Measuring Biodiversity and Fungal Biomass

Exam Tip: If a question asks you to compare two plots where Plot A has 10 species of 10 individuals each, and Plot B has 10 species but one species has 91 individuals, Plot A has higher evenness, even though they have the same species richness.

Chemical Biomass Indicators

While counting individual mushrooms is possible in a forest, most fungi are microscopic or live within substrates like soil or wood. To estimate the total "amount" of fungal life—known as biomass—scientists often turn to chemical biomarkers. Instead of counting bodies, they measure specific molecules that are unique to fungal cells.

Two of the most common indicators are chitin and ergosterol:

1. Chitin: A long-chain polymer of N-acetylglucosamine that forms the structural component of fungal cell walls. Because chitin is a major constituent of the fungal cell wall, its concentration in a soil sample can serve as a proxy for total fungal presence.
2. Ergosterol: A sterol found in the cell membranes of most fungi (similar to cholesterol in animals). It is highly specific to fungi, making it an excellent indicator for quantifying live fungal biomass.

However, these chemical methods have significant limitations. Chitin is extremely stable and can persist in the environment long after the fungus has died, potentially leading to an overestimation of living biomass. Conversely, ergosterol can degrade due to oxidation or microbial activity, which might lead to an underestimation.

Estimating Fungal Species

The final layer of complexity involves determining how many species actually exist on Earth. There is a significant gap between "conservative" estimates and "phylogenetic" estimates of fungal diversity.

Conservative estimates rely on traditional methods, such as morphological identification (looking at what the fungus looks like under a microscope or with the naked eye). These methods tend to produce lower numbers because they group organisms together if they appear similar.

Phylogenetic estimates, however, use DNA sequencing to look at evolutionary relationships. This approach has revealed the existence of cryptic species—organisms that look identical to the human eye but are genetically distinct and cannot interbreed. Because DNA reveals these hidden lineages, phylogenetic estimates of global fungal diversity are significantly higher than morphological ones.

The mathematical relationship between the number of samples taken and the number of new species discovered often follows a power law, suggesting that as our sampling effort increases, our understanding of the "true" diversity continues to expand.

Key Concept: A cryptic species is a group of organisms that are morphologically indistinguishable but are genetically distinct. This is a primary reason why modern DNA-based studies report much higher fungal diversity than historical studies.

In summary, measuring life requires a tiered approach: ecological metrics to understand community structure, chemical indicators to estimate invisible biomass, and genetic tools to uncover the hidden diversity of the fungal kingdom.