so abundant in the ocean's nutrient-poor 'marine deserts.' By understanding the nature of such an extreme organism, experts can gain more insight into the evolution of life on our planet and perhaps elsewhere in the universe.
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