🐨 Possum Impact Investigation
Case Study: Understanding Pest Impact on Native Ecosystems
Investigation: How do possums impact native forest ecosystems?
Learning Focus: Analysing the effects of introduced species on native biodiversity
Background | Te Kōrero Tīmatanga
🦘 The Brushtail Possum in Aotearoa
- Introduced from Australia in 1837 for fur trade
- Population: approximately 30 million in New Zealand
- Eat 21,000 tonnes of vegetation per night
- No natural predators in NZ
- Also prey on native birds, eggs, and invertebrates
- Carry bovine tuberculosis (Tb) — threat to farming
Unlike in Australia where possums are protected natives, in Aotearoa they are one of our most damaging introduced pests. This case study examines how possums impact our native ngahere (forest) ecosystems.
1. Predictions | Ngā Whakapae
Before looking at the data, answer these questions:
a) What types of food do you think possums prefer to eat?
b) How might possum feeding affect the structure of a forest?
c) What native species might be most affected by possums? Why?
2. Data Analysis | Te Tātari Raraunga
Study 1: Possum Diet Analysis
Researchers examined the stomach contents of possums from different forests:
| Food Type | % of Diet (Podocarp Forest) | % of Diet (Beech Forest) |
|---|---|---|
| Leaves (native trees) | 65% | 55% |
| Flowers & fruit | 15% | 10% |
| Fern fronds | 10% | 20% |
| Invertebrates | 5% | 8% |
| Bird eggs/nestlings | 3% | 5% |
| Other | 2% | 2% |
Analysis Questions:
a) What is the main component of possum diet?
b) Why is possum predation on bird eggs particularly concerning for kererū (native pigeon)?
c) How might the loss of flowers and fruit affect other forest species?
Study 2: Forest Canopy Health
Scientists compared rātā trees in areas with and without possum control:
| Measurement | Possum Control Area | No Possum Control |
|---|---|---|
| Average canopy cover (%) | 85% | 45% |
| Rātā trees flowering | 78% | 12% |
| Dead/dying trees | 5% | 35% |
| Seedling regeneration | High | Very Low |
| Native bird count (per hectare) | 42 | 15 |
Analysis Questions:
a) Calculate the difference in canopy cover between the two areas:
b) Why do you think fewer rātā trees flower in the uncontrolled area?
c) How might reduced flowering affect tūī and kererū populations?
d) Explain the connection between possum browsing and reduced seedling regeneration:
3. Trophic Cascade | Te Arawhata Kai
A trophic cascade is when changes at one level of the food web cause chain reactions throughout the ecosystem.
Task: Create a diagram showing how possum browsing creates a trophic cascade in a native forest:
Include:
- Primary producers (trees like rātā, kāmahi)
- Primary consumers (possums, kererū, invertebrates)
- Secondary consumers (native predators, kārearea)
- Arrows showing energy flow
- Notes explaining the cascade effect
4. Case Study: Kōkako Recovery
🐦 The Kōkako Story
The kōkako (blue-wattled crow) was once common throughout New Zealand's North Island forests. By 1999, only about 400 pairs remained. Possums were identified as a major threat because they:
- Eat the same foods (fruit, flowers, leaves)
- Prey on eggs and chicks in nests
- Destroy nesting habitat
After intensive possum control, kōkako populations have increased to over 1,800 pairs!
Response Question:
Explain how possum control helped kōkako recovery. Use at least TWO ecological concepts in your answer.
5. Conclusion | Whakakapinga
Summarise how possums impact native forest ecosystems:
Kaitiakitanga Reflection:
As kaitiaki (guardians) of te taiao (the environment), what responsibilities do we have regarding introduced pests?
Extension Challenge
🎙️ Prepare for Class Debate
Topic: "Should 1080 poison be used for possum control in New Zealand?"
Research arguments FOR and AGAINST this question. Consider:
- Effectiveness of 1080 vs. alternatives
- Impact on non-target species
- Cost and practicality
- Māori perspectives on poison use in ngahere
- Scientific evidence from DOC and regional councils
See the Debate Preparation Guide for help structuring your arguments.