UK Biodiversity Net Gain accounting (Environment Act 2021 habitat metric) scores a site in habitat units = area × distinctiveness × condition. Development destroys the baseline units the moment clearance begins — the loss is instant. A created offset site, however, does not start at full quality: it matures toward its target condition over years, so the metric applies a temporal discount that rewards ratios high enough to outrun the lag.
Delivered(t) = 0 for t ≤ delay
Delivered(t) = Target · (1 − e^(−(t−delay)/τ)) · (1 − risk) · difficulty
for t > delay
Target = Units_lost × offset ratio
τ = time-to-target ÷ 3 (≈95% mature by time-to-target)
Net(t) = Delivered(t) − Units_lost
Debt = ∫ max(0, −Net(t)) dt over 0…year (unit-years of deficit)
- Habitat type sets three real discount inputs: time-to-target (how long full condition takes to establish), risk-of-failure (chance the offset never reaches target), and a difficulty-of-creation multiplier — woodland is slow, risky, and hard to recreate; grassland is fast, low-risk, and easy.
- Offset ratio is the nominal units created per unit lost — regulators require ratios above 1:1 precisely because of this discounting.
- Implementation delay models the gap between a development's clearance and the offset site's planting/restoration start.
- Biodiversity debt is the running area between the zero-line and the net-balance curve while it is negative — a proxy for real, temporary biodiversity loss the landscape carries even in a scheme that eventually reaches "no net loss".
- Habitat grid (top panel) — each site is drawn as an 8×8 field of growing/shrinking squares: red squares vanish the instant clearance starts; green squares grow in one by one as the offset matures toward its target, mirroring the 3D version's instanced marker field in a flat top-down view.
- Net-balance chart (bottom panel) — drag left/right anywhere on it to scrub the monitoring year directly; the shaded red band is the running biodiversity debt.
Real-world relevance: this temporal-lag problem is why most net-gain standards (UK BNG, several US wetland-mitigation banking rules) mandate offset ratios well above 1:1 and favour habitat banking that starts maturing years before the paired loss occurs.