Biodiversity is not one thing you can count. It is variety, measured at three different scales, and each scale protects an ecosystem in a different way. Get the three levels straight now and everything else in this topic falls into place.
📚 What you need to know
Biodiversity is the total variety of life — all the variation within and between living things 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 richness (how many species) with evenness (how evenly they are spread).
Genetic diversity is the range of gene forms within a species, between and within populations.
High biodiversity increases resilience: the ecosystem resists change and avoids tipping points.
Low diversity systems, such as monocultures, can collapse from a single pest or disease.
The three levels
DefinitionBiodiversity is the total diversity of living systems, including the variety of habitats, the variety of species, and the genetic variation within each species.
The levels are nested. Lose genetic diversity inside a species and you eventually lose the species, and with it a slice of the habitat’s diversity too.
Habitat diversity
This is the range of different habitats found within a particular ecosystem or biome.
Many different habitats means many different niches to be exploited, so more species can live there.
A coral reef has high habitat diversity: crevices, overhangs, sand patches, lagoons and open water all within a few metres.
A large sandy desert has low habitat diversity, because conditions are much the same across the whole area.
Species diversity
Species richness is simply the number of species present in a community or defined area.
Species evenness is how evenly the individuals are shared out between those species.
Species diversity takes both into account, which is why it is more informative than richness alone.
A tropical rainforest is described as species-rich; a field of a single crop is not.
Genetic diversity
Individuals of the same species carry the same set of genes, but those genes come in different forms.
Genetic diversity exists between populations of the same species, and also within a single population.
It matters because it lets a population adapt to and survive environmental change.
That change can be biotic — new predators, new pathogens, competition — or abiotic, such as temperature, rainfall or humidity.
Genetic diversity is the level students forget, and it is often the one worth the most marks. If every individual is genetically identical, then whatever kills one of them can kill all of them. That is the whole argument in a sentence.
Why diversity gives resilience
Back in 1.2 you met resilience: a system’s ability to absorb a disturbance and stay in the same state instead of tipping into a new one. Biodiversity is one of the two things that supply it, alongside the size of storages.
Count the lines, not the dots. It is the number of alternative routes through the web that keeps the whole thing standing.
🧩 The chain to write in an exam
High biodiversity means many species and many links in the food web.
If a disturbance removes one species, others can take over its role.
So energy and nutrients keep flowing and the ecosystem resists change.
The system stays in its steady-state equilibrium and does not approach a tipping point.
In a low-diversity system there is no alternative route, so one loss can cause collapse.
The monoculture problem
Some forests are dominated by only one or two tree species. If a pathogen arrives that attacks a dominant species, a huge part of the ecosystem can be wiped out at once.
Agricultural monocultures are the extreme case: a single species, often with very low genetic diversity too, so a single pest or disease can destroy an entire crop.
The same disturbance hitting a diverse system does far less damage, because most of the system is unaffected and can carry on.
Two kinds of vulnerability. A crop field is vulnerable at the species level, because only one species is there. A field of genetically identical cloned plants is vulnerable at the genetic level as well, because every individual has exactly the same weaknesses. Naming which level is at risk is what turns a general answer into a precise one.
Worked examples
WE 1
Distinguish the three levels of biodiversity
Outline the three levels at which biodiversity can be assessed, giving an example of each. (3 marks)
Level 1: habitat diversity
The range of different habitats within an ecosystem or biome — a coral reef has many microhabitats, a sandy desert has very few.
Level 2: species diversity
The number of species and how evenly individuals are spread between them — high in a rainforest, low in a wheat field.
Level 3: genetic diversity
The range of gene forms within a species, between and within its populations — high in a wild population, low in a cloned crop.
Habitats, species, genes — three scales of the same ideagive a named example at each level; unsupported definitions rarely score full marks
WE 2
Link biodiversity to resilience
Explain why an ecosystem with high biodiversity is more resilient than one with low biodiversity. (4 marks)
Point 1: more links
High biodiversity means more species and a more complex food web, so there are many alternative feeding routes.
Point 2: substitution
If a disturbance removes one species, others can take over its role, so energy and nutrient flows continue.
Point 3: the outcome
The system resists change and returns to its average state, staying away from a tipping point.
Point 4: the contrast
In a low-diversity system such as a monoculture, one pest or pathogen can remove the dominant species and the ecosystem may collapse.
Alternative routes are what stop a loss becoming a collapseuse “tipping point” and “steady-state equilibrium” — they link this straight back to 1.2
WE 3
Apply genetic diversity
A farmer plants a single variety of banana, all grown from cuttings of one parent plant. Suggest why this is risky. (3 marks)
Point 1: no genetic variation
Plants grown from cuttings are genetically identical, so the crop has almost no genetic diversity.
Point 2: the consequence
Every individual has the same weaknesses, so a pathogen able to infect one can infect all of them.
Point 3: no adaptation
There is no variation for selection to act on, so the population cannot adapt to a new pest or a change in conditions.
Identical plants means one disease can take the whole cropname the level — this is a genetic diversity question, not a species diversity one
💡 Exam tips
Learn the three levels as a list and always say which one a question is about.
For species diversity, mention richness and evenness together.
Link biodiversity to resilience and tipping points — the examiner wants that connection.
Use niche and microhabitat when explaining why habitat diversity raises species diversity.
Have one high-diversity and one low-diversity example ready for each level.
Split biotic and abiotic changes when explaining why genetic diversity helps.
⚠ Common mistakes
Saying biodiversity just means the number of species. That is richness, one part of one level.
Forgetting genetic diversity. It is the level most often left out and most often worth a mark.
Confusing habitat with ecosystem. Habitat diversity is the range of habitats inside an ecosystem.
Claiming diverse systems never change. They are disturbed too; they just recover.
Saying deserts have no biodiversity. They have low biodiversity, with well-adapted specialists.
Vague resilience answers. Explain the mechanism: alternative species take over the role.
Up next: How Evolution Generates Diversity — all of this variety came from somewhere, and the process that built it is still running.
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