Variety of life: genetic diversity, species diversity and habitat (ecosystem) diversity.
Species richness
The number of different species.
Species evenness
How evenly individuals are spread among the species.
Simpson's reciprocal index
D = N(N − 1) ÷ Σn(n − 1): higher D = more diverse.
Natural selection
Individuals with favourable variations survive and reproduce more.
Speciation
Formation of new species, often after geographical isolation.
\(N\) = total number of organisms, \(n\) = number of each species. Compare D values for similar habitats; a high D suggests a stable, healthy ecosystem.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: A diversity index only makes sense when compared: say whether D is higher or lower than in another site or time, and why.
3.2Human impact on biodiversity
Key terms
HIPPO
Main threats: Habitat loss, Invasive species, Pollution, Population (human), Overexploitation.
Invasive species
A non-native species that spreads and harms native ecosystems.
IUCN Red List
Assesses the extinction risk of species (from least concern to extinct).
Habitat fragmentation
Breaking a habitat into smaller, isolated patches.
Habitat loss and fragmentation is the biggest threat: small patches support fewer species, have more edge effects and isolate populations.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Use a named species or ecosystem case study with specific facts (location, numbers, threats, protection).
3.3Conservation and regeneration
Key terms
In situ conservation
Protecting species in their natural habitat (e.g. national parks).
Ex situ conservation
Protecting species outside their habitat (zoos, seed banks, botanic gardens).
Species-based conservation
Focused on individual species, e.g. flagship or keystone species.
Habitat-based conservation
Protecting whole ecosystems.
Rewilding
Restoring natural processes, sometimes by reintroducing species.
Reserve design: large, circular (less edge), connected by wildlife corridors, buffer zones around a protected core.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: The strongest answers combine species- and habitat-based approaches and link them to a specific case study.