How Scientists Measure Biodiversity: Alpha, Beta, and Gamma Diversity

The Living Planet Index Fell 69% Between 1970 and 2018

The 2022 WWF Living Planet Report documented a 69% average decline in monitored vertebrate wildlife populations between 1970 and 2018—a figure that has become one of the most cited statistics in conservation biology. The headline number is striking, but understanding what it means requires understanding what biodiversity is, how scientists measure it, and why different metrics reveal different aspects of the biodiversity crisis. A 69% decline in average population size does not mean 69% of species have gone extinct; it means that the typical wildlife population monitored is less than a third its 1970 size. Both facts are alarming, but they describe different dimensions of a multifaceted problem that scientists have developed increasingly sophisticated tools to quantify.

Measuring something you cannot count is genuinely hard.

Species Richness vs Species Evenness

The most intuitive measure of biodiversity is species richness: the number of species in a defined area. A forest with 50 species of birds is richer than one with 30. But species richness ignores an important dimension: how evenly are individuals distributed among those species? A community with 50 species where one species accounts for 95% of individuals and the remaining 49 species are represented by single individuals is ecologically very different from one where all 50 species are equally abundant—even though both have the same species richness.

The Shannon-Wiener Index (H') combines both richness and evenness into a single metric, calculated as H' = -Σ(pᵢ × ln pᵢ), where pᵢ is the proportion of all individuals belonging to species i. Higher H' values indicate greater diversity considering both the number of species and their relative abundance. Values typically range from 0 (single species dominance) to above 3–4 for very diverse communities. The Simpson's Index (D = Σpᵢ²) offers an alternative that emphasizes the probability that two randomly selected individuals belong to the same species, with higher D values indicating lower diversity.

The Alpha, Beta, Gamma Framework

Robert Whittaker in 1960 proposed a three-level framework for describing biodiversity at different spatial scales that remains central to ecology:

• Alpha diversity (α): Species diversity within a single site or local habitat. The number of species in a particular forest patch, lake, or grassland plot. Alpha diversity is what most people think of when they consider local biodiversity.
• Beta diversity (β): The difference in species composition between sites—turnover as you move across a landscape. High beta diversity means different locations support very different communities; low beta diversity means the same species occur everywhere. Beta diversity is critical for understanding why protecting multiple different sites preserves more species than protecting multiple similar sites of the same total area.
• Gamma diversity (γ): Total species diversity across a landscape or region; the total species pool from which local communities are assembled. Gamma diversity is approximately the product of alpha and beta diversity.

Conservation planning must account for all three levels. A region with high alpha diversity but low beta diversity (the same species everywhere) may need fewer protected sites than a region with lower alpha but high beta diversity, where many species are site-specific and absent from adjacent areas.

The IUCN Red List Categories

As of 2023, the IUCN Red List has assessed approximately 157,000 species, of which more than 44,000 are threatened with extinction. The assessed species represent only a fraction of the estimated 8–10 million species on Earth; the vast majority—particularly invertebrates and plants—have never been formally assessed.

The Species-Area Relationship

One of ecology's most robust empirical patterns is the species-area relationship: larger areas contain more species, following the power law S = cAz. The exponent z typically falls between 0.2 and 0.35 for species within the same habitat type, meaning that a tenfold increase in area roughly doubles the number of species. The relationship, formalized by Frank Preston in the 1960s, has practical conservation implications: reducing habitat area by 90% is predicted to reduce species richness by approximately 50% at equilibrium—though local extinctions often take decades to centuries to fully manifest after habitat loss (the "extinction debt").

Biodiversity Hotspots

Norman Myers identified the biodiversity hotspot concept in 1988, defining hotspots as regions that support exceptionally high concentrations of endemic species (species found nowhere else) and have lost at least 70% of their original habitat. The current canonical list recognizes 36 hotspots globally. Together they cover approximately 2.5% of Earth's land surface but contain more than 50% of the world's plant species and 42% of terrestrial vertebrates as endemic species. Hotspot regions include the Mediterranean Basin, the Cape Floristic Region of South Africa, Madagascar, the Indo-Burma region, and the tropical Andes—all characterized by high species turnover (high beta diversity) driven by topographic complexity and long evolutionary isolation.

The eDNA Revolution in Biodiversity Assessment

Environmental DNA (eDNA) sampling—the collection and analysis of genetic material shed by organisms into their environment (water, soil, air)—has transformed biodiversity monitoring over the past decade. By filtering water from a lake or stream and sequencing all DNA fragments present, scientists can detect the presence of hundreds of species simultaneously without seeing a single individual. eDNA methods are particularly powerful for detecting rare or cryptic species that are difficult to observe directly, monitoring invasive species at early stages of establishment, and conducting large-scale biodiversity surveys in remote regions.

eDNA metabarcoding (matching DNA sequences against reference databases of known species) enables the simultaneous identification of entire communities—from fish to amphibians to invertebrates—from a single water sample. The method is revolutionizing freshwater biodiversity assessment, and aerial eDNA sampling (capturing organisms' shed DNA from air filters) is beginning to enable similar surveys for terrestrial communities including insects and mammals.

Evidence for the Sixth Mass Extinction

Earth has experienced five mass extinctions in the fossil record, each eliminating more than 75% of species in a geologically brief interval. Paleontologists estimate that background extinction rates (the rate at which species normally go extinct between mass events) average approximately 0.1 extinctions per million species-years. Current observed extinction rates for well-studied groups (mammals, birds, amphibians) are estimated at 100–1,000 times this background rate—and the pace is accelerating. A 2022 study in Biological Reviews estimated that nearly one-third of all terrestrial vertebrate species have declining populations. The drivers—habitat loss, overexploitation, invasive species, pollution, and climate change—are all human-caused, leading many scientists to characterize the current extinction crisis as the sixth mass extinction and the first driven by a single species.