Biodiversity is more than a list of species. This guide explains how to measure biodiversity in an ecosystem by combining species richness, relative abundance, evenness, population trends and the Shannon diversity index. It includes a comparison table, a worked example and a practical field-survey checklist for students, teachers and anyone monitoring a habitat over time.
Overview
In ecology, biodiversity usually describes the variety of living organisms in a defined area. A useful measurement begins by stating exactly what is being measured: plants, birds, invertebrates, fungi or another group; a particular habitat; and a specific sampling period. Without that context, two biodiversity figures may not be meaningfully comparable.
The simplest measure is species richness, meaning the number of different species recorded. Richness is easy to understand, but it gives every species the same weight. A meadow containing 10 species is recorded as having the same richness whether individuals are distributed evenly among them or 90% belong to one highly dominant species.
Relative abundance shows how individuals are distributed among species. Evenness summarises how similar those abundances are. A habitat with high richness and high evenness often has a different ecological structure from one with the same richness but one dominant species and several rare ones.
For a fuller picture, biodiversity surveys may also track population size, occurrence across sampling locations, habitat condition and change through time. These measures answer different questions. A species can remain present while its population declines, or the number of species can increase temporarily because generalist species have entered a disturbed site.
Species richness versus evenness
| Measure | What it tells you | Main limitation |
|---|---|---|
| Species richness | How many species were recorded | Does not show whether one species dominates |
| Relative abundance | What proportion of individuals belongs to each species | Depends on reliable counts and consistent sampling |
| Evenness | How evenly individuals are distributed among species | Can be difficult to compare if rare species are missed |
| Population trend | Whether abundance or occupancy is rising, falling or stable | Needs repeated surveys using comparable methods |
| Shannon index | A combined measure of richness and relative abundance | Its value depends on sampling effort and the logarithm used |
What to track
A practical biodiversity monitoring sheet should record both the observations and the conditions under which they were made. At minimum, include the survey date, location, habitat description, weather, observer or survey team, sampling method and time spent surveying.
For each species, record the number of individuals where counting is realistic. For plants, this might be the number of stems or occupied quadrats; for birds, it might be individuals detected during a fixed observation period; for insects, it could be the number caught in standardised traps. If individuals cannot be counted reliably, record presence or absence and label the method clearly.
Track these core variables:
- Richness: the total number of species detected.
- Abundance: the count of individuals, observations or occupied sampling units.
- Relative abundance: each species' count divided by the total count.
- Evenness: how close the species' relative abundances are to one another.
- Occupancy: the proportion of sampled plots or locations where a species occurs.
- Population or occupancy trend: the direction and size of change across comparable surveys.
Where identification is uncertain, use a consistent identification level rather than forcing a precise name. For example, recording an organism as “unidentified grasshopper” may be more scientifically useful than assigning an unreliable species name. Photographing uncertain records can support later verification.
Shannon diversity index explained
The Shannon index combines richness and relative abundance. A commonly used form is:
H′ = −Σ(pi × ln pi)
Here, pi is the proportion of all recorded individuals belonging to species i, and ln is the natural logarithm. Higher values generally indicate a community with more species and/or a more even distribution, but the index should only be compared between surveys with similar sampling design.
Worked sample dataset
Suppose a quadrat survey records 100 individuals from four species:
| Species | Count | Relative abundance (p) | −p × ln(p) |
|---|---|---|---|
| A | 40 | 0.40 | 0.367 |
| B | 30 | 0.30 | 0.361 |
| C | 20 | 0.20 | 0.322 |
| D | 10 | 0.10 | 0.230 |
| Total | 100 | 1.00 | 1.280 |
The species richness is 4. Adding the final column gives a Shannon index of approximately H′ = 1.28. A related measure, Shannon evenness, can be estimated as J′ = H′ ÷ ln(S), where S is species richness. In this example, J′ is approximately 1.28 ÷ ln(4), or 0.92. Values closer to 1 indicate a more even distribution within this calculation, while the result should not be treated as a universal quality score.
The same method can be calculated with a spreadsheet. Keep full precision during the calculation and round only the final result. For guidance on reporting rounded ecological calculations, see Significant Figures Rules.
Cadence and checkpoints
The right survey interval depends on the organisms and question. A school project may use weekly or monthly observations during a defined season. A conservation monitoring programme may use seasonal or annual surveys. The key requirement is consistency: survey the same locations, use the same effort and, where possible, work in comparable weather and time windows.
Use a simple monitoring cycle:
- Baseline: complete enough repeated surveys to describe normal variation rather than relying on one visit.
- Routine checks: repeat observations at a planned monthly, seasonal or quarterly interval.
- Quality checkpoint: review missing records, identification uncertainty, changes in observers and differences in effort.
- Comparison: calculate the same measures for each period and display richness, total abundance and key species trends separately.
- Review: investigate notable changes before describing them as ecological decline or recovery.
For a field notebook, record any event that could affect results, such as mowing, flooding, drought, fire, construction, grazing or a change in survey access. These observations help distinguish a biological change from a sampling or habitat-management effect.
How to interpret changes
A falling Shannon index can result from fewer species, greater dominance by one species, or both. A rising index can result from new species being detected, a more even distribution or increased sampling effort. The number alone cannot identify the cause.
Interpret several measures together. If richness falls, total abundance falls and occupancy of several species declines across repeated standardised surveys, the pattern is more concerning than a one-off reduction in a single index. If richness rises but evenness falls sharply, the community may have become dominated by one or a few species. If richness rises only because survey effort increased, the apparent change may reflect detection rather than ecology.
Sampling completeness matters. Rare or inconspicuous species are easy to miss, while conspicuous species may be overrepresented. More quadrats, longer observation periods or repeated visits can increase the number of species detected. Avoid comparing a highly intensive survey directly with a brief walkover survey.
Also consider taxonomic scope. A bird survey and a plant survey measure different components of biodiversity. A whole-ecosystem conclusion should not be based on one group alone unless the limitation is clearly stated. For wider context on habitat patterns, see the guide to biomes of the world, and for species whose effects extend beyond their abundance, read about keystone species.
When to revisit
Revisit a biodiversity record on the schedule set by the project: monthly for short-term classroom monitoring, seasonally for many habitat comparisons, or quarterly when a recurring tracker is intended to identify broader changes. Recalculate the measures whenever new observations are added, but do not overwrite the original dataset. Preserve the raw counts, dates and field notes so that later results can be checked.
Update the method when the survey question changes, the habitat is altered, the target species list is revised or the available identification guidance improves. If monitoring standards or assessment frameworks are updated, document what changed and avoid treating results from incompatible methods as one continuous series.
Field-survey checklist
- Define the habitat, taxonomic group and survey area.
- Choose fixed plots, transects or observation points.
- Set a repeatable survey duration and sampling effort.
- Record date, time, weather and habitat conditions.
- Use consistent identification rules and flag uncertain records.
- Count individuals or record presence using the same method each time.
- Calculate richness, relative abundance and, where useful, the Shannon index.
- Compare trends across repeated surveys rather than isolated visits.
- Record disturbance, management and unusual events.
- Save the raw data, calculations and final interpretation together.
The most useful biodiversity measurement is not necessarily the most complicated one. A clearly defined survey repeated consistently can reveal more than a sophisticated index based on incomplete or incomparable observations.
