How To Calculate Biodiversity | Count Species Right

Biodiversity sounds big and abstract until you put numbers on it. Once you do, it becomes a practical way to compare one field, pond, forest patch, reef, or garden with another. You can use it to track habitat change, check restoration work, compare school survey sites, or make sense of species lists from a field notebook.

The catch is simple: there is no single “biodiversity number” that fits every job. Some measures count how many species are present. Some also track how evenly individuals are spread across those species. A site with ten species where one species makes up 90% of all individuals is not the same as a site with ten species split into similar numbers.

That’s why good biodiversity math starts with a clean count, then picks the index that matches the question. The Convention on Biological Diversity defines biological diversity as variation within species, between species, and across ecosystems. In day-to-day fieldwork, the part most people calculate first is species diversity within a sample or site using abundance data from a survey. See the Convention on Biological Diversity definition for the formal wording.

What Biodiversity Means In A Measurable Way

When people say “calculate biodiversity,” they’re usually measuring one of three things:

• Species richness: the number of species present.
• Species evenness: how evenly individuals are spread among species.
• Diversity index: a single value that blends richness and evenness.

Richness is the easiest place to start. If Site A has 12 species and Site B has 8, Site A is richer. Still, richness alone can hide a lopsided sample. If one species swamps the count, the site may feel less diverse on the ground than the species total suggests.

That is where Shannon and Simpson indices earn their keep. Both use proportions, not just raw species totals. Educational material from UC Davis frames species diversity around those two building blocks: richness and evenness. Their teaching page is handy if you want the logic behind the formulas as well as the calculation steps: species richness and species evenness.

How To Calculate Biodiversity In Four Clear Steps

This method works for a classroom survey, a field transect, a bird count, a quadrat study, or a camera-trap species tally, as long as you have counts for each species from the same sampling effort.

Step 1: Build A Species Count Table

List every species you recorded and the number of individuals for each one. Use the same survey method across sites. If one site was sampled for ten minutes and another for two hours, the comparison will be shaky from the start.

Step 2: Find Total Individuals

Add all individuals across all species. This total is usually written as N.

Step 3: Turn Counts Into Proportions

For each species, divide its count by the total count. This gives the species proportion, often written as p. These proportions are what most diversity formulas use.

Step 4: Choose The Right Metric

Use richness when you only need a species count. Use Shannon when you want a balanced view of richness and evenness. Use Simpson when you want an index that reacts strongly to dominant species.

Here is the workflow in one place.

Task	What You Do	Why It Matters
Define the sample	Pick one site, date, method, and sampling area	Keeps site-to-site results comparable
List species	Write each species once	Prevents double counting
Count individuals	Tally each species carefully	Creates the abundance data you need
Calculate total N	Add all individuals together	Needed for proportions
Calculate p	Divide each species count by N	Turns raw counts into comparable shares
Measure richness	Count the number of species	Shows how many species are present
Measure Shannon index	Use H′ = -Σ(p × ln p)	Balances richness and evenness
Measure Simpson index	Use D = Σ(p²), or 1 – D	Shows dominance and diversity
Compare sites	Use the same index for each site	Makes the comparison fair

Worked Example With Real Numbers

Say you sampled a small meadow and counted four species:

• Butterfly A: 20
• Butterfly B: 15
• Beetle C: 10
• Bee D: 5

Total individuals, N, equals 50. The species proportions are 0.40, 0.30, 0.20, and 0.10.

Species Richness

Richness is just the number of species present. In this sample, richness equals 4.

Shannon Diversity Index

The Shannon formula is H′ = -Σ(p × ln p). Using the meadow sample:

• 0.40 × ln(0.40) = -0.3665
• 0.30 × ln(0.30) = -0.3612
• 0.20 × ln(0.20) = -0.3219
• 0.10 × ln(0.10) = -0.2303

Add them together, then remove the minus sign: H′ ≈ 1.28. A higher Shannon value means more diversity within the set you are comparing.

Simpson Diversity Index

One common Simpson form is D = Σ(p²). For the same sample:

• 0.40² = 0.16
• 0.30² = 0.09
• 0.20² = 0.04
• 0.10² = 0.01

Add them: D = 0.30. In this form, lower values mean more diversity. Many teachers and field guides also report 1 – D, which here is 0.70, so higher values mean more diversity. NIST gives the standard Simpson formula as the sum of squared proportions on its Simpson diversity index page.

Which Biodiversity Formula Should You Use

The right answer depends on what you want the number to tell you. Richness is great for a plain species total. Shannon is often the better teaching and survey choice when you care about both variety and balance. Simpson is handy when one or two dominant species may hide what is happening in the rest of the sample.

Metric	Best For	Reading The Result
Species richness	Quick species totals	Higher count = more species present
Shannon index (H′)	Balanced site comparison	Higher value = richer and more even sample
Simpson index (D)	Dominance checks	Lower D = more diversity
Simpson diversity (1 – D)	Easy-to-read reporting	Higher value = more diversity

Common Mistakes That Skew The Result

A biodiversity score is only as good as the sample behind it. A few slips can throw off the whole comparison.

• Mixing survey effort: counts from different plot sizes, trap numbers, or observation times do not compare cleanly.
• Using vague species IDs: “small brown bird” and “sparrow” should not sit in the same table unless your ID rules are clear.
• Comparing different index forms: Simpson’s D and 1 – D move in opposite directions.
• Ignoring seasonality: spring and dry-season samples may tell different stories from the same site.
• Relying on one tiny sample: small samples can miss rare species and make a site look poorer than it is.

If you are comparing habitats, keep your method boring in the best way: same area, same timing, same effort, same taxonomic rules. That makes the math honest.

How To Read Your Final Biodiversity Number

A biodiversity number means little on its own. It gains value when you compare it with another site, another year, or another treatment plot. If Meadow A has a Shannon index of 1.28 and Meadow B has 0.72, Meadow A has the more varied and even sample under the same survey setup.

Use plain language when you report the result:

• “This site had 14 species and a Shannon index of 2.1.”
• “Species richness stayed flat, but Simpson diversity dropped, which points to stronger dominance by a few species.”
• “The restored plot had fewer species than the old plot, yet evenness was better.”

That style tells the reader what changed instead of tossing out a lone decimal and calling it a day.

Simple Rule For Most Readers

If you are new to the topic, start with species richness, then add Shannon for a fuller picture. If you suspect one species dominates the site, calculate Simpson too. Between those three numbers, you can describe most basic biodiversity patterns with enough detail to be useful and still keep the math manageable.