What biodiversity means

· Biodiversity = the total diversity of living systems. It exists at 3 levels: habitat diversity, species diversity and genetic diversity.
· Higher biodiversity usually increases resilience because ecosystems with more variety are generally better able to withstand disturbance and maintain stability.
· In exams, link low biodiversity to lower resilience, greater vulnerability to change and a higher risk of ecosystem instability.

Evolution and natural selection

· Evolution = cumulative change in the heritable characteristics of a population or species over time.
· Natural selection is the mechanism that drives evolutionary change.
· The process of natural selection requires:
· Variation within a population.
· Overproduction of offspring.
· Competition for limited resources.
· Differential survival and reproduction: individuals with advantageous adaptations are more likely to survive and reproduce.
· Because advantageous traits are heritable, their frequency increases over generations.
· Strong exam phrasing: the environment selects, so individuals do not adapt because they “need” to.

This diagram shows the core stages of natural selection: variation, overproduction, selection and reproduction. It is useful for explaining how advantageous traits become more common in a population over time. Use it to structure a full exam explanation of natural selection. Source

Speciation

· Speciation = the formation of new species through evolution.
· It happens when populations become isolated, experience different selective pressures, and eventually become unable to interbreed to produce fertile offspring.
· Core exam sequence: isolation → different adaptations → reproductive isolation → new species.
· Be clear that speciation is not just evolutionary change within a species; it is the formation of a separate species.

This diagram shows allopatric speciation, where a population is split by a geographic barrier and later becomes reproductively isolated. It is ideal for explaining the sequence from isolation to the formation of two distinct species. Use it when answering speciation questions. Source

Measuring biodiversity: richness, evenness and Simpson’s reciprocal index

· Species diversity depends on both richness and evenness.
· Richness = the number of species present.
· Evenness = how similar the population sizes of those species are.
· A community with high richness and high evenness has higher species diversity.
· Simpson’s reciprocal index is used to quantify species diversity, compare ecosystems and monitor change over time.
· Formula: $D = \frac{N(N-1)}{\sum n(n-1)}$
· $N$ = total number of individuals of all species.
· $n$ = number of individuals of one species.
· A higher $D$ value means greater diversity.
· 1 is the lowest possible value.
· Exam tip: when interpreting results, always comment on whether the value suggests greater or lower richness/evenness, not just “more biodiversity”.
· Practical link: collect valid sample data, then calculate and compare Simpson’s reciprocal index between sites or across time.

Why biodiversity knowledge matters

· Knowledge of global and regional biodiversity is essential for effective conservation management.
· Biodiversity data may be gathered by citizen science, voluntary organizations, government agencies, indigenous communities and parabiologists.
· In exam answers, connect biodiversity monitoring to better management strategies, conservation priorities and more accurate identification of areas at risk.

This map shows the global distribution of biodiversity hotspots, helping explain why biodiversity is unevenly distributed across Earth. It is especially useful for linking hotspot concentration to tropical regions and to conservation priority. Use it to support HL discussion of hotspot-based conservation. Source

Earth history, fossils, mass extinctions and the Anthropocene

· Earth history extends over about 4.5 billion years.
· The evolution of life is understood using the fossil record.
· Earth history is divided into the geological timescale: eons, eras, periods and epochs.
· Boundaries between time units are marked by major geological and biological changes, often visible in fossils.
· Mass extinctions are followed by rapid speciation because many niches become available.
· The previous five mass extinctions had natural causes such as tectonic movement, supervolcanic eruptions, climatic change, sea-level change and meteorite impact.
· The current crisis is often described as the anthropogenic sixth mass extinction.
· The Anthropocene is a proposed geological epoch marked by major human impact on Earth systems.
· There is debate over whether it should be formally recognized and when it began.
· Proposed golden spikes include:
· 1610: a dip in atmospheric CO₂ linked to the arrival of Europeans in the Americas.
· 1950: spherical fly ash particles.
· 1964: Carbon-14 markers from nuclear testing.
· Evidence humans are leaving a geological signature includes:
· chemical pollution preserved in strata
· mixing of native and non-native species in the fossil record
· nuclear deposits
· modified terrestrial and marine sedimentary systems
· human-made minerals
· Exam tip: distinguish Anthropocene evidence from the broader idea of human impact on biodiversity.

This diagram summarizes the geological timescale and marks major events in Earth history, including important biological changes. It helps students place mass extinctions, long-term evolution, and the idea of the Anthropocene into a deep-time context. Use it when revising fossils and geological time divisions. Source