Unit 1 Introduction to biodiversity
Key unit competence: Analyse factors that threaten biodiversity.
Key ideas
- Biodiversity is the variety of life in an area. It has three levels: genetic diversity (variety of alleles within a species), species diversity (number of species and how evenly individuals are spread among them) and ecosystem (habitat) diversity (variety of habitats and ecosystems).
- Key terms: species (organisms that can interbreed to produce fertile offspring), population, community, ecosystem, habitat, niche (the role of a species: what it eats, where and when it lives, how it interacts), species richness (number of species) and species evenness (how equal the numbers of each species are).
- Simpson's index of diversity: D = 1 − Σ(n/N)², where n = number of individuals of each species and N = total number of all individuals. D ranges from 0 (no diversity) to almost 1 (very high diversity). A high D shows a stable ecosystem that resists change.
- Sampling is used because counting every organism is impossible. Samples must be random to avoid bias, and repeated to be reliable. Methods: quadrats (frame quadrats for plants and slow animals; percentage cover or frequency), transects (line and belt transects to study changes along a gradient, e.g. from forest edge to grassland), pitfall traps, sweep nets, pooters, Tullgren funnels (soil animals), light traps (moths), kick sampling (stream animals), and capture-mark-release-recapture.
- Lincoln index: estimated population = (number marked in first sample × total in second sample) ÷ number marked in second sample.
- Importance of biodiversity: food, medicines (e.g. quinine from cinchona), timber, fuel, genetic resources for breeding, ecosystem services (pollination, clean water, soil formation, climate regulation, decomposition), tourism (gorillas in Volcanoes, Akagera wildlife), and cultural and aesthetic value.
- Threats (HIPPO): Habitat destruction (deforestation, draining wetlands, farming, urbanisation), Invasive species (water hyacinth, Lantana), Pollution (pesticides, eutrophication, plastics), Population growth of humans, Overexploitation (overhunting, poaching, overfishing, overharvesting of trees); plus climate change and disease.
- Conservation: in situ (in natural habitat: national parks such as Nyungwe, Volcanoes, Akagera, Gishwati-Mukura; forest reserves; protected wetlands) and ex situ (outside the habitat: zoos, botanical gardens, seed banks, gene banks, captive breeding). Also laws (CITES), community-based tourism revenue sharing, education, reforestation.
Common misconceptions
- "Biodiversity just means the number of species." No: it includes genetic, species and ecosystem diversity, and evenness as well as richness.
- "A Simpson's index of 0.9 means low diversity." No: values close to 1 show high diversity.
- "Placing quadrats where there are many plants gives a good sample." No: quadrats must be placed randomly (e.g. using random coordinates) to avoid bias.
- "Ex situ conservation is always better than in situ." No: in situ keeps whole ecosystems and natural behaviour; ex situ is a backup for very endangered species.
Worked examples
Example 1. In a sample, species A has 10 individuals, B 10 and C 20. Calculate Simpson's index of diversity.
Solution
N = 40. n/N: A 0.25, B 0.25, C 0.5. Squares: 0.0625 + 0.0625 + 0.25 = 0.375. D = 1 − 0.375 = 0.625.
Example 2. Explain why a belt transect is better than random quadrats to study plants from a river bank up a hillside.
Solution
The environment changes gradually along a line (moisture, light, soil). A belt transect places quadrats at regular intervals along that line, so it shows how distribution and abundance change with the gradient; random quadrats would mix the zones and hide the pattern.
Practice questions
Define the three levels of biodiversity.
Show answer
Genetic diversity: variety of alleles within a species. Species diversity: number of species and evenness of their numbers. Ecosystem diversity: variety of habitats and ecosystems in an area.
Common mistake: Giving only species richness.
Calculate Simpson's index for: species P 15, Q 5, R 5, S 25.
Show answer
N = 50; n/N = 0.3, 0.1, 0.1, 0.5; squares 0.09 + 0.01 + 0.01 + 0.25 = 0.36; D = 1 − 0.36 = 0.64.
Common mistake: Forgetting to subtract from 1.
Describe how to use quadrats to estimate the population of a plant in a field.
Show answer
Use random coordinates to place quadrats; count the plants (or percentage cover) in each; repeat many times; calculate the mean per quadrat; divide by quadrat area to get density; multiply by the area of the field.
Common mistake: Placing quadrats where plants are dense.
State four importances of biodiversity.
Show answer
Food and medicines; genetic resources for crop and animal breeding; ecosystem services (pollination, clean water, soil fertility, climate regulation); tourism income; cultural and aesthetic value.
Common mistake: Listing threats instead.
Explain five threats to biodiversity in Rwanda.
Show answer
Habitat loss (farming, settlements, draining wetlands); invasive species (water hyacinth); pollution (pesticides, plastics, eutrophication); overexploitation (poaching, charcoal, overfishing); climate change and human population growth.
Common mistake: Giving threats without explanation.
Distinguish between in situ and ex situ conservation with two examples each.
Show answer
In situ: in the natural habitat (national parks, forest reserves). Ex situ: outside the habitat (zoos, botanical gardens, seed banks, captive breeding).
Common mistake: Calling national parks ex situ.
Name a suitable method to sample: (a) earthworms; (b) beetles on the ground; (c) stream invertebrates.
Show answer
(a) Digging/soil samples or Tullgren funnel for small ones; (b) pitfall traps; (c) kick sampling with a net.
Common mistake: Using a light trap for beetles on the ground.
Explain why a high Simpson's index suggests a stable ecosystem.
Show answer
Many species with even numbers mean complex food webs; if one species declines, consumers have alternatives, so the ecosystem resists disturbance.
Common mistake: Saying high index means one dominant species.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Sampling is repeated many times to: a. improve reliability 2. Volcanoes National Park mainly protects: b. mountain gorillas 3. The variety of alleles within a species is: c. species richness 4. The number of different species in an area is: d. genetic diversity Show answer
1 → a (improve reliability) · 2 → b (mountain gorillas) · 3 → d (genetic diversity) · 4 → c (species richness)
- A mean of many samples is more reliable.
- It is in the Virunga massif.
- It allows adaptation to change.
- Evenness measures how equal their numbers are.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
200 · ex situ conservation · outcompeting them for light and space · making the soil too fertile- In a capture-recapture study, 30 beetles were marked; later 40 were caught, of which 6 were marked. The population is .
- A seed bank is an example of .
- Lantana spreading in a national park reduces native plants mainly by .
Show answer
1: 200 · 2: ex situ conservation · 3: outcompeting them for light and space
- (30 × 40) ÷ 6 = 200.
- Seeds are stored outside the habitat.
- Invasive plants have few natural enemies.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The role of a species in its ecosystem is its: a. new medicines from plants 2. Small animals in leaf litter and soil are extracted using a: b. Tullgren funnel 3. In Simpson's index D = 1 − Σ(n/N)², N is the: c. niche 4. A benefit of biodiversity is: d. total number of individuals of all species Show answer
1 → c (niche) · 2 → b (Tullgren funnel) · 3 → d (total number of individuals of all species) · 4 → a (new medicines from plants)
- It includes feeding and interactions.
- Heat drives animals downward.
- n is for each species.
- Many drugs come from plants.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
too high, because of bias · high diversity · too low, because of bias · 0.62- A sample gives n/N values 0.5, 0.3 and 0.2. Simpson's index is .
- A learner places quadrats only where many flowers grow. The estimate will be .
- Simpson's index of diversity close to 1 indicates .
Show answer
1: 0.62 · 2: too high, because of bias · 3: high diversity
- 1 − (0.25 + 0.09 + 0.04) = 0.62.
- Non-random placement biases results.
- Values range from 0 to almost 1.