Interpreting a Field Trial of a Biological Control Agent
A biocontrol beetle is released to control an invasive weed on a 200-hectare reserve. Monitoring over five years records weed cover and beetle density:
| Year | Weed cover (%) | Beetle density (per m²) |
|---|---|---|
| 0 | 62 | 0 (release) |
| 1 | 58 | 0.4 |
| 2 | 40 | 1.8 |
| 3 | 22 | 3.1 |
| 4 | 24 | 1.2 |
| 5 | 35 | 0.3 |
Rainfall in years 3–5 was below average. A native beetle that feeds on the same weed is present at low density throughout. The reserve also contains a rare native plant that the introduced beetle has occasionally been observed on in laboratory choice tests.
1. Describe the relationship between weed cover and beetle density over years 0–3, and propose an explanation for the reversal in years 4–5.
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Years 0–3: weed cover falls from 62% to 22% as beetle density rises from 0 to 3.1 per m². The pattern is consistent with the beetle suppressing the weed - a numerical response (beetle population grows as food increases) followed by a functional response (more beetles consume more weed). The decline in weed is the dependent variable responding to the predator.
Years 4–5 reversal: weed cover rises again (22% → 35%) while beetle density collapses (3.1 → 0.3 per m²). Several explanations must be weighed:
- Resource depletion: the beetle reduced its own food supply, so its population crashed, releasing the weed from control.
- Abiotic stress: below-average rainfall in years 3–5 may have reduced beetle survival or reproduction directly, independent of weed availability.
- Delayed density dependence: the beetle's peak in year 3 overshot the sustainable density, and the subsequent crash is a lagged response.
The data alone cannot separate these; a strong answer notes that correlation does not establish which driver dominates and that rainfall is a confounding variable requiring controlled comparison.
2. Evaluate the proposed biocontrol program against an alternative (repeated herbicide application), identifying at least one ecological risk and one piece of evidence that would strengthen or weaken the case for the beetle.
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Case for the beetle: it is self-perpetuating, targets one weed species, avoids repeated chemical input, and the year 0–3 data show real suppression. Cost per hectare is likely lower once established.
Case against / risks:
- Non-target effects: laboratory choice tests show the beetle can feed on a rare native plant. Laboratory preference does not prove field attack, but it is a genuine risk requiring field monitoring before large-scale release.
- Persistence and reversibility: once released, the beetle cannot easily be recalled, and the year 4–5 crash shows control is not stable.
- Competition with the native beetle feeding on the same weed could displace it.
Evidence that would strengthen the case: field surveys showing no feeding damage on the rare native plant over multiple seasons, and replicated plots where weed cover stays low without repeated release.
Evidence that would weaken it: confirmed field damage to the native plant, or data showing the weed rebounds to pre-release levels whenever rainfall is low - indicating the beetle, not the weed, is the fragile element. Herbicide use has its own risks (non-target spray drift, resistance), so the comparison is risk-versus-risk, not risk-versus-safety.