Biodiversity is not one thing you count. It is three separate levels of variety, and an ecosystem can score high on one while being dangerously low on another — which is exactly how a forest full of trees can still collapse when a single disease arrives.
📘 What you need to know
Biodiversity is the variation within and between all forms of life — the range of habitats, species and genes in a region.
It is assessed at three levels: habitat diversity, species diversity and genetic diversity.
Habitat diversity is the range of different habitats within an ecosystem or biome.
Species diversity combines species richness (how many species) with evenness (how equally abundant they are).
Genetic diversity is the variety of genes within a species.
Biodiversity is crucial for resilience: it lets ecosystems resist environmental change and avoid tipping points.
The three levels of biodiversity
Biodiversity can be thought of as the study of all the variation that exists within and between all forms of life. It looks at the range and variety of habitats, species and genes within a particular region, and is assessed at three levels.
The nesting is deliberate: habitats contain species, and species contain genetic variation. Lose the outer level and you lose the inner ones with it.
Habitat diversity
This is the range of different habitats within a particular ecosystem or biome. A large number of different habitats in an area means high biodiversity; only one or two means low.
Coral reefs are the standard high example. They are very complex, with lots of microhabitats and niches to be exploited.
Large sandy deserts are the standard low example. Conditions are essentially the same throughout the whole area.
Species diversity
An ecosystem with a very high number of different species — a tropical rainforest, say — is described as species-rich.
Two terms, kept apartSpecies richness — the number of species within an ecosystem. Species diversity — the number of species and the evenness of abundance across them.
The greater the number of species, and the more evenly organisms are distributed among those species, the greater the species diversity. Ecosystems with high species diversity are usually more stable than those with low diversity, because they are more resilient to environmental change.
Genetic diversity
Genetic diversity is the diversity of genes found within different individuals of a species. Individuals of the same species share the same set of genes, but those genes can take a variety of different forms — so genetic diversity occurs both between populations of the same species and within a single population.
This matters because it helps a population adapt to and survive environmental change, whether that change is biotic (new predators, pathogens, competition) or abiotic (temperature, humidity, rainfall).
Why biodiversity drives resilience
Biodiversity is very important for the resilience of ecosystems, because it allows them to resist changes in the environment and avoid ecological tipping points.
Both panels start with a similar number of individuals. The difference is entirely in how many species those individuals are spread across.
This is 1.2 applied to ecology. Resilience was defined there as a system’s ability to absorb disturbance and avoid tipping points, and it depended on diversity and storage size. Biodiversity is simply what diversity means when the system is an ecosystem. The rainforest-versus-monoculture contrast is the same argument in different clothing.
Worked examples
WE 1
The three levels
Outline the three levels at which biodiversity can be assessed. (3 marks)
Level 1: habitats
The number and range of different ecosystems and habitats in an area — a coral reef has many microhabitats, a sandy desert very few.
Level 2: species
The number of species present and their relative abundance, combining richness and evenness.
Level 3: genes
The genetic variation within each species, both between populations and within a single population.
Habitats, species, genes — from largest scale to smallestgiving one example per level turns three statements into three secure marks
WE 2
Diversity and resilience
Explain, using a named example, why low species diversity reduces ecosystem resilience. (4 marks)
Step 1: the principle
High species diversity gives an ecosystem more ways to respond to a disturbance, so it can resist environmental change and avoid tipping points.
Step 2: the example
The pine forests of Florida are dominated by only one or two tree species.
Step 3: the vulnerability
If a pathogen arrives that targets one of those dominant species, a very large proportion of the forest is affected at once.
Step 4: the outcome
The whole population could be wiped out and the ecosystem could collapse, because no other species is abundant enough to fill the role left behind.
Few species means few alternatives when one of them fails“avoid tipping points” links this straight back to systems in 1.2
WE 3
Genetic diversity
Explain why genetic diversity within a species is important. (3 marks)
Step 1: what it is
Genetic diversity is the variety of gene forms found within different individuals of a species. All individuals share the same set of genes, but those genes take different forms.
Step 2: why it matters
It helps a population adapt to and survive changes in its environment, because some individuals will carry forms suited to the new conditions.
Step 3: the two kinds of change
Those changes may be biotic — new predators, pathogens or competitors — or abiotic, such as shifts in temperature, humidity or rainfall.
Variation is the raw material a population needs to survive changenaming both biotic and abiotic changes is worth doing explicitly
💡 Exam tips
Learn all three levels and be able to give an example of each.
Keep species richness and species diversity distinct. Diversity includes evenness.
Use coral reef (high) and sandy desert (low) for habitat diversity.
Use Florida pine forest as your low-diversity, low-resilience example.
Link diversity to resilience using the phrase avoid ecological tipping points.
Mention both biotic and abiotic changes when discussing genetic diversity.
⚠ Common mistakes
Treating biodiversity as just a species count. Habitat and genetic levels count too.
Using richness and diversity interchangeably. They are examined as different terms.
Saying more organisms means more biodiversity. A million individuals of one species is low diversity.
Ignoring genetic diversity. It is the level students most often forget.
Claiming high diversity guarantees stability. It makes an ecosystem more resilient, not immune.
Forgetting evenness. Two areas can have identical richness and very different diversity.
Up next: How Evolution Generates Diversity. You now know what biodiversity is and why it matters. The obvious next question is where all that variety came from in the first place.
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