Q2:

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

Initial Setup & The Biodiversity Problem

Currently, the total number of scientifically named and described species on Earth is approximately $1.5$ to $1.7$ million. However, the actual number of species is significantly higher, with extreme estimates ranging from $20$ to $50$ million. To arrive at a more scientifically rigorous and conservative estimate—such as evolutionary biologist Robert May’s widely accepted estimate of approximately $7$ million global species—ecologists must rely on indirect statistical and mathematical methodologies. Direct counting is impossible due to the vastness of unexplored habitats (such as deep oceans and tropical rainforest canopies) and the cryptic nature of many species.

Step 1: Statistical Extrapolation via Tropical-to-Temperate Ratios

The primary methodology utilized by ecologists to estimate global biodiversity involves statistical extrapolation based on exhaustively studied groups of organisms, most commonly insects.

• The Baseline: Ecologists determine the exact species richness of a specific, well-documented taxonomic group (like beetles or butterflies) in a well-studied temperate region.
• The Comparison: They then sample the same taxonomic group in a highly biodiverse, less-studied tropical region (e.g., through canopy fogging to collect all insects in a specific volume of tropical rainforest).
• The Extrapolation: By calculating the ratio of tropical species to temperate species for this known group, ecologists generate a scaling multiplier. Assuming this ratio holds relatively constant across different forms of life, they extrapolate this multiplier to other, less-studied groups of plants and animals globally to estimate the total worldwide species richness.

Step 2: Taxonomic Scaling (Ratio Mapping)

Another powerful estimation tool involves using the known ratio of species between different taxonomic groups in a well-studied region and applying that ratio globally.

For example, in regions where biological inventories are complete (like Great Britain or parts of North America), ecologists have observed a consistent ratio between fungal species and plant species. Typically, there are approximately $6$ species of fungi for every $1$ species of vascular plant.
By taking the widely accepted estimate of global vascular plant species (roughly $300,000$) and multiplying it by this $6:1$ empirical ratio, ecologists mathematically estimate that there are approximately $1.8$ million species of fungi worldwide, even though only a fraction of these have been discovered.

Step 3: The Species-Area Relationship (SAR) Model

Ecologists heavily utilize spatial modeling, specifically the Species-Area Relationship proposed by Alexander von Humboldt. This principle states that within a region, species richness increases with the explored area, but only up to a limit. This relationship mathematically manifests as a rectangular hyperbola.

The equation governing this relationship is:
$S = cA^z$

Where:

• $S = \text{Species Richness}$
• $A = \text{Area}$
• $z = \text{Slope of the line (regression coefficient)}$
• $c = \text{Y-intercept}$

By applying a logarithmic transformation, the relationship becomes linear, allowing for easier statistical extrapolation across vast geographic scales:
$\log S = \log c + z \log A$

By determining the $z$-value (which typically ranges from $0.1$ to $0.2$ for small areas but steepens to $0.6$ to $1.2$ for entire continents), ecologists can input the total habitable surface area of the Earth into the equation to estimate global species limits.

Step 4: The Limitation of Prokaryotes

It is vital to note that these estimations inherently exclude prokaryotes (bacteria and archaea). Conventional taxonomic methods (based on morphological traits or interbreeding capabilities) are fundamentally unsuited for microorganisms. If molecular and biochemical techniques (such as DNA barcoding and 16S rRNA sequencing) are utilized to define microbial species, the global species estimate scales exponentially, potentially reaching into the billions.

Conclusion

Final Solution: Ecologists estimate the total number of species globally not by counting, but by utilizing rigorous statistical extrapolation. They primarily achieve this by calculating the ratio of species in exhaustively studied temperate regions to those in heavily undersampled tropical regions for specific taxonomic groups (like insects), and projecting this ratio across all life forms. This is further supported by mathematical models like the Species-Area Relationship ($S = cA^z$) and constant taxonomic scaling ratios, ultimately yielding conservative global estimates of approximately 7 million eukaryotic species.