A degraded site (cleared, farmed, or mined and then abandoned) recovers through successional stages: bare soil → pioneer weeds → grass/shrub → early-secondary thicket → late-secondary forest → mature climax forest. Pioneer species tolerant of poor soil colonize first and slowly build soil structure and shade — but advancing past the early-secondary stage needs a later-successional key species to establish, and that requires either a nearby intact ecosystem supplying steady seed rain, or a human hand.
Passive restoration removes the disturbance and waits. If a nearby seed source exists, the key species eventually arrives by natural dispersal and succession resumes toward maturity — slowly, but for free. If no nearby seed source exists (isolated site, poor disperser, missing pollinator/animal vector), the probability of natural arrival is zero: the site is permanently arrested at the early-secondary stage no matter how many decades pass. Try "+50 yr" repeatedly with the seed source off — the stage never moves.
Active restoration pays an upfront cost to directly plant/reintroduce the key species, bypassing the need for natural dispersal entirely. This breaks a stall instantly and lets succession resume at a faster rate toward the mature target — at the cost of nursery stock, labor and site prep money, regardless of whether a seed source exists.
- Seed source toggle — flips whether the wider landscape can supply the key species on its own; only matters for passive restoration.
- Replant key species — pays the active-restoration cost and instantly unlocks progression past the early-secondary bottleneck, from whichever stage the site is currently at.
- +50 yr — fast-forwards the clock to demonstrate that passive restoration without a seed source truly never advances, however long you wait.
Real-world relevance: restoration ecologists check for seed-source proximity and disperser presence before recommending passive recovery — arrested succession from missing key species is a well-documented failure mode of "just let nature do it" strategies.