Two things make species diversity: Species diversity is a product of two things: richness and evenness. Both can be measured, so communities can be compared.
The points to remember
- Richness = the number of species in a community.
- Evenness = how similar the population sizes of each species are.
- Species diversity depends on both: many species, and none dominating.
- Many species is not enough: if one species dominates, diversity is still low.
- To define species diversity, name both: richness and evenness (relative abundance).
Both meadows have the same richness (4 species) and 100 plants. In Meadow A the numbers are even; in Meadow B grass dominates, so B has the lower species diversity.
Rich but uneven is still low: Picture a field of lavender with just one sunflower. Richness is 2, but evenness is very low, and the lone sunflower is unlikely to reproduce, so diversity stays low.
Real example: in Borneo, rainforest with hundreds of tree species has been cleared for oil palm plantations, where almost every tree is an oil palm: low richness and very low evenness.
Remember it as: Richness counts the species; evenness checks the shares.
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Richness and evenness matter because they decide how well a community copes when something goes wrong.
Why richness matters
- Species depend on each other, so with many species the loss of one has less impact.
- More species make more branched food webs, so the system recovers faster from disease or disturbance.
- Many niches are filled, leaving fewer gaps for invasive species.
- A wide variety keeps the ecosystem productive, providing life support such as clean water and oxygen.
Real example: in the 1970s Dutch elm disease killed about 25 million elms in Britain. Hedges and streets planted almost only with elms lost nearly every tree; mixed woods with many tree species kept most of theirs.
Why evenness matters
- When no single species dominates, the ecosystem is more stable and resilient.
- Species in similar numbers complement each other: they use resources in different ways.
- Competition between similar-sized populations keeps each species fit.
- Richness and evenness both raise the diversity index and biodiversity.
Real example: zebra and wildebeest graze the Serengeti in similar huge numbers. Zebra crop the tall, tough tops of the grass and wildebeest the shorter leaves below, so neither dominates and the grass is fully used.
Not another level of biodiversity: Richness and evenness are part of species diversity. Asked for another level of biodiversity, give genetic or habitat diversity, not richness or evenness.
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How this comes up: Define species diversity [1]; distinguish richness from evenness [1]; explain why both matter for biodiversity [3].
A farmer is turning a field back into a traditional hay meadow.
Explain why both species richness and species evenness matter for the biodiversity of the meadow.
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Both words, every time: A definition of species diversity with only richness, or only evenness, loses the mark. Name both.