What is Biodiversity?
Biodiversity has three components:
- [[Species and Taxonomy|Species]] richness - the number of different species in a community (Q3.1 in your paper). Simple count of species.
- Species evenness - how evenly individuals are distributed among species. A community with 100 species but 99% of individuals belonging to one species has low evenness.
- [[Genetic Diversity and Adaptation|Genetic diversity]] - variation at the allele level within populations.
The Simpson's Index of Diversity
AQA requires you to be able to use the formula:
$$
D = 1 - \frac{\sum n(n-1)}{N(N-1)}
$$
Where:
- n=number of individuals of each species
- N=total number of individuals of all species
- D ranges from 0 (no diversity) to 1 (infinite diversity)
A higher D value indicates a more diverse, stable community. Ecosystems with high biodiversity tend to be more resilient because the loss of one species has a less catastrophic effect when other species can fill similar ecological roles.
Factors Affecting Biodiversity
Human activities that reduce biodiversity:
- Habitat destruction (deforestation, urbanisation)
- Monoculture farming
- Pollution
- Introduction of invasive species
- Climate change
Conservation strategies:
- Protected areas (national parks, nature reserves)
- Seed banks
- Captive breeding programmes
- International agreements (e.g. CITES)
3.4.7 - Investigating Diversity
Random Sampling
To measure biodiversity, samples must be random to avoid bias. Techniques include:
- Random quadrats - coordinates generated randomly, quadrats placed at intersections to count organisms
- Transects - a line is laid across a habitat; quadrats placed at regular intervals to detect change across an environmental gradient (e.g. from a rockpool to the shore)
Why random? If you choose where to sample, you unconsciously introduce bias (e.g. sampling only where you see lots of organisms), making results unrepresentative of the whole area.
Statistical Testing - the t-test and Chi-squared
For biodiversity investigations, you may need to compare means between two habitats using the t-test, or test whether observed ratios differ significantly from expected using the chi-squared (χ²) test.
Q6.2 - 6.3 in your paper used chi-squared to test whether the proportion of plants producing 2n gametes changed between breeding cycles. The calculated χ² (350) was far greater than the critical value (3.841) at p=0.05, meaning the difference between observed and expected was statistically significant - the null hypothesis was rejected, and the scientists could conclude the proportion of plants producing 2n gametes did change significantly between cycles.
Directional Selection (Q6.4)
This links biodiversity to evolution. [[Overview|Directional selection]] occurs when an extreme phenotype at one end of the range is favoured by the environment. Over successive generations, the allele frequency shifts in that direction.
In Q6.4, the scientists were selecting plants that produced 2n gametes and using only those to breed. Plants producing 2n gametes had a selective advantage (they were chosen to reproduce). Over breeding cycles 0→3, the proportion producing 2n gametes increased from 4/54 to 56/56 - the allele(s) for 2n gamete production became fixed in the population because individuals without them were not selected for reproduction.
Summary
- Biodiversity has three components: species richness (number of species), species evenness (how evenly distributed), genetic diversity (allele variation within populations)
- Simpson's Index: D = 1 − Σn(n−1)/N(N−1). Range: 0 (no diversity) to 1 (maximum). Higher D = more stable, resilient ecosystem.
- Human activities reduce biodiversity: habitat destruction, monoculture, pollution, invasive species, climate change
- Conservation strategies: protected areas, seed banks, captive breeding, international agreements (CITES)
- Random sampling: quadrats at random coordinates; transects for change across a gradient. Random to avoid bias.
- χ² test: compares observed vs expected frequencies; significant result (calculated > critical value at p=0.05) → reject null hypothesis → difference is not due to chance
- Directional selection: extreme phenotype favoured→allele frequency shifts over generations
AQA Exam Tips
- Simpson's Index formula: D = 1 − Σn(n−1)/N(N−1). Remember: n = individuals of each species; N = total. A higher D means more diversity. A result close to 1 = highly diverse community.
- Why random sampling: to avoid bias - if you choose where to sample, you may unconsciously select more or fewer organisms than the true average.
- χ² interpretation: if calculated χ² > critical value at p=0.05 (with correct degrees of freedom), the null hypothesis is rejected and the difference is statistically significant. Always state what the null hypothesis is first.
- Three components of biodiversity: AQA may ask you to state what biodiversity means. State all three: species richness, species evenness, genetic diversity.
- Conservation - give reasons: protected areas protect habitat; seed banks preserve genetic diversity for future use; captive breeding prevents extinction of critically endangered species.