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Species richness is the number of species, while evenness describes how equally individuals are distributed among them. Two sites can have equal richness but different diversity if one is dominated by one species. Genetic diversity within a species also matters for adaptation and conservation. A sample describes the sampled place, time and method; it does not automatically give the complete biodiversity of an entire region.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Distinguish richness and evenness
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does species richness count?
Number of species. Evenness describes the abundance distribution.
After watching 2: What must be checked before comparing two published diversity values?
The index formula and sampling method. Different conventions have different scales.
What you'll be able to do
Species richness is the number of species, while evenness describes how equally individuals are distributed among them. Two sites can have equal richness but different diversity if one is dominated by one species. Genetic diversity within a species also matters for adaptation and conservation. A sample describes the sampled place, time and method; it does not automatically give the complete biodiversity of an entire region.
For OCR Biology A the commonly specified Simpson diversity form is , where n is the number of individuals of each species and N is the total. A larger value indicates greater diversity in this convention. Other sources use reciprocal or finite-sample forms, which give different numerical scales. Write the supplied formula and define its symbols before substituting; never import a value from another convention and compare it directly without conversion.
Use comparable sampling effort, season, area and identification methods when comparing sites. Random or stratified sampling can reduce bias and reveal spatial variation. Conservation can protect habitats in situ or preserve material ex situ through seed banks, collections or breeding programmes. Genetic bottlenecks, reintroduction conditions and human needs affect success. A higher sample index supports one part of a conservation argument, but ecological roles, threats and sampling uncertainty also matter.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Another sample has 9 and 1 individuals. Compare using the same formula.
It is less diverse by this index despite equal richness.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.