Q.How do ecologists estimate the total number of species present in the world?

It is practically impossible to count every single species present on Earth. Ecologists employ a more systematic and statistically sound approach to estimate global species richness. They conduct intensive studies on species richness for well-studied groups, such as insects, in both tropical and temperate regions. By statistically comparing the species richness observed in these regions for these groups, they derive a ratio. This ratio is then extrapolated to estimate the total number of species for other groups of plants and animals, thereby arriving at an estimate for the world's total species richness.

Estimating the total number of species on Earth is a monumental challenge, akin to counting every grain of sand on a vast beach. The sheer scale of biodiversity, coupled with the inaccessibility of many habitats like deep oceans or dense tropical rainforests, makes a direct, exhaustive count practically impossible. Ecologists, therefore, employ sophisticated indirect methods, relying on statistical approaches rather than attempting to catalog every single organism.

The primary strategy involves a two-pronged approach: first, conducting intensive surveys in specific, well-defined areas where species richness can be accurately measured; and second, using these findings to extrapolate to the wider, less-explored world. For instance, scientists might meticulously identify and count all species of a particular group, such as insects, within a small, manageable forest plot. They then analyze the patterns observed in how species richness changes with the area surveyed – a concept known as the species-area relationship.

Once these patterns are established in thoroughly studied regions, ecologists apply statistical models to project these numbers onto much larger, less-investigated territories. A common example involves estimating insect diversity in tropical regions. Since tropical areas are incredibly diverse but often less explored than temperate zones, ecologists might extrapolate from the more complete inventories available for insects in temperate countries. This allows them to arrive at a reasonable estimate for the vast, uncatalogued insect populations in the tropics.

Based on such comprehensive statistical analyses and extrapolations, particularly those involving species-area relationships, the British ecologist Robert May provided a widely accepted estimate for the total number of species on Earth. He conservatively placed this figure at about 7 million. It is crucial to understand that this is an estimate, a scientific consensus derived from the best available data and methodologies, rather than a precise count.

Further insights into global biodiversity, based on these estimations, reveal interesting distributions:

• More than 70% of all recorded species are animals.
• Among animals, insects are by far the most species-rich group, constituting over 70% of all animals. This means that for every 10 animals on Earth, 7 are insects.
• Plants, which include algae, fungi, mosses, ferns, gymnosperms, and angiosperms, account for no more than 22% of the total species.
• Interestingly, the number of fungal species in the world is greater than the combined total of all species of fishes, amphibians, reptiles, and mammals.
• It is also important to note that prokaryotes (like bacteria and archaea) are not well-represented in these estimates. Traditional taxonomic methods struggle with their identification, and if biochemical and molecular criteria were used, their diversity alone could run into millions.

First state why a direct headcount is impossible (huge uncatalogued tropical diversity), then name the actual two-step method the chapter describes: measure species richness for a well-studied group like insects across both temperate and tropical zones, compute the ratio, and extrapolate that ratio to other taxa — landing on Robert May's ~7 million global estimate as the concrete number to anchor the answer.