Rewilding, Trophic Cascades and a Carbon Claim
A 40,000-hectare reserve reintroduces wolves after a 70-year absence. Baseline data: elk density 8 per km², willow saplings heavily browsed along streams, and stream banks eroding. Five years after reintroduction, elk density falls to 5 per km² and their distribution shifts away from open stream banks; willow height increases 3-fold in the first two years; beaver colonies rise from 2 to 9. A carbon-offset company claims the wolves have sequestered an extra 50,000 tonnes of CO₂ because 'predators create forests'. Meanwhile, a drought in year four reduces willow growth across the region, including outside the reserve.
1. Construct the trophic cascade argument linking wolves to stream-bank recovery, and identify the evidence that supports each link.
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Chain of reasoning.
- Wolves are apex predators; reintroduction increases predation pressure on elk.
- Elk numbers fall (8 → 5 per km²) and, crucially, their behaviour changes - they avoid open stream banks where they are more vulnerable. This is a trait-mediated effect, not just a density effect.
- Reduced browsing pressure lets willow saplings grow (3-fold height increase in two years).
- Taller willow stabilises banks, reducing erosion, and provides food and dam-building material for beaver (colonies 2 → 9).
- Beaver ponds slow water, trap sediment and raise the water table - further benefiting riparian vegetation.
Evidence quality. The elk density and distribution data support links 1–2; the willow height data support link 3; the beaver increase and reduced erosion support links 4–5. The cascade is plausible and internally consistent, but the data are correlational - no control reserve without wolves is described, so the drought comparison is the key check.
2. Evaluate the carbon-offset company's claim and the 50,000-tonne figure.
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Evaluating the claim. The phrase 'predators create forests' overstates the mechanism: wolves do not create forests, they can allow recovery of riparian vegetation where browsing was the limiting factor. The claim is directionally plausible but the number is unsupported.
Problems with the figure.
- No biomass or carbon stock measurements are given - only willow height and beaver counts. Extrapolating 50,000 tonnes from these requires assumptions about wood density, area of recovery and permanence that are not stated.
- The drought in year four reduced willow growth regionally, including outside the reserve. Using the outside area as a control reveals a climate signal that could explain part of the pattern and would reduce any wolf-attributable gain.
- Carbon must be counted against baseline and against permanence; fire, future drought or wolf removal could reverse gains.
Verdict. The trophic cascade is well supported; the carbon figure is not. A defensible claim would require measured carbon stocks inside and outside the reserve, a before-after-control-impact design, and a permanence assessment.