Species Richness

“A count can stay the same while the living community changes completely. ”

Species richness is one of biodiversity's most familiar measures because it answers an apparently simple question: how many species are present? The measure is useful, intuitive and often necessary. It is also easy to overinterpret. Two sites can contain the same number of species while differing profoundly in abundance, identity, ecological role and conservation value.

Richness is a count, not a complete description of diversity. It does not reveal whether one species dominates while the others are rare, whether native specialists have been replaced by widespread generalists or whether the species perform similar functions. A community of ten common disturbance-tolerant species can have the same richness as a community containing ten endemic or threatened species.

Sampling determines the count. More time, larger plots and greater taxonomic expertise usually reveal more species.

Seasonal surveys may miss migrants or organisms active only during rains. Richness therefore cannot be compared credibly unless effort, area, method and timing are comparable. An apparent increase may reflect better detection rather than ecological improvement. Global analyses of ecological time series have exposed another limitation.

Studies led by Maria Dornelas and, later, Helmut Hillebrand found extensive turnover in local communities even where average local species richness showed no consistent decline. Species were arriving and disappearing, and composition was changing. A stable count could therefore coexist with substantial ecological reorganisation. This matters in agricultural landscapes.

A simplified farm may gain several open-country or invasive species while losing forest-dependent species.

Total richness can remain constant or rise, allowing a positive headline even though the community has become more homogeneous and less distinctive. Richness alone cannot tell whether the change is desirable. Scale further complicates interpretation. Alpha richness describes species within a site. Beta diversity describes differences among sites. Gamma richness describes the wider region.

A programme may raise richness in individual plots by encouraging the same common species everywhere, while reducing differences among plots and leaving regional diversity unchanged. The local number improves while the landscape becomes more uniform. Richness should therefore be paired with other information. Abundance shows how populations are faring. Composition identifies which species are present.

Evenness shows how individuals are distributed. Functional and phylogenetic diversity reveal differences in ecological roles and evolutionary history. Threat status and endemism help interpret conservation significance. Richness should therefore be paired with abundance and evenness.

A site containing twenty species, nineteen of which are represented by one individual, is ecologically different from a site where populations are distributed more evenly. It may also be less resilient if the rare observations are transient visitors rather than viable populations. Community composition, functional traits and conservation status help explain what the count contains. Standardisation is essential.

Surveys should use comparable effort, season, area and method, and report detection limits.

Rarefaction and occupancy models can help compare samples of different intensity, while environmental DNA can reveal organisms that visual surveys miss. Each method also has biases. The goal is not to maximise the number detected but to make change interpretable. A stable richness total can even conceal degradation.

Native specialists may disappear while disturbance-tolerant or introduced species arrive, leaving the count unchanged. This is why claims such as 'no net loss of species' require scrutiny. Arithmetic equality does not prove ecological equivalence, particularly when unique or irreplaceable species are replaced by common generalists. The point is not to abandon richness. It is to report it with discipline.

A credible claim names the taxonomic group, spatial unit, sampling method and comparison.

It explains whether the change reflects colonisation, recovery, replacement or improved detection. Species richness is a useful doorway into biodiversity evidence, but it should not be mistaken for the whole room.

Practical application

Define the sampling unit and effort before collecting data. Use repeatable methods, cover relevant seasons and document taxonomic uncertainty. Where detection probability differs across sites or years, use appropriate statistical methods rather than treating raw counts as directly comparable. Report richness alongside composition and abundance for priority groups.

Identify which species were gained or lost and whether they are native, specialist, threatened or invasive. At landscape scale, examine whether sites are becoming more similar even when local counts appear stable.

Why it matters

Species richness offers a clear measure of one dimension of biodiversity and can reveal changes that require attention. Its simplicity becomes dangerous only when organisations use it to imply healthy populations, intact communities or restored ecosystems without supporting evidence.

Common misconception

A higher species count is often assumed to mean a healthier ecosystem. Richness can increase through invasive species, disturbance-tolerant generalists or stronger sampling effort. The ecological meaning depends on which species changed, in what abundance and across what scale.

Connections

Biodiversity includes genetic, species and ecosystem dimensions. Habitat determines whether species can persist, while ecological integrity considers composition and function. Pollination may depend less on total richness than on the presence and abundance of species able to provide the service when and where it is needed.

A question worth asking

If your biodiversity indicator increased by five species, do you know which five arrived and which species may have disappeared?

Selected references

Magurran, A. E. 2004. Measuring Biological Diversity. Dornelas, M. et al. 2014. Assemblage Time Series Reveal Biodiversity Change but Not Systematic Loss. Science 344: 296-299. Hillebrand, H. et al. 2018. Biodiversity Change Is Uncoupled from Species Richness Trends: Consequences for Conservation and Monitoring. Journal of Applied Ecology 55: 169-184. IPBES. 2019.

Global Assessment Report on Biodiversity and Ecosystem Services. Gotelli, N. J. and Colwell, R. K. 2001. Quantifying Biodiversity: Procedures and Pitfalls in the Measurement and Comparison of Species Richness. Ecology Letters 4: 379-391.