VCE Environmental Science (Year 11) · Unit 1 — Earth's Systems and Ecosystems · 16m read

Biodiversity — Measurement and Threats

Ever wondered why losing one species might trigger a chain reaction that collapses an entire ecosystem? Biodiversity is the variety of all living things in a place, and you'll learn how scientists actually measure it using tools like species counts and genetic diversity indexes. This matters because we're losing species faster than ever before, and understanding these threats—from habitat destruction to climate change—helps you see why protecting ecosystems isn't just environmentally important, it's essential for human survival and VCE success in environmental science.

Learning objective: Measure and analyse biodiversity and evaluate the threats to it.

By the end of this lesson you can:

• I can explain the three levels of biodiversity (genetic, species, ecosystem).
• I can calculate species richness and the Simpson's Diversity Index.
• I can identify and explain the main threats to biodiversity (HIPPO).

Biodiversity is a Jenga tower. Each species is a block; remove too many and the tower collapses.

Biodiversity — levels, measurement and threats

### Three levels of biodiversity

Genetic diversity: Variation in genes within a species.
- Important for: adaptation to environmental change; disease resistance.
- Threat: small isolated populations lose genetic diversity (inbreeding).

Species diversity: Number and relative abundance of species in an area.
- Measured by: species richness, species evenness, diversity indices.

Ecosystem diversity: Variety of ecosystems (forest, grassland, wetland, coral reef) in a region.

### Measuring species diversity

Species richness: Simple count of species in an area.

Simpson's Diversity Index (D):

$$D = 1 - \frac{\sum n_i(n_i - 1)}{N(N-1)}$$

Where $n_i$ = number of individuals of species $i$; $N$ = total individuals.
- $D$ close to 1 = high diversity
- $D$ close to 0 = low diversity (dominated by one species)

### Threats to biodiversity — HIPPO

H — Habitat loss: Deforestation, wetland drainage, urbanisation. #1 threat globally.

I — Invasive species: Introduced species outcompete or prey on natives.
- Australia: feral cats kill 1.5B animals/year; foxes, rabbits, cane toads.

P — Pollution: Pesticides, plastic, nutrient runoff, air and water pollution.

P — Population growth: More humans = more land, resources, and waste.

O — Over-exploitation: Overfishing, poaching, unsustainable harvest.

+ Climate change: Increasingly amplifies all other threats — shifts habitat zones, intensifies disturbances, causes coral bleaching.

### Keystone species

A keystone species has a disproportionately large effect on its ecosystem relative to its abundance.

*Example:* Sea otters eat sea urchins. Without otters (overhunted for fur), urchins explode in numbers and devour kelp forests — destroying an entire ecosystem. Restoring otters restores the kelp forest.

AARI TIP: Ask Aari to explain the trophic cascade concept using the reintroduction of wolves to Yellowstone.

Biodiversity Measurement

• Species richness: S = \text{total number of species} (Simple count; no abundance info)
• Simpson's index: D = 1 - \dfrac{\sum n_i(n_i-1)}{N(N-1)} (D→1 = high diversity; D→0 = low diversity)

Quick reference

| Level | Meaning |
|---|---|
| Genetic | Variation within species |
| Species | Number of species |
| Ecosystem | Variety of habitats |
| Measurement | Quadrats, transects, Simpson's index |
| Threats | Habitat loss, invasive species |

Key ideas at a glance

• Measure systematically: Sampling.
• Threats: HIPPO.
• Conservation: In situ and ex situ.
• Indices: Compare diversity.

Worked example (Medium)

Three sites each have 3 species with counts: Site A: (20, 20, 20); Site B: (58, 1, 1); Site C: (10, 20, 30). Compare species richness and diversity.

1. Species richness. All sites have 3 species — same richness (S = 3).
2. Site A: Simpson's D. N = 60. $\sum n(n-1) = 20(19) \times 3 = 1140$ D_A = 1 - \frac{1140}{60 \times 59} = 1 - \frac{1140}{3540} \approx 0.68
3. Site B: Simpson's D. N = 60. $\sum = 58(57) + 1(0) + 1(0) = 3306$ D_B = 1 - \frac{3306}{3540} \approx 0.07
4. Conclusion. Site A: high diversity (even distribution). Site B: very low diversity (almost a monoculture). Same richness, very different diversity.

Answer: Species richness fails to capture diversity — evenness matters. Site A ($D \approx 0.68$) is far more diverse than Site B ($D \approx 0.07$) despite both having 3 species.

Worked example (Medium)

A reserve has 40 species in 2014 and 28 species in 2024 with no immigration. Calculate the percentage loss in species richness, and identify two likely causes.

1. Loss. 40 − 28 = 12 species lost.
2. Percentage. 12 / 40 × 100 = 30% loss.
3. Cause 1. Habitat fragmentation — patches too small to sustain breeding populations.
4. Cause 2. Invasive species displacing natives.

Answer: 30% loss of species richness over 10 years; likely drivers include fragmentation and invasive competition.

Common mistakes

• Equating species richness with biodiversity Students count species and assume high richness = high biodiversity, ignoring evenness and other levels. Fix: Biodiversity has three levels: genetic, species, and ecosystem. Species diversity includes both richness AND evenness — use Simpson's D to capture both dimensions.
• Listing HIPPO threats without explaining mechanisms Students memorise the acronym but cannot explain HOW each threat reduces biodiversity in a specific context. Fix: For each HIPPO threat, explain the mechanism: e.g., habitat fragmentation isolates populations → reduced gene flow → inbreeding depression → reduced adaptive capacity → local extinctions.

Minimum viable populations — how small is too small?

Conservation biology uses the concept of minimum viable population (MVP) — the smallest population size that can survive long-term (typically defined as having a 95% probability of persisting for 100 years) without intervention.

Small populations face several threats that large populations can absorb:

Genetic drift: In small populations, random chance can eliminate alleles, reducing genetic diversity regardless of natural selection. Traits that would be neutral or even beneficial in large populations can be lost by accident.

Inbreeding depression: When a small population breeds among close relatives, harmful recessive alleles are more likely to be expressed. This reduces fitness — lower birth rates, higher disease susceptibility, reduced survival.

Demographic stochasticity: Random variation in births and deaths has bigger impacts in small populations. A few bad breeding seasons could push a population of 50 to extinction, while a population of 5,000 would easily recover.

Research suggests MVPs for vertebrates typically range from 500 to several thousand individuals, though this varies by species. Many Australian threatened mammals exist in populations well below these thresholds.

For Unit 1 Environmental Science, MVP connects biodiversity threats to management responses: captive breeding, translocation programs, habitat corridors, and predator-proof enclosures all aim to maintain populations above minimum viable levels.

Practice questions

1. According to HIPPO, the most significant threat to global biodiversity is:

   1. Over-exploitation
   2. Habitat loss
   3. Invasive species
   4. Pollution

   (1 marks)

2. Explain why Australia has the world's highest rate of mammal extinctions since European settlement.

   (3 marks)

Summary

• Three biodiversity levels: genetic (gene variation), species (number/evenness), ecosystem (variety of ecosystems).
• Simpson's D = $1 - \sum n_i(n_i-1)/N(N-1)$. Close to 1 = diverse; 0 = mono-dominated.
• HIPPO: Habitat loss (primary), Invasives, Pollution, Population growth, Over-exploitation.
• Climate change amplifies all HIPPO threats.
• Australia: highest mammal extinction rate — isolation + invasive species + habitat loss.