8.6Ecology

Biodiversity — AP BIOLOGY Revision Notes & Topical Questions

5 min read 8 parts Lesson 7 of 8 1 quiz

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Lesson summary

8.6 Biodiversity — Notes — AP Biology notes: Describe the relationship between ecosystem diversity and its resilience to changes in the environment.Biodiversity = the variation of genes, species and habitats in a region.Genetic diversity — variation within each species.Species di…

Keystone species
Trophic cascade
Adaptation
Heterozygote advantage
Ecosystem service
Species diversity — number of species and relative abundance (richness and evenness)

Part 1 of 6What biodiversity is

Biodiversity = the variation of genes, species and habitats in a region.

Genetic diversity — variation within each species.

Species diversity — number of species and relative abundance (richness and evenness).

Ecosystem diversity — number and range of ecosystems and habitats.

Part 2 of 6Resilience

Principle — ecosystems with more components and greater biodiversity are more resilient to environmental change because functional redundancy lets more species fill similar roles.

Fewer components — an ecosystem may be unable to adapt,

causing loss of species, instability and possible collapse, broken feeding relationships and reduced nutrient cycling and productivity.

Genetic diversity — low genetic diversity reduces the ability to adapt and raises extinction risk.

Diverse ecosystems are more resilient, not immune.

Part 3 of 6Maintaining ecosystem diversity

Biotic and abiotic factors — some ecosystems rely on particular factors, e.g. rainforests depend on a functional water cycle.

Producers — form the base of all food chains.

Keystone species — have a disproportionate effect on structure and function (any species can qualify, not only large animals); removal often causes loss of other species or collapse,

and protecting them maintains stability.
Elephants eat shrubs and small trees, maintaining grassland for zebras and so lions.
Beavers slow water flow and create wetland habitat for many species.
Sea otters control sea urchins in Pacific Northwest kelp forests, which urchins would otherwise overgraze.

Trophic cascade — removing a top predator has knock-on effects; reintroducing wolves to Yellowstone reduced elk overgrazing and restored riverbank vegetation.

Part 4 of 6Adaptation, mutation and heterozygote advantage

Adaptation = a heritable genetic variation favoured by selection that appears as a trait giving an advantage in a particular environment.

Source — adaptations arise from mutations, which occur randomly during DNA replication and not because of environmental pressure.

Selection — advantageous mutations are favoured and passed on, becoming widespread (natural selection).

Heterozygote advantage = heterozygous individuals have higher fitness than either homozygote.

HbS/HbS individuals develop sickle cell disease, and HbA/HbA individuals are susceptible to malaria.
HbA/HbS carriers are largely symptom-free and resistant to malaria, so the allele persists where malaria is common.

Part 5 of 6Threats, services and conservation

Threats — habitat destruction (the main threat), invasive species, overexploitation, pollution and climate change.

Ecosystem services — pollination, nutrient cycling, water purification, medicines and food.

Conservation — protected reserves and habitat restoration, habitat corridors linking habitats so populations can exchange genes, captive breeding that manages matings to avoid inbreeding, seed banks, controlling invasives and regulating hunting and fishing.

Population size — populations are kept large enough to retain alleles, because small populations lose alleles through drift.

Level	Measures	Importance	Threat	Protection
Genetic	Allele variety	Adaptation to change	Inbreeding, small populations	Seed banks
Species	Number and evenness	Stable food webs	Extinction	Protected species laws
Ecosystem	Variety of habitats	Many services	Habitat loss	Reserves

Part 6 of 6More from the specification

Significant

Less than 5% probability that the difference is due to chance.

Native species

A species that naturally lives in an ecosystem.

Key terms

Term	Meaning
Biodiversity	variation of genes, species and habitats
Resilience	ability of an ecosystem to resist and recover from disturbance
Keystone species	species with a disproportionate effect on its ecosystem
Trophic cascade	knock-on effect through food web from changing a top predator
Adaptation	heritable trait favoured by selection
Heterozygote advantage	higher fitness of heterozygotes than either homozygote
Ecosystem service	benefit provided by an ecosystem

Avoid these mistakes

Saying mutations arise because an organism needs them.

Thinking keystone species are the most abundant — their effect is disproportionate to abundance.

Saying diverse ecosystems never change — they are more resilient, not immune.

Equating biodiversity with the number of individuals — it is variation of genes, species and habitats.

Last updated: 14 October 2026